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Title: Texas Rocks and Minerals: An Amateur's Guide

Author: Roselle M. Girard

Illustrator: Bill M. Harris

Release date: August 18, 2016 [eBook #52839]

Language: English

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*** START OF THE PROJECT GUTENBERG EBOOK TEXAS ROCKS
AND MINERALS: AN AMATEUR'S GUIDE ***

BUREAU OF ECONOMIC GEOLOGY
The University of Texas
Austin, Texas
Peter T. Flawn, Director

Guidebook 6

TEXAS ROCKS AND MINERALS
An Amateur’s Guide

By
ROSELLE M. GIRARD
Sketches by Bill M. Harris

February 1964
Second Printing, April 1972
Third Printing, April 1976
Fourth Printing, May 1979

iii

Contents

Page
Preface vii
Introduction 1
Earth’s outer crust 2
Geologists 2
Time and rock units 2
Geologic map 6
What are rocks and minerals? 7
Chemical elements 7
Minerals 7
Rocks 8
Igneous rocks 9
Extrusive or volcanic igneous rocks 9
Intrusive igneous rocks 9
Sedimentary rocks 10
Soils 10
Sedimentary rock materials in broken fragments 11
Sedimentary rock materials in solution 12
Cementing materials and chemical sediments 12
Sedimentary rocks formed by plants and animals 12
Metamorphic rocks 12
Static metamorphism 13
Contact metamorphism 13
Dynamic metamorphism 14
Occurrence and properties of minerals 14
How minerals occur 14
Crystalline minerals 14

Crystals 14
Imperfect crystals 14
Amorphous minerals 15
Some distinguishing properties of minerals 15
Color 16
Luster 16
Transmission of light 16
Hardness 16
Streak or powder 17
Cleavage 17
Parting 17
Fracture 17
Specific gravity 18
Effervescence in acid 18
Some special occurrences of minerals 18
Cave deposits 18
Concretions 19
Geodes 19
Petrified wood 20
Collecting rocks and minerals 22
Rock and mineral identification charts 24
How to use the mineral identification charts 24
Key to mineral identification charts 25
Mineral identification charts 26
How to use the rock identification charts 39
Rock identification charts 40
Descriptions of some Texas rocks and minerals 43
Anhydrite 43
Asbestos 43
Barite 44
Basalt 45
Calcite 46
Cassiterite 47
Celestite 48
Cinnabar 49
Clay 51
Copper minerals (chalcocite, chalcopyrite, malachite, azurite) 52
Dolomite 54

Feldspar 55
Fluorite 56
Galena 57
Garnet 58
Gneiss 59
Gold 59
Granite 61
Graphite 62
Gypsum 63
Halite 65
Hematite 66
Limestone 68
Limonite 70
Llanite 71
Magnetite 72
Manganese minerals (braunite, hollandite, pyrolusite) 73
Marble 75
Mica 76
Obsidian and vitrophyre 77
Opal 78
Pegmatite 79
Pyrite 80
Quartz 81
Quartzite 84
Rhyolite 85
Sand and sandstone 85
Schist 87
Serpentine 87
Shale 88
Silver minerals (argentite, cerargyrite, native silver) 89
Sulfur 90
Talc and soapstone 93
Topaz 94
Tourmaline 94
Uranium minerals (carnotite, uranophane, pitchblende) 95
Volcanic ash (pumicite) 97
Composition, hardness, and specific gravity of some Texas minerals 99
Books about rocks and minerals 100

Nontechnical books for beginners 100
Textbooks and other reference books 100
Selected references on Texas rocks and minerals 100
Glossary 102
Index 104

Illustrations

Page
Guadalupe Peak and El Capitan in the Guadalupe Mountains, Culberson
County, Texas
1
Earth’s outer crust 2
Geologic time scale 3
Generalized geologic map of Texas 4-5
A mineral is made up of chemical elements 7
A rock is made up of minerals 8
Extrusive igneous rocks form at the earth’s surface 9
Intrusive igneous rocks form beneath the earth’s surface 10
Soils develop from weathered rock and associated organic material 11
Conglomerate from Webb County, Texas 11
Precipitated sediments lining a teakettle 12
Contact metamorphism 13
A scalenohedron 14
Barite specimen showing radial form 15
Chalcedony showing botryoidal form 16
Transparent mineral 16
Streak plate 17
Conchoidal fracture 18
Stalactites and stalagmites in the Caverns of Sonora, Sutton County, Texas 19
Calcite geode from Travis County, Texas 20
Petrified wood from Texas Gulf Coastal Plain 20
Prospector’s hammer 22
Hand lens 22
Physiographic outline map of Texas 42

Massive anhydrite 43
Amphibole asbestos from Gillespie County, Texas 44
Barite cleavage fragment from west Texas 44
Basalt from Brewster County, Texas 45
Calcite has perfect rhombohedral cleavage 46
Calcite crystals (dog-tooth spar) from the Terlingua area of Brewster County,
Texas
47
Celestite cleavage fragment from Lampasas County, Texas 48
Cinnabar and calcite crystals from the Terlingua area of Brewster County,
Texas
50
Bentonite is used as a drilling-fluid additive 51
Hazel copper-silver mine, Culberson County, Texas 53
Dolomite rock from Burnet County, Texas 54
Feldspar cleavage fragment from Llano County, Texas 55
Microcline feldspar crystals from Llano County, Texas 56
Fluorite has octahedral cleavage 57
Galena has perfect cubic cleavage 57
Garnet crystal forms 58
Gneiss from Blanco County, Texas 59
Placer gold in stream gravels 60
Granite from Gillespie County, Texas 61
Texas State Capitol building at Austin is made of Burnet County granite 62
Graphite is used in pencil lead, generator brushes, and lubricants 63
Selenite gypsum crystal from Bastrop County, Texas 64
Selenite gypsum rosettes from Nolan County, Texas 64
Fibrous gypsum from Terlingua area, Brewster County, Texas 65
Salt domes occur on the Gulf Coastal Plain 66
Specular hematite from Carrizo Mountains, Hudspeth County, Texas 67
Limestone from Travis County, Texas 68
Limestone quarry at Georgetown, Williamson County, Texas 69
Limonite ore is changed to metallic iron in a blast furnace 71
Metallic iron is made into steel in an open-hearth furnace 72
Magnetite, Llano County, Texas 73
Hollandite from Jeff Davis County, Texas 74
Precambrian metamorphic marble from Llano County, Texas 75
Mica minerals have perfect cleavage in one direction 76

Obsidian arrowheads 77
Opalized wood from Washington County, Texas 78
Quartz-feldspar pegmatite from Burnet County, Texas 79
Pyrite veins in white marble from Llano County, Texas 80
Cubic crystals of pyrite 80
Quartz crystal from Burnet County, Texas 81
Amethyst geode from the Alpine area of Brewster County, Texas 82
Milky quartz from Burnet County, Texas 82
Smoky-quartz crystals from Burnet County, Texas 83
Polished agate from Rio Grande gravels of Zapata County, Texas 83
Jasper from Uvalde County, Texas 84
Sandstone from Zavala County, Texas 86
Prospector 89
Sulfur is obtained by the Frasch process 92
Talc schist from the Allamoore area of Hudspeth County, Texas 93
Topaz crystal from Mason County, Texas 94
Black tourmaline crystals with milky quartz from Llano County, Texas 95
A Geiger counter is used to detect radioactivity 96

vii

PREFACE

This booklet has been designed to serve as a brief, simple guide that will be
of help to school children, amateur collectors, and others who are just
beginning to develop an interest in the rocks and minerals of Texas. It is a
companion volume to Texas Fossils by William H. Matthews III published
as Guidebook No. 2 by the Bureau of Economic Geology.

Numerous present and former staff members of The University of Texas
contributed time and talents to the preparation of this book, and their help is
gratefully acknowledged: Peter T. Flawn, Director of the Bureau of
Economic Geology, Thomas E. Brown, John W. Dietrich, Alan Humphreys,
Elbert A. King, Jr., Peter U. Rodda, and others, including the late John T.
Lonsdale, made many helpful suggestions; John S. Harris and Miss
Josephine Casey edited the manuscript; Cader A. Shelby prepared a number
of the photographs; Bill M. Harris made the illustrative sketches under the
direction of James W. Macon; and Cyril Satorsky designed the cover.

1

Texas Rocks and Minerals
An Amateur’s Guide
Roselle M. Girard

INTRODUCTION

Texas has a great variety of rocks and minerals—some are common and
others are not. This book is designed to acquaint you with some of them and
to tell you in a nontechnical way what they are like, some of the places
where they are found, and how they are used. Although we do not know
exactly how all of the rocks and minerals formed, some of the ideas about
their origin are mentioned.

If you would like to learn more about rocks and minerals in general, the
names of several reference books are listed on page 100. In addition,
scientific reports that describe in detail many of the rocks and minerals of
Texas have been published by the Bureau of Economic Geology of The
University of Texas, the United States Geological Survey, and other
organizations. A selected list of these reports is given on pages 100-101.

Rocks and minerals are familiar objects to all of us. We pick up attractive or
unusual pebbles for our collections, we admire rocky mountain peaks, we
speak of the mineral resources of our State and Nation. Rocks and minerals
enter, either directly or indirectly, into our daily living. From them come the
soils in which grow the grains, the fruits, and the vegetables for our food, the
trees for our lumber, and the flowers for our pleasure. The iron, copper, lead,
gold, silver, and manganese, the sulfur and salt, the clays and building
stones, and the other metals and nonmetals that we require for our way of
living were once a part of the earth’s crust.

Texas’ highest mountain is Guadalupe Peak, right, with an elevation of 8,751 feet. El Capitan,
left, has an elevation of 8,078 feet. These peaks in the Guadalupe Mountains in Culberson
County consist largely of Capitan reef limestone, which formed during the Permian Period.

2

Earth’s Outer Crust
Rocks and minerals make up most of the outer layer or crust of our earth—
the actual ground beneath our feet. The crust is approximately 18 to 30 miles
thick beneath the continents. In general, the outermost part consists of many
layers of stratified rocks, one above another. The older rocks normally make
up the bottom or the deeper layers, and the younger rocks form the upper
layers. Not all the layers are perfectly flat and parallel—some are lenticular
(lens-shaped), some are tilted, some are partly eroded away, and some are
present in one place and absent in another. Beneath the continents, the layers
of rock rest on ancient metamorphic rocks and on great masses of igneous
rock such as granite. These lower rocks are known as the basement.

Earth’s outer crust (thickness not drawn to scale).

Over much of the land surface of the earth, the outermost layer is made up of
layers of rock

On the continents, the layers of rock rest on metamorphic rocks and on
igneous rocks such as granite

Geologists
Those who study the earth’s crust—its origin, history, rocks, minerals,
fossils, and structure—are known as geologists. The geologists who are
especially interested in a particular phase of geology, as this science is
called, are given special names: those who study fossils are called
paleontologists; those who study minerals are called mineralogists; those
who study rocks are called petrologists.

Time and Rock Units
The earth’s crust is believed to be at least 3¼ billion years old. In order to
deal with this vast stretch of time, geologists have divided the billions of
years into various time units and have given each unit a name. The great
divisions of geologic time, called eras, are Early Precambrian, Late
Precambrian, Paleozoic, Mesozoic, and Cenozoic. These eras are divided

into smaller units of time called periods, and the periods are divided into
epochs. The [xx time scale] shows the geologic time divisions. Earliest
geologic time is shown at the bottom of the scale; most recent is shown at
the top.

By examining and studying the different rocks and rock layers, geologists try
to discover in which unit of geologic time these rocks formed. Those rocks
that formed during a period of geologic time are called a system of rocks;
those that formed during an epoch are called a series. For example, the
Cambrian System of rocks formed during the Cambrian Period; the
Cretaceous System of rocks formed during the Cretaceous Period; the
Tertiary System of rocks formed during the Tertiary Period. We are now in
the younger epoch (called Recent) of the Quaternary Period of the Cenozoic
Era. The rocks that are forming now are the Recent Series of rocks.

3

Geologic time scale

ERA
PERIOD
EPOCH
CENOZOIC
QUATERNARY (lasted 0-1 million years)
Recent
Pleistocene
TERTIARY (lasted 62 million years)
Pliocene
Miocene
Oligocene
Eocene
Paleocene
—63 million years ago—
MESOZOIC
CRETACEOUS (lasted 72 million years)
JURASSIC (lasted 46 million years)
TRIASSIC (lasted 49 million years)
—230 million years ago—
PALEOZOIC
PERMIAN (lasted 50 million years)
PENNSYLVANIAN (lasted 30 million years)
MISSISSIPPIAN (lasted 35 million years)
DEVONIAN (lasted 60 million years)
SILURIAN (lasted 20 million years)
ORDOVICIAN (lasted 75 million years)
CAMBRIAN (lasted 100? million years)
—600? million years ago—
LATE PRECAMBRIAN
EARLY PRECAMBRIAN
These time estimates are from the paper, Geologic Time Scale, by J. Lawrence Kulp, published in
Science, Vol. 133, No. 3459, April 14, 1961. (The time divisions are not drawn to scale)

4

Plate 10. GENERALIZED GEOLOGIC MAP OF TEXAS
Modified from Geologic Map of Texas, 1933

This map in a higher resolution

EXPLANATION
CENOZOIC
1 Quaternary
2 Tertiary (Oligocene, Miocene, and Pliocene)
3 Tertiary (Eocene)
4 Volcanic (extrusive) igneous rocks
MESOZOIC
5 Upper Cretaceous (Gulf series)

6 Lower Cretaceous (Comanche series)
7 Jurassic
8 Triassic
PALEOZOIC
9 Permian
10 Mississippian and Pennsylvanian
11 Cambrian, Ordovician, Silurian, Devonian and undivided
Paleozoic
12 Rocks (Precambrian) older than Paleozoic
13 Intrusive igneous rocks (Precambrian, Mesozoic or
Cenozoic)
These rocks are found either at the surface or directly beneath the soils and subsoils
which cover most of Texas.

Geologists also subdivide rocks into lesser units. One of these, called a 6
group, is made up of two or more formations. A formation comprises
rocks or strata (layers of rock) that are recognized and mapped as a unit.
Some formations consist of layers of one particular type of rock, such as
limestone or shale. Formations are named after a nearby geographic locality,
and in some formation names, the type of rock is included. For example,
three of the Texas geologic formations are called Buda Limestone, Del Rio
Clay, and Eagle Ford Shale.

Geologic Map
The geologic map (pp. 4-5) shows the rocks that are found at the surface in
Texas. Some of these are extremely old. Some, geologically speaking, are
very young.

7

WHAT ARE ROCKS AND MINERALS?

Although rocks and minerals are often mentioned together, and to some
people they have similar meanings, geologists make a distinction between
the two words. In general, rocks are made up of minerals, and minerals are
made up of chemical elements.

Chemical Elements
The chemical elements include oxygen, silicon, calcium, sulfur, carbon,
gold, silver, and many others. There are 90 naturally occurring elements.
Each is made up of molecules that consist of only one kind of atom.
Chemical elements may either be combined with each other or occur alone.
They are the building blocks of our world for they make up all the gases, all
the liquids, all the minerals, all the plant and animal life, and all the other
physical matter. Some of the chemical elements that occur in the rocks and
minerals mentioned in this book are listed below.

Aluminum Al
Barium Ba
Beryllium Be
Boron B
Calcium Ca
Carbon C
Cerium Ce
Chlorine Cl

Copper Cu
Fluorine F
Gold Au
Hydrogen H
Iron Fe
Lead Pb
Magnesium Mg
Manganese Mn
Mercury Hg
Molybdenum Mo
Oxygen O
Potassium K
Silicon Si
Silver Ag
Sodium Na
Strontium Sr
Sulfur S
Thorium Th
Tin Sn
Uranium U
Vanadium V
Yttrium Y
Zinc Zn
Zirconium Zr

We can compare the chemical elements to the letters of our alphabet. The
letters, like the chemical elements, are fundamental building blocks, and
they can be brought together in various combinations to form words.

Minerals

A mineral can be compared to a word of our language. We combine letters to
form a word, and nature combines certain chemical elements to form each
particular mineral. For example, calcite, a mineral that is abundant in Texas,
is always made up of the same proportions of the same three elements:
calcium, carbon, and oxygen.

A mineral is made up of chemical elements. The mineral calcite, for example, always consists of
the same proportions of calcium, carbon, and oxygen.

Each mineral has its own characteristic internal structure and other
properties. At ordinary temperatures, nearly all the minerals are solids rather
than gases or liquids. (Water and mercury are the principal exceptions.) In
addition, minerals are inorganic rather than being composed of plant or
animal matter.

When a single chemical element is found alone in nature as a solid, it is 8
considered to be a mineral, too. Gold, silver, copper, lead, and sulfur are
some of the chemical elements that can occur alone as solid minerals. When
they occur this way, we refer to them as native silver, native copper, or
native sulfur. Although the element mercury is a liquid rather than a solid at
ordinary temperatures, it too is a mineral when it occurs alone in nature. It is
then called native mercury.

Rocks
We have already compared the chemical elements to the alphabet and the
minerals to words. We can now go a step further and compare rocks to

sentences. We put words together to make sentences; nature puts minerals
together to make rocks. A sentence does not have to be made up of a definite
number of words, nor does a rock have to be made up of a definite number
of minerals. Some rocks, such as granite, may be composed of several
minerals. Others, such as dolomite and rock gypsum, consist of only one
mineral.

Minerals do not lose their identities when they make up a rock. Instead, they
are merely associated together in varying proportions. Some rocks, as we
will find later, instead of being composed of the minerals themselves, are
made up of fragments of earlier-formed rocks.

Ordinarily, we think of rocks as hard and solid substances, such as limestone
and granite, but some geologists consider loose and uncemented materials,
such as sand, gravel, or volcanic ash, to be rocks also. The words sediments
or deposits are often used to describe this uncemented or loose material.

Rocks are commonly grouped, according to how they formed, into three
great classes known as igneous, metamorphic, and sedimentary.

A rock is made up of minerals. The igneous rock granite, for example, consists chiefly of quartz
and feldspar; other minerals such as mica and hornblende are commonly present.

9
IGNEOUS ROCKS

Igneous rocks result from the cooling of hot, molten rock material or
magma. Magma that reaches the surface through volcanoes is called lava.
Magma comes from deep within the earth and is made up of a mixture of
molten mineral materials. Igneous rocks have been forming throughout the
geologic past and are still forming today. We can understand how they form
when we look at pictures of hot, molten lava flowing from volcanoes, such
as Mauna Loa in Hawaii. As lava cools, it hardens into rock.

Extrusive or Volcanic Igneous Rocks

The igneous rocks that form on the earth’s surface are called extrusive or
volcanic igneous rocks. When magma flows to the surface, it cools and

hardens quickly. The mineral grains that form during this fast cooling may
be too small to be distinguished from each other. Some lava cools too
quickly for minerals to crystallize—then the rock is volcanic glass.

Extrusive igneous rocks form at the earth’s surface from lava that cools and hardens relatively
quickly.

No volcanic igneous rocks are forming in Texas now. However, during
Tertiary time, in the Big Bend area and in other parts of the Trans-Pecos
country of west Texas, lava came to the surface and hardened. (The
physiographic outline map, p. 42, shows where these areas are located.)

Intrusive Igneous Rocks

The cooling and hardening of hot, molten magma also takes place below the
earth’s surface. Here, the magma cools slowly to form rocks made up of

mineral grains that are large enough to be readily visible. These rocks are
known as intrusive igneous rocks. We know that they are present below the
surface in Texas because of wells drilled in many areas of the State. In Pecos
County, a well reached granite, an intrusive igneous rock, at a depth of
16,510 feet. Other wells in Texas have reached the granite basement rocks at
much shallower depths. But not all intrusive igneous rocks in Texas are
found underground. In the Trans-Pecos country of west Texas, in the 10
Balcones fault zone, and in the Llano uplift of central Texas, some are
now seen at the surface. They, like all intrusive rocks, were formed below
the ground, but earth’s processes of uplift and erosion have gradually
uncovered them.

Intrusive igneous rocks form from molten rock material (magma) that cools and hardens
beneath the earth’s surface.

SEDIMENTARY ROCKS

Sedimentary rocks are made up of sediments, which are rock and mineral
grains that have come from weathered rocks of all kinds. Rocks are
weathered when water, ice, snow, wind, and other agents cause them either
to dissolve, as table salt does when put in water, or to break apart, as old
pavement commonly does.

Soils

Some of the broken-down rocks, along with associated plant and animal
matter, develop into soils. When you examine soil with a magnifying glass,
you may be able to see some of the small rock and mineral grains that still
remain in it. Some soils have formed on top of the rocks from which 11
they came, and some have been moved in from another place.

Soils develop from weathered rock and associated organic material.

SOIL
SUBSOIL
WEATHERED ROCK
BEDROCK

Sedimentary Rock Materials in Broken Fragments

Water and wind not only weather the rocks and soils but also move the
weathered materials (the sediments) and deposit them in other places.
Whenever you see a dust or sand storm, or a muddy creek or river, you are
observing the movement of sediments by wind and water to other land areas
or to the sea. The combination of weathering and movement is called
erosion.

Conglomerate from Webb County, Texas, is composed of rounded gravel that has been cemented
together.

Some of the rock fragments carried by water are still fairly large when they
reach their destinations. On the basis of size, they are called boulders,
cobbles, pebbles, and granules. Loose deposits of these larger-size sediments
make up what is known as gravel. Nature cements gravels together to 12
form rocks such as conglomerates (made up of rounded gravel) and
breccias (made up of sharp-cornered gravel).

The finer sediments are called sand, silt, mud, and clay. When cemented, the
sand grains become sandstones, the silt particles become siltstones, and the
mud and clay particles become shale. The sedimentary rocks that are made
up of these rock fragments are called clastic or fragmental rocks.

Sedimentary Rock Materials in Solution

As they are weathered, some rocks dissolve and go into solution. For
example, a number of the Texas creeks and rivers carry calcium carbonate in
solution because they flow through areas where limestone rocks, which
consist mostly of calcium carbonate, are being weathered. (Water that
contains a large amount of dissolved rock material is called hard water.)

Cementing materials and chemical sediments.—

Some of the waters containing dissolved rock material seep through loose
sediments where the dissolved material may come out of solution and form a
cement, which binds the sediments together. For example, when loose sand
sediments are cemented, they form sandstone. Three of the most common
cements are iron oxide, calcium carbonate, and silicon dioxide, although a
number of other materials also serve as cements.

Dissolved rock materials come out of solution not only to serve as cementing
agents but to form the chief mineral of some sedimentary rocks as well.
Sedimentary rocks of this kind form mostly in lakes and seas into which
much dissolved material is carried by rivers. When the dissolved material
comes out of solution, it is said to be precipitated and the mineral sediments
it forms are the chemical sediments. Some limestones originate this way.
You can see examples of precipitated materials by noting the crust-like
deposits that form inside some water pipes and teakettles, as dissolved
material in the water comes out of solution.

Precipitated sediments are commonly observed lining a teakettle.

Sedimentary rocks formed by plants and animals.—

The dissolved rock material can come out of solution in another way. Some
plants and animals are able to take dissolved calcium carbonate out of the
sea water and use it to build their shells and other structures. Some of these
organisms, such as corals and algae, can grow upward from the sea floor in
large groups to form reefs that later become reef limestones. Other
limestones are made up of the remains of plants and animals that collect on
the sea floor and become cemented together.

METAMORPHIC ROCKS

Metamorphic rocks come from earlier-formed rocks that have undergone a
change or a metamorphosis. All igneous and sedimentary rocks, and earlier-
formed metamorphic rocks too, can be changed, without being moved to
some other place, into new and different rocks. As they are changed, 13
they may become harder, new minerals may form, and they may look
entirely different. For example, granite, an igneous rock, can be changed into
the metamorphic rock known as gneiss; limestone, a sedimentary rock, can
be changed into marble; shale, a sedimentary rock, can be changed into
slate. These changes occur because the earth is a big and complex chemical
system. The agents that bring about these changes, which always occur
below the surface of the earth, are heat, pressure, and fluids—both liquids

and gases. Several different kinds of change or metamorphism can take
place.

Static Metamorphism

Some of the changes occur because the rocks are at great depths. As more
and more younger rocks are deposited on top of them, the older rocks
become deeply buried. The great thicknesses of younger rocks are heavy,
and they squeeze and press down on the rocks beneath them. The deeply
buried rocks are also hotter than surface rocks. In general, the temperature
increases about 1° Fahrenheit for each 50 feet of depth below the surface.
The change of deeply buried rocks into new rocks by pressure and heat is
known as static metamorphism.

Contact Metamorphism

Another method of change or metamorphism involves molten igneous rock
material. When hot magma moves up through rocks, it not only heats and
pushes them, but it also may soak them with liquids and gases, causing the
nearby rocks to change into new rocks, by a process called contact
metamorphism.

Some rocks are altered by heat and fluids when they are invaded by hot magma in a process
called contact metamorphism.

UNALTERED ROCK
METAMORPHIC ROCK
MAGMA

Dynamic Metamorphism 14

Still another rock-changing process is one that is associated with mountain
building. When mountains are formed, heat and great pressures develop deep
within the earth’s crust. The flat layers of rock are then slowly pushed and
squeezed so that they bend up into arches, fracture, or slide over each other.

These forces cause great changes in the rocks in widespread areas. This
process of change is known as dynamic metamorphism.

Occurrence and Properties of Minerals

HOW MINERALS OCCUR

Rocks are made up of minerals. In addition, minerals are associated with
rocks in other ways. For example, minerals fill or coat cracks and cavities
that have developed in some of the rocks. Minerals are either crystalline or
amorphous.

Crystalline Minerals

Most minerals are crystalline. In crystalline minerals, combinations of atoms
are arranged in ordered patterns, which are repeated over and over. This
orderly internal structure of atoms is a characteristic of each crystalline
mineral, as mineralogists are able to determine by using X-rays and special
microscopes.

Crystals.—

When a mineral occurs as a well-formed individual crystal, it has a definite,
precise shape. The kind of crystal shape it has depends on its own type of
crystalline internal structure. A well-formed crystal has smooth, flat, outer
surfaces called crystal faces, which are arranged together to form prisms,
cubes, pyramids, and many other geometric shapes. For example, quartz, a
common Texas mineral, is commonly found as a six-sided, prism-shaped
crystal that is topped by pyramid-like forms. Pyrite, another common
mineral, occurs as cube-shaped crystals. We can identify some minerals
more readily by learning to recognize their crystal shapes.

A scalenohedron, one of the many crystal forms of calcite.

Imperfect crystals.—

A crystalline mineral commonly forms under conditions that do not permit it
to become a well-shaped crystal. Although the mineral may show a few
crystal faces, it does not have a complete crystal shape and so is described as
massive, or is said to occur in masses. Some of the minerals that make up
rocks occur as crystalline masses. For example, calcite is a crystalline
mineral that occurs in the metamorphic rock marble without its normal
crystal shape.

Many crystalline minerals occur as incomplete and imperfect crystals that
are grouped together in various arrangements. If these incomplete crystals
are arranged around a common center like the spokes of a wheel, they 15
are said to be radial or radiated. If the groups of incomplete crystals
look like bundles of strings or fibers, they are described as fibrous. If they
are in rounded masses that resemble bunches of grapes, they are called
botryoidal. If they look like fish scales, they are described as scaly. Some
crystalline minerals are made up of tiny grains that are grouped together like
the grains in a lump of sugar. A mineral occurring in this way is described as
granular. More descriptions of crystalline minerals are found in the section
on Texas rocks and minerals (pp. 43-98).

Barite specimen showing radial form.

Amorphous Minerals

An amorphous mineral, unlike a crystalline mineral, does not have a definite,
orderly arrangement of its atoms. Because of this lack of internal structure,
the mineral occurs in masses that have no regular geometric shapes, and it
has no crystal form of its own. Only a few minerals are amorphous.

SOME DISTINGUISHING PROPERTIES OF MINERALS

We use our senses of sight, hearing, smell, touch, and taste to become aware
of the world around us. For example, we recognize a flower by noting its
color, its fragrance, and the texture, shape, and arrangement of its petals.
These are some of its characteristic properties. A mineral also has
distinguishing properties, among them color, luster, and hardness, which
help us identify it. Some minerals have a single outstanding property, such as
the magnetism of magnetite, that makes them easier to recognize. But to
identify most minerals, we need to determine not just one, but several
properties.

16

Chalcedony showing botryoidal form.

Color

Color is one of the properties we notice first. The color of some minerals is
always the same, and it helps us to identify them. But it is not a dependable
property to use in identifying all minerals, because some contain impurities
that change or hide the real color.

Luster

The luster is the way the surface of a mineral reflects light. The luster of a
mineral may be nonmetallic, submetallic, or metallic. Mineral metals such as
gold, silver, galena, and pyrite have a metallic luster. A few minerals have a
luster that is almost, but not quite metallic—their luster is submetallic. A
mineral with a nonmetallic luster may look vitreous (glassy), silky, resinous
(like resin), greasy, earthy (dull), pearly, or adamantine (brilliant).

Transmission of Light

Some minerals allow light to pass through them; others do not. A mineral is
transparent if you can see both light and objects through it, as through clear
glass. If you can see only light, but no objects, as through frosted glass, the
mineral is translucent. When you hold an opaque mineral up to the light, it
looks dark. No light at all comes through it, even through the thin edges.

Transparent mineral.

Hardness

Some minerals are soft and can be scratched easily. Others, which are harder,
are resistant to scratching. To measure a mineral’s hardness, we try to find
out which substances will scratch it and which substances will not scratch it.
To do this in a general way, several ordinary objects—such as a fingernail, a
copper penny, a pocket knife, a piece of window glass, and a steel file—can
be used. For a more exact way of testing hardness, we can use ten minerals
that make up what is known as Mohs scale. Each mineral in this scale has a
different hardness, and each one has been given a number that represents its
hardness. For example, talc, the softest mineral in this scale, is given a
hardness of 1. Gypsum, the next softest mineral in the scale, has a hardness
of 2. Diamond, the hardest mineral known, is given the top hardness of 10 in
this scale. These ten minerals are listed below. Alongside them are five 17
common objects with their hardnesses.

1—Talc
2—Gypsum Fingernail—slightly over 2
3—Calcite Copper penny—about 3
4—Fluorite
5—Apatite Pocket knife—slightly over 5
6—Orthoclase Window glass—5½
7—Quartz Steel file—about 6½
8—Topaz
9—Corundum
10—Diamond

Suppose, for example, that a mineral can be scratched by fluorite, which has
a hardness of 4 on Mohs scale, but cannot be scratched by calcite, which has
a hardness of 3. We then know that this mineral is softer than fluorite, but
harder than calcite; therefore, it has a hardness of about 3½. In the same way,
if a mineral can be scratched by a pocket knife, which is slightly more than 5
in hardness, but not by a copper penny, which has a hardness of about 3, we
know then that its hardness is between 3 and 5.

Streak or Powder

The streak is the mark, made of fine powder, that a mineral leaves as you rub
it across a streak plate. A streak plate is a flat piece of white tile or porcelain
that has a dull, unglazed surface. The streak plate is about as hard as quartz,
which is 7 on Mohs scale, and you will not be able to use it for minerals that
have a greater hardness. For these, you can obtain the powder by scratching
the mineral or by crushing a small piece of it.

A streak plate is used to determine the color of the streak or powder of a mineral.

The color of the streak or powder is extremely helpful in identifying some
minerals. For example, hematite is a mineral that may be any one of several
different colors, but its streak or powder is always reddish brown.

Cleavage

As they break, some crystalline minerals always split along a smooth, flat
surface. This property is known as cleavage. Some cleavages are smooth and
perfect; others are not so perfect. The cleavage surfaces, because of the
mineral’s crystalline internal structure, are parallel to possible crystal faces,
even though the mineral itself may occur as a crystalline mass without a
perfect crystal shape.

Some minerals will cleave in only one direction; some, in several directions.
For example, galena, a mineral found in Texas, has perfect cubic cleavage. It

cleaves in three directions that are at right angles to each other. These
cleavage directions are parallel to possible cubic crystal faces, and some of
the cleavage fragments are cubes.

Parting

A few minerals sometimes show a kind of false cleavage known as parting.
Parting, unlike cleavage, is not constant and does not occur in every
specimen of a particular mineral. For this reason, it is not a very dependable
means of identification.

Fracture

Minerals also break in another way. When the break is in a different
direction from that of the cleavage or parting, it is known as the fracture. A
fracture is called conchoidal if the mineral’s broken surface is curved like
the inside of a spoon or shell. Thick pieces of glass break with this
conchoidal fracture. A fracture is described as hackly if the broken surface
has sharp, jagged edges; as even, if the surface is generally flat; and as
uneven, if it is rough and not flat. If the mineral breaks into splinters, 18
its fracture is called splintery.

Conchoidal fracture.

Specific Gravity

The specific gravity is a measure of whether a mineral is heavy or light. It is
a comparison of the weight of a piece of the mineral with the weight of an
equal volume of water. The mineral quartz, for example, has a specific
gravity of 2.65. This means that a piece of quartz is a little more than 2½
times as heavy as an equal volume of water. Accurate measurements of
specific gravity can be made in a laboratory. You can, however, learn to
estimate specific gravities just by lifting various minerals and judging
whether they are heavy or light.

Effervescence in Acid

This is a property that depends on the chemical composition of the mineral.
Carbonate minerals, which contain (in addition to at least one other element)
three parts of oxygen and one part of carbon, can be tested with dilute
hydrochloric acid. When a drop or two of this acid is put on a carbonate
mineral such as calcite (calcium carbonate, CaCO₃), the acid begins to
bubble and fizz. The fizzing or effervescence is caused by the carbon dioxide
gas that is formed when the acid and mineral come in contact with each
other. This test is also helpful in identifying rocks, such as limestone and
marble, that contain carbonate minerals.

SOME SPECIAL OCCURRENCES OF MINERALS

Cave Deposits

Beautiful mineral deposits occur in some natural caves. Deposits that look
like icicles, called stalactites, are found hanging from the ceiling of a cave.
Other deposits, stalagmites, are like the stalactites except that they jut
upward from the floor. Columns are formed from stalactites and stalagmites
that have joined together. In addition, some caves contain sheet-like deposits
that are spread along the ceiling, floor, and walls. These deposits are called
flowstone. Calcite is one of the minerals that commonly form cave deposits.

Just a few of the caves in Texas contain these deposits. They occur mostly in
the limestone rocks that are south and southwest of the Llano uplift area of
central Texas. Some of the commercial caves that contain good examples of
calcite deposits are located near Boerne in Kendall County and near Sonora
in Sutton County. Calcite deposits also occur in Longhorn Cavern, a large
cave located in the Longhorn Cavern State Park of Burnet County. These
caves were formed by underground waters that moved through cracks and
pores in the limestone rocks and dissolved passageways in them. After the
cave passages were made, water containing dissolved calcium carbonate
dripped into the cave. As it evaporated, this water left behind a deposit of
calcium carbonate—the mineral calcite.

You can better understand how the cave deposits are formed by watching
icicles grow in wet, freezing weather. First, small hanging drops of water

freeze, and a small icicle forms. Then, as more water drips over it and 19
freezes, the icicle grows longer and wider. Some of the water drips
completely over the icicle and falls to the ground. There, it either freezes
into a sheet of ice, or it begins to build upward to form an upside-down
icicle. The water dripping down in the caves evaporates instead of freezing,
and in doing so it leaves behind a deposit of calcite.

Calcite stalactites and stalagmites in the Caverns of Sonora, Sutton County, Texas. Photograph
courtesy of the Travel and Information Division of the Texas Highway Department.

Concretions

Limestone, shale, and other sedimentary rocks commonly have scattered
throughout them masses of other rocks and minerals, such as limonite, chert,
and pyrite. These masses are called concretions. Concretions may be round
or oval, or they may have odd, irregular shapes. They—such as some of the
limonite concretions of east Texas—even may look like gourds or sweet

potatoes. Concretions generally are harder than the surrounding rocks. Some
are smaller than peas, but others are several feet wide. (The word nodule is
used to describe small, rounded concretions as well as other small, rounded
mineral occurrences.)

It is believed that some concretions form at the same time as the rocks in
which they occur. Other concretions develop after the rocks themselves have
formed. These are deposited by underground water that contains dissolved
mineral matter. The water seeps through the rocks and deposits mineral
matter around an object in the rock, such as a fossil or a grain of sand, to
form a concretion.

Geodes

Geodes are rounded, generally hollow masses that occur mostly in
limestones. They are scattered through the rocks and can be lifted or dug out.
Some geodes are as small as walnuts, and some are as large as 20
basketballs. Most of them have a rough, dull-looking outer surface. If
you break geodes open, you will find that many are lined with beautiful
crystals of calcite, celestite, or quartz that point inward toward the hollow
center.

Calcite geode found in Lower Cretaceous strata of western Travis County, Texas.

It is thought that a geode forms when water, carrying dissolved mineral
material, seeps into a cavity in the rock, then deposits the mineral material as
a lining in the cavity. This lining becomes the outer part of the geode. Thus a
geode—unlike a concretion, which grows from the center outward—forms
from outside to inside.

Some of the Lower Cretaceous limestone rocks of Travis, Williamson, and
Lampasas counties contain calcite and celestite geodes. Celestite geodes
have also been found in Permian rocks in parts of Coke, Fisher, and Nolan
counties.

Petrified Wood

Petrified wood from Texas Gulf Coastal Plain.

We often find some minerals occurring as petrified wood. (Petrified wood
includes silicified wood, opalized wood, agatized wood, and carbonized
wood.) Petrified wood forms when plant material, such as a tree or a bush, is
replaced by a mineral. It is formed by underground water carrying 21
dissolved mineral matter. As this water seeps through sediments in
which the plants are buried, it gradually deposits agate, chalcedony, calcite,
opal, chalcocite, or some other mineral in the place of each fiber of the
wood. By this slow change from plant to mineral matter, the original shape
and structure of the wood remain unchanged.

Petrified wood is commonly found in some of the Tertiary, Permian, and
Lower Cretaceous rocks of Texas. (See Opal, Quartz, Copper Minerals, pp.
78, 84, 52).

22

COLLECTING ROCKS AND MINERALS

Perhaps you would like to start your own collection of rocks and minerals.
For this purpose you will need a hammer (a prospector’s hammer with a pick
on one end of it is a good tool), some newspapers to wrap around the
specimens to keep them from breaking, and a cloth bag in which to carry the
specimens.

Prospector’s hammer.

Before you start to collect, be sure to ask the owner’s permission to go on his
property. If he agrees to let you come on his land, be careful about closing
gates, and do not leave holes into which his livestock might step and be
injured. Look out for snakes. Plenty of rattlers, copperheads, and moccasins

are still left in Texas. And, incidentally, collecting is not allowed in State or
National parks.

To identify the rocks and minerals that you collect, you probably will need
several articles with which to make simple tests. The following can be easily
obtained:

1. A pocket knife, a copper penny, a piece of window glass, a steel
file, and a piece of quartz to test the hardness. If you prefer to use a
group of minerals of known hardness, such as those of Mohs scale
described on pages 16-17, you can either collect your own or buy a
prepared set from a mineral supply house.

2. A streak plate to test the color of the mineral’s streak. Mineral
streak plates can be purchased, or a piece of unglazed tile can be
used.

3. A magnifying glass to examine small cleavage surfaces, crystals,
and rock grains. A number of different kinds can be bought, from the
simple reading glass to the precisely made hand lens. A lens with ten-
power magnification is good for general use.

4. A small magnet to test whether or not a mineral is magnetic.

5. Dilute (10%) hydrochloric acid (HCl), also known as muriatic
acid, to test carbonate rocks and minerals. You can buy a small bottle
at a drug store. Be extremely careful in handling this acid, and keep it
away from small children—it is a POISON. If you spill any on
yourself, it will burn your skin and eat holes in your clothes.

Hand lens.

The rock and mineral identification charts on pages 24-41 will help you to
make the simple identification tests in a methodical way.

It is a good idea to have some system of labeling your rock and mineral 23
specimens. Some collectors carry note paper with them on field trips.
Then they can write down the location and, if possible, the name of the rock
or mineral. This information is either wrapped with the specimen or stuck to
it with tape. One way to label large collections is to put a small spot of paint
or fingernail polish on each of the rock and mineral specimens. When the
paint has dried, a number can be written on it in black India ink. Then, on a
file card, the name and the number of the specimen can be written, together
with the place where it was found, the date of collection, and the name of the
collector.

24

ROCK AND MINERAL IDENTIFICATION
CHARTS

To help you identify them, various Texas rocks and minerals are listed
together in the following charts according to properties that they have in
common. Although useful, the identification charts may not always give you
perfect results. For example, hardness, which is used as a guide, is not to be
completely relied upon in the identification of rocks.

The charts on the following pages pertain only to the rocks and minerals that
are described in this book. It is quite possible that you will find rocks and
minerals in Texas that are not included in these charts.

If you find a rock or a mineral that you are unable to identify, you can check
your local library for reference books that may aid you (several such
references are noted on pages 100-101). If you need further help, possibly
the science teacher at a nearby public school will be able to identify the
specimen for you. Or if a college or university is located in your area
(especially one that has a department of geology), you can obtain help there.
In Texas, the Bureau of Economic Geology is a mineral information center.
Most other states have similar geological research and public-service
organizations. Other sources of information might be the gem and mineral
societies that are found in a number of communities. Many of the members
of these organizations are experts in the identification of rocks and minerals.

How To Use the Mineral Identification Charts

In the mineral identification charts (pp. 26-38), the minerals have been
grouped, first of all, on the basis of luster: the first group includes the
minerals that appear metallic and almost metallic (submetallic); the second
group includes those that appear nonmetallic. Next, the minerals have been
arranged within the two groups according to color.

After you have determined the luster and the color of an unknown mineral,
turn to the Key to Mineral Identification Charts on page 25. It will direct you
to the proper mineral chart.

Mineral Charts 1 through 5, which include the minerals of various colors
with metallic and submetallic lusters, are subdivided according to the
hardness of the minerals. To determine the hardness of a mineral that has
one of these lusters, you can make the following tests:

1. Will the mineral readily leave a mark on paper?

2. If it will not readily leave a mark on paper, will an ordinary pocket
knife scratch it?

3. Is it too hard to be scratched by an ordinary pocket knife?

Mineral Charts 6 through 15 are for the nonmetallic minerals of various
colors. They, too, are subdivided according to the hardness of the minerals,
as follows:

1. Can the mineral be scratched by a fingernail?

2. If it cannot be scratched by a fingernail, can it be scratched by a
copper penny?

3. If it cannot be scratched by a copper penny, can it be scratched by
an ordinary pocket knife?

4. If it cannot be scratched by an ordinary pocket knife, can it be
scratched by a piece of quartz?

5. Is it too hard to be scratched by quartz?

When the luster, color, and hardness of a mineral have been determined, you
may find that several minerals on the charts fit the description. To narrow
your choice, you can then test other properties of the mineral. Notice the
“remarks” column on the charts. In it, is mentioned anything that is
distinctive about the mineral.

For more complete mineral identification lists and tables, you can use
textbooks, such as Dana’s Manual of Mineralogy, revised by C. S. Hurlbut,
Jr., or Mineralogy, by E. H. Kraus, W. F. Hunt, and L. S. Ramsdell.

25

Key to Mineral Identification Charts
If the mineral has a metallic or submetallic luster,

and is: Consult Mineral Chart
white 1
gray 2
yellow 3
brown 4
black 5

If the mineral has a nonmetallic luster,

and is: Consult Mineral Chart
white 6
gray 7
yellow 8
brown 9
black 10
green 11
blue 12
red or pink 13
purple or violet 14
colorless 15

26

Mineral Identification Charts
Chart Mineral Streak Remarks Hardness
No.
1. METALLIC luster, WHITE color
A. Does not readily leave mark on paper but can be scratched by
ordinary pocket knife
Native silver Shiny silver Silver-white color that 2½-3
white, tarnishes to gray, black, or
unless yellowish brown; heavy; can
tarnished be flattened when hit with
hammer
2. METALLIC or SUBMETALLIC luster, GRAY color
A. Will leave mark on paper
Argentite Shiny, Lead-gray color that 2-2½
blackish to tarnishes to dull black; knife
lead gray cuts it smoothly; heavy; may
occur as masses and coatings
Galena Grayish Shiny lead-gray color; heavy; 2½
black cube-shaped fragments and
crystals
Graphite Black Steel-gray color; greasy feel; 1-2
very soft; splits into thin
flakes
B. Does not readily leave mark on paper but can be scratched by
ordinary pocket knife
Chalcocite Grayish Shiny lead-gray color that 2½-3
black tarnishes to dull black; knife
cuts it smoothly; may have
black sooty coating;
commonly occurs as compact
or granular masses
Hollandite Black Silvery-gray color; may 4-6
occur as rounded masses

C. Cannot be scratched by ordinary pocket knife
Braunite Steel gray or Dark steel-gray color and 6-6½
black submetallic luster
Hematite Dark reddish Steel-gray color; commonly 5½-6½
brown occurs as granular or (may be
compact masses; shiny, scaly softer)
variety is specular hematite;
notice streak
Hollandite Black Silvery-gray color; may 4-6
occur as rounded masses
3. METALLIC luster, YELLOW color
A. Does not readily leave mark on paper but can be scratched by
ordinary pocket knife
Chalcopyrite Greenish Brass-yellow or golden- 3½-4
black yellow color that may tarnish
and show rainbow-like
colors; commonly massive;
notice streak
Gold Shiny Shiny yellow color; 2½-3
golden extremely heavy; flattens
yellow when hit with hammer;
notice streak
B. Cannot be scratched by ordinary pocket knife
Pyrite Black, Shiny, pale golden-yellow or 6-6½
greenish brass-yellow color that may
black, or tarnish; occurs as grains, as
brownish masses, or as cubes or other
black crystal shapes; notice
hardness and streak
27

4. METALLIC or SUBMETALLIC luster, BROWN color
A. Does not readily leave mark on paper but can be scratched by
ordinary pocket knife
Limonite Rusty Dark-brown color; some 5-5½
yellowish specimens have a shiny black

brown surface; notice streak
B. Cannot be scratched by an ordinary pocket knife
Cassiterite Pale brown, Brown; submetallic; heavy; 6-7
pale yellow notice streak
or white
Hematite Dark reddish Dark brown color; 5½-6½
brown commonly occurs as granular (may be
or compact masses; notice softer)
streak
Limonite Rusty, Dark brown color; some 5-5½
yellowish specimens have a shiny black
brown surface; notice streak
5. METALLIC or SUBMETALLIC luster, BLACK color
A. Will leave mark on paper
Argentite Shiny, Lead-gray color that 2-2½
blackish to tarnishes to dull black; knife
lead grey cuts it smoothly; heavy; may
occur as masses and coatings
Graphite Black Greasy feel; very soft; splits 1-2
into thin flakes
Pyrolusite Black Very soft; will soil fingers; 1-2
may be powdery
B. Does not readily leave mark on paper but can be scratched by an
ordinary pocket knife
Chalcocite Grayish Shiny lead-gray color that 2½-3
black tarnishes to dull black; knife
cuts it smoothly; may have a
black sooty coating;
commonly occurs as compact
or granular masses
Hollandite Black May occur as rounded 4-6
masses
Limonite Rusty, Some specimens have shiny 5-5½
yellowish black surface; notice streak
brown

C. Cannot be scratched by an ordinary pocket knife
Braunite Steel gray or Luster is submetallic 6-6½
black
Cassiterite Pale brown, Submetallic luster; heavy; 6-7
pale yellow, notice streak
or white
Hematite Dark reddish Notice streak; commonly 5½-6½
brown occurs as granular or (may be
compact masses softer)
28

Hollandite Black May occur as rounded 4-6
masses
Limonite Rusty Some specimens have shiny 5-5½
yellowish black surface; notice streak
brown
Magnetite Black Fragments cling to a magnet 6
Pitchblende Brownish Brownish black, greenish 5½
black black, or black; radioactive;
heavy; may appear dull or
greasy
6. NONMETALLIC luster, WHITE color
A. Can be scratched by a fingernail
Cerargyrite Shiny white Appears waxy; knife cuts it 1-1½
or gray smoothly; turns violet brown
to black when exposed to
light
Gypsum White Soft; occurs as crystals or as 2
fibrous, granular, compact, or
earthy masses
Talc White Knife cuts it smoothly; feels 1
soapy or greasy; splits into
thin flakes
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny

Anhydrite White Commonly occurs as sugary- 3-3½
looking masses
Barite White Rather heavy; cleavage 3-3½
fragments may be flat and
slab-like
Calcite White Dilute hydrochloric acid 3
fizzes on calcite; perfect
cleavage in 3 directions gives
rhomb-shaped fragments
Celestite White Not quite as heavy as barite; 3-3½
crystals commonly prism-
shaped or flat-looking; some
cleavage fragments are flat
and slab-like
Halite White Salty taste; dissolves in 2½
water; cube-shaped cleavage
fragments
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Anhydrite White Commonly occurs as sugary- 3-3½
looking masses
Barite White Rather heavy; cleavage 3-3½
fragments may be flat and
slab-like
Celestite White Not quite as heavy as barite; 3-3½
crystals commonly prism-
shaped or flat-looking; some
cleavage fragments are flat
and slab-like
29

Dolomite White Commonly occurs as 3½-4
granular masses and as
rhomb-shaped crystals; dilute
hydrochloric acid may fizz
slightly on dolomite

Fluorite White Cleavage in 4 directions can 4
give fragments that are
shaped like octahedrons;
crystals commonly cubes
Opal White Curved, conchoidal fracture; 5-6
may appear glassy, greasy,
resinous, or dull; milky white
and bluish-white precious
opal shows plays of colors
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly luster; good 6
cleavage in 2 directions that
meet at an angle of 90° or
near 90°; common in granite
and pegmatite rocks
Opal White Curved, conchoidal fracture; 5-6
may appear glassy, greasy,
resinous, or dull; milky white
and bluish-white precious
opal shows plays of colors
Quartz White Curved conchoidal fracture; 7
occurs as milky quartz, chert,
and chalcedony; crystals
commonly 6-sided prisms
with pyramid-like ends
7. NONMETALLIC luster, GRAY color
A. Can be scratched by a fingernail
Amphibole White Made up of slender, flexible 1-2½
asbestos fibers that can be pulled apart
Cerargyrite Shiny white Appears waxy; knife cuts it 1-1½
or gray smoothly; turns violet brown
to black when exposed to
light
Gypsum White Soft; occurs as crystals or as 2
fibrous, granular, compact, or

earthy masses
Sulfur White or Will burn with a blue flame; 1½-2½
pale yellow commonly found as crystals,
crusts, or grains
Talc White Knife cuts it smoothly; feels 1
soapy or greasy; splits into
thin flakes
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Amphibole White Made up of slender, flexible 1-2½
asbestos fibers that can be pulled apart
Anhydrite White Commonly occurs as sugary- 3-3½
looking masses
30

Calcite White Dilute hydrochloric acid 3
fizzes on calcite; perfect
cleavage in 3 directions gives
rhomb-shaped fragments
Celestite White Crystals commonly prism- 3-3½
shaped or flat-looking; some
cleavage fragments are flat
and slab-like
Halite White Salty taste; dissolves in 2½
water; cube-shaped cleavage
fragments
Sulfur White or Will burn with a blue flame; 1½-2½
pale yellow commonly found as crystals,
crusts, or grains
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Anhydrite White Commonly occurs as sugary- 3-3½
looking masses
Celestite White Crystals commonly prism- 3-3½
shaped or flat-looking; some

cleavage fragments are flat
and slab-like
Dolomite White Commonly occurs as 3½-4
granular masses and as
rhomb-shaped crystals; dilute
hydrochloric acid may fizz
slightly on dolomite
Opal White Curved, conchoidal fracture; 5-6
may appear glassy, greasy,
resinous, or dull
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly luster; good 6
cleavage in 2 directions that
meet at an angle of 90° or
near 90°; common in granite
and pegmatite rocks
Opal White Curved, conchoidal fracture; 5-6
may appear glassy, greasy,
resinous, or dull
Quartz White Curved conchoidal fracture; 7
occurs as chert and
chalcedony
8. NONMETALLIC luster, YELLOW color
A. Can be scratched by a fingernail
Carnotite Yellow Bright canary yellow or 2
lemon yellow; radioactive;
occurs as crusts and powdery
masses
Gypsum White Yellowish; soft; occurs as 2
crystals or as fibrous,
granular, compact or earthy
masses
Limonite Rusty Brownish-yellow color; may 1+
yellowish be soft and earthy
brown

31

Muscovite White Light colored; splits into 2-2½
(white mica) thin, flat, transparent sheets
that will bend without
breaking
Sulfur White or Will burn with a blue flame; 1½-2½
pale yellow commonly found as crystals,
crusts, or grains
Uranophane Light yellow Yellow to yellow-orange 2-3
to light color; radioactive
yellow
orange
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Barite White Rather heavy; cleavage 3-3½
fragments may be flat and
slab-like
Calcite White Yellowish; dilute 3
hydrochloric acid fizzes on
calcite; perfect cleavage in 3
directions gives rhomb-
shaped fragments
Muscovite White Light colored; splits into 2-2½
(white mica) thin, flat, transparent sheets
that will bend without
breaking
Sulfur White or Will burn with a blue flame; 1½-2½
pale yellow commonly found as crystals,
crusts, or grains
Uranophane Light yellow Yellow to yellow-orange 2-3
to light color; radioactive
yellow
orange
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife

Barite White Rather heavy; cleavage 3-3½
fragments may be flat and
slab-like
Opal White Curved, conchoidal fracture; 5-6
may appear glassy, greasy,
resinous, or dull
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly luster; good 6
cleavage in 2 directions that
meet at an angle of 90° or
near 90°
Garnet White Commonly occurs as crystals 6½-7
Opal White Curved, conchoidal fracture; 5-6
may appear glassy, greasy,
resinous, or dull
Quartz White Curved conchoidal fracture; 7
brownish-yellow smoky
quartz crystals commonly 6-
sided prisms with pyramid-
like ends; chalcedony and
jasper may be a shade of
yellow, too
32

9. NONMETALLIC luster, BROWN color
A. Can be scratched by a fingernail
Gypsum White Brownish; soft; occurs as 2
crystals or as fibrous,
granular, compact or earthy
masses
Limonite Rusty May be soft and earthy 1+
yellowish
brown
Muscovite White Light colored; splits into 2-2½
(white mica) thin, flat, transparent sheets

that will bend without
breaking
Sulfur White or Will burn with a blue flame; 1½-2½
pale yellow commonly found as crystals,
crusts, or grains
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Barite White Rather heavy; cleavage 3-3½
fragments may be flat and
slab-like
Biotite (black White Dark brown; splits into thin, 2½-3
mica) flat sheets that will bend
without breaking
Calcite White Dilute hydrochloric acid 3
fizzes on calcite; perfect
cleavage in 3 directions gives
rhomb-shaped fragments
Halite White Salty taste; dissolves in 2½
water; cube-shaped cleavage
fragments
Muscovite White Light colored; splits into 2-2½
(white mica) thin, flat, transparent sheets
that will bend without
breaking
Sulfur White or Will burn with a blue flame; 1½-2½
pale yellow commonly found as crystals,
crusts, or grains
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Barite White Rather heavy; cleavage 3-3½
fragments may be flat and
slab-like
Dolomite White Commonly occurs as 3½-4
granular masses and as
rhomb-shaped crystals; dilute

hydrochloric acid may fizz
slightly on dolomite
Fluorite White Cleavage in 4 directions can 4
give fragments that are
shaped like octahedrons;
crystals commonly cube-
shaped
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Cassiterite Pale brown, Brown, reddish brown, or 6-7
pale yellow, yellowish brown; heavy; dull
or white to brilliant luster
33

Feldspar White Glassy or pearly luster; good 6
cleavage in 2 directions that
meet at an angle of 90° or
near 90°; common in granite
and pegmatite rocks
Garnet White Commonly occurs as crystals 6½-7
Quartz White Curved conchoidal fracture; 7
brown smoky quartz crystals
commonly 6-sided prisms
with pyramid-like ends;
chalcedony, chert, and jasper
may be a shade brown, too
Tourmaline White Dark brown variety is 7-7½
dravite; notice hardness,
striations on crystals, and
triangular cross section of
some crystals
E. Cannot be scratched by quartz
Tourmaline White Dark brown variety is 7-7½
dravite; notice hardness,
striations on crystals, and

triangular cross section of
some crystals
10. NONMETALLIC luster, BLACK color
A. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Biotite (black White Splits into thin, flat sheets 2½-3
mica) that will bend without
breaking
B. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Garnet White Commonly occurs as crystals 6½-7
Quartz White Curved conchoidal fracture; 7
brownish-black smoky quartz
crystals commonly 6-sided
prisms with pyramid-like
ends; chalcedony and chert
may be black, too
Tourmaline White Black variety is schorl; 7-7½
notice hardness, striations on
crystals, and triangular cross
section of some crystals
C. Cannot be scratched by quartz
Tourmaline White Black variety is schorl; 7-7½
notice hardness, striations on
crystals, and triangular cross
section of some crystals
11. NONMETALLIC luster, GREEN color
A. Can be scratched by a fingernail
Amphibole White Made up of slender, flexible 1-2½
asbestos fibers that can be pulled apart
34

Cerargyrite Shiny white Light greenish color; appears 1-1½
or gray waxy; knife cuts it smoothly;
turns violet brown to black
when exposed to light
Muscovite White Light colored; splits into 2-2½
(white mica) thin, flat, transparent sheets
that will bend without
breaking
Sulfur White or Greenish; will burn with a 1½-2½
pale yellow blue flame; commonly found
as crystals, crusts, or grains
Talc White Light greenish color; knife 1
cuts it smoothly; feels soapy
or greasy; splits into thin
flakes
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Amphibole White Made up of slender, flexible 1-2½
asbestos fibers that can be pulled apart
Biotite (black White Dark green; splits into thin, 2½-3
mica) flat, translucent sheets that
will bend without breaking
Calcite White Dilute hydrochloric acid 3
fizzes on calcite; perfect
cleavage in 3 directions gives
rhomb-shaped fragments
Halite White Greenish tint; salty taste; 2½
dissolves in water; cube-
shaped cleavage fragments
Muscovite White Light colored; splits into 2-2½
(white mica) thin, flat, transparent sheets
that will bend without
breaking
Serpentine White Two kinds: silky and fibrous, 2½-4
waxy and platy

Sulfur White or Greenish; will burn with a 1½-2½
pale yellow blue flame; commonly found
as crystals, crusts, or grains
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Fluorite White Cleavage in 4 directions can 4
give fragments shaped like
octahedrons; crystals
commonly cubes
Malachite Green Bright green color; dilute 3½-4
hydrochloric acid will fizz on
malachite
Serpentine White Two kinds: silky and fibrous, 2½-4
waxy and platy
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly luster; good 6
cleavage in 2 directions that
meet at an angle of 90° or
near 90°
Garnet White Commonly occurs as crystals 6½-7
35

12. NONMETALLIC luster, BLUE color
A. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Anhydrite White Commonly occurs as sugary- 3-3½
looking masses
Barite White Rather heavy; cleavage 3-3½
fragments may be flat and
slab-like
Calcite White Dilute hydrochloric acid 3
fizzes on calcite; perfect
cleavage in 3 directions gives
rhomb-shaped fragments

Celestite White Not quite as heavy as barite; 3-3½
crystals commonly prism-
shaped or flat-looking; some
cleavage fragments are flat
and slab-like
Halite White Salty taste; dissolves in 2½
water; cube-shaped cleavage
fragments
B. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Anhydrite White Commonly occurs as sugary- 3-3½
looking masses
Azurite Blue Bright, intense blue color; 3½-4
dilute hydrochloric acid will
fizz on azurite
Barite White Rather heavy; cleavage 3-3½
fragments may be flat and
slab-like
Celestite White Not quite as heavy as barite; 3-3½
crystals commonly prism-
shaped or flat-looking; some
cleavage fragments are flat
and slab-like
Fluorite White Cleavage in 4 directions can 4
give fragments that are
shaped like octahedrons;
crystals commonly cube-
shaped
Opal White Curved, conchoidal fracture; 5-6
may appear glassy, greasy,
resinous, or dull; milky white
and bluish-white precious
opal shows plays of colors
C. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz

Feldspar White Glassy or pearly luster; good 6
cleavage in 2 directions that
meet at an angle of 90° or
near 90°
Opal White Curved, conchoidal fracture; 5-6
may appear glassy, greasy,
resinous, or dull; milky white
and bluish-white precious
opal shows plays of colors
36

Quartz White Curved conchoidal fracture; 7
occurs as crystalline quartz
and as bluish chalcedony
D. Cannot be scratched by quartz
Topaz White Perfect basal cleavage gives 8
flat, plate-like fragments;
notice hardness
13. NONMETALLIC luster, RED or PINK color
A. Can be scratched by a fingernail
Gypsum White Reddish; soft; occurs as 2
crystals or as fibrous,
granular, compact, or earthy
masses
Hematite Dark reddish Brownish-red color; soft and 1+
brown earthy
Sulfur White or Reddish; will burn with blue 1½-2½
pale yellow flame; commonly found as
crystals, crusts, or grains
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Anhydrite White Pinkish tint; commonly 3-3½
occurs as sugary-looking
masses
Barite White Pinkish tint; rather heavy; 3-3½
cleavage fragments may be

flat and slab-like
Calcite White Pink color; dilute 3
hydrochloric acid fizzes on
calcite; perfect cleavage in 3
directions gives rhomb-
shaped fragments
Cinnabar Dark red Dark red or bright yellowish- 2½
red color; shiny, brilliant
luster when pure; dull and
earthy when impure; heavy
Halite White Reddish tint; salty taste; 2½
dissolves in water; cube-
shaped cleavage fragments
Sulfur White or Reddish; will burn with blue 1½-2½
pale yellow flame; commonly found as
crystals, crusts, or grains
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Anhydrite White Pinkish tint; commonly 3-3½
occurs as sugary-looking
masses
Barite White Pinkish tint; rather heavy; 3-3½
cleavage fragments may look
flat and slab-like
Dolomite White Pink color; commonly occurs 3½-4
as granular masses and as
rhomb-shaped crystals; dilute
hydrochloric acid may fizz
slightly on dolomite
37

Fluorite White Pink color; cleavage in 4 4
directions can give fragments
that are shaped like
octahedrons; crystals
commonly cubes

Opal White Reddish color; curved, 5-6
conchoidal fracture; may
appear glassy, greasy,
resinous, or dull
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly luster; good 6
cleavage in 2 directions that
meet at an angle of 90° or
near 90°
Garnet White Commonly occurs as crystals 6½-7
Opal White Reddish color; curved, 5-6
conchoidal fracture; may
appear glassy, greasy,
resinous, or dull
Quartz White Curved, conchoidal fracture; 7
occurs as rose quartz, as pink
chert, and as agate and
jasper
14. NONMETALLIC luster, PURPLE or VIOLET color
A. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Fluorite White Cleavage in 4 directions can 4
give fragments that are
shaped like octahedrons;
crystals commonly cubes
B. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Quartz, White Curved, conchoidal fracture; 7
variety: amethyst crystals commonly
amethyst 6-sided prisms with pyramid-
like ends
15. NONMETALLIC luster, COLORLESS
A. Can be scratched by a fingernail

Cerargyrite Shiny white Appears waxy; knife cuts it 1-1½
or gray smoothly; turns violet brown
to black when exposed to
light
Gypsum White Transparent selenite variety 2
commonly occurs as flat,
diamond-shaped crystals;
splits into thin, flat sheets
that will not bend without
breaking
Muscovite White Splits into thin, flat, 2-2½
(white mica) transparent sheets that will
bend without breaking
38

B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Barite White Rather heavy; cleavage 3-3½
fragments may be flat and
slab-like
Calcite White Dilute hydrochloric acid 3
fizzes on calcite; perfect
cleavage in 3 directions gives
rhomb-shaped fragments
Celestite White Not quite as heavy as barite; 3-3½
crystals commonly prism-
shaped or flat-looking; some
cleavage fragments are flat
and slab-like
Halite White Salty taste; dissolves in 2½
water; cube-shaped cleavage
fragments
Muscovite White Splits into thin, flat, 2-2½
(white mica) transparent sheets that will
bend without breaking

C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Barite White Rather heavy; cleavage 3-3½
fragments may be flat and
slab-like
Celestite White Not quite as heavy as barite; 3-3½
crystals commonly prism-
shaped or flat-looking; some
cleavage fragments are flat
and slab-like
Dolomite White Commonly occurs as 3½-4
granular masses and as
rhomb-shaped crystals; dilute
hydrochloric acid may fizz
slightly on dolomite
Fluorite White Cleavage in 4 directions can 4
give fragments that are
shaped like octahedrons;
crystals commonly cubes
Opal White Curved, conchoidal fracture; 5-6
transparent hyalite variety
resembles ice
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Opal White Curved, conchoidal fracture; 5-6
transparent hyalite variety
resembles ice
Quartz White Curved, conchoidal fracture; 7
rock crystal quartz
commonly 6-sided prism
with pyramid-like ends
E. Cannot be scratched by quartz
Topaz White Perfect basal cleavage gives 8
flat, plate-like fragments;
notice hardness

39

How To Use the Rock Identification Charts
In the rock identification charts on pages 40-41, the Texas rocks described in
this book are arranged in four major groups according to their texture.

1. Glassy (the rocks are smooth, dark, and shiny)
2. Compact, dull, or stony (the rocks are smooth and dull, but the individual
grains are too small to be recognized)
3. Granular (at least some of the individual grains of the rocks are large
enough to be seen without a magnifying glass)
4. Fragmental (the rocks are made up of fragments that are either loose or
cemented together)

Consult Rock Chart 1, if the rock is glassy; Chart 2, if it is compact, dull, or
stony; Chart 3, if it is granular; and Chart 4, if it is fragmental.

Two of the rock charts are subdivided. In Rock Chart 2, the compact, dull, or
stony rocks are arranged according to hardness as follows:

A. Rocks that can be scratched by a fingernail
B. Rocks that cannot be scratched by a fingernail but can be scratched by an
ordinary pocket knife
C. Rocks that cannot be scratched by an ordinary pocket knife

In Rock Chart 3, the granular rocks also are arranged according to hardness
into:

A. Rocks that can be scratched by an ordinary pocket knife
B. Rocks that cannot be scratched by an ordinary pocket knife
These harder rocks are subdivided into three groups:
1. Those that have grains of about equal size
2. Those with large easily seen grains that are scattered through a mass of
finer grains
3. Those rocks whose grains are arranged in layers

In the “remarks” column of the rock identification charts are included further
tests that will aid you in identifying the rock.

For a more complete rock determination chart, you can consult a textbook,
such as Rocks and Rock Minerals, by L. V. Pirsson and A. Knopf.

40

Rock Identification Charts
Chart Rock Remarks
No.
1. GLASSY appearance (rock is dark, smooth, and shiny)
Obsidian Entire rock is glassy
Vitrophyre Crystalline grains are scattered through the dark
glassy mass
2. COMPACT, DULL, OR STONY appearance (individual grains too
small to be recognized)
A. Can be scratched by a fingernail
Chalk Dilute hydrochloric acid fizzes on it
Clay Earthy odor when breathed on
Diatomite Crumbly
Rock gypsum Made up of the mineral gypsum
Soapstone Soapy or greasy feel
B. Cannot be scratched by a fingernail but can be scratched by
ordinary pocket knife
Dolomite Dilute hydrochloric acid may fizz slightly on it;
will fizz if rock is powdered
Limestone Dilute hydrochloric acid fizzes on it
Serpentine rock Commonly some shade of green
Shale Breaks in flat, thin flakes; earthy odor
C. Cannot be scratched by an ordinary pocket knife
Basalt Dark colored and heavy
Chert Hard, smooth, and porcelain-like
Rhyolite Light to dark colored; may show streaks or flow
structure
3. GRANULAR appearance (at least some of the individual grains are
large enough to be seen without a magnifying glass)

A. Can be scratched by an ordinary pocket knife
Limestone Dilute hydrochloric acid will fizz on it
Marble Dilute hydrochloric acid will fizz on calcite
marble, and it may fizz slightly on dolomite
marble
Rock gypsum Made up of the mineral gypsum
Rock salt Has a salty taste; made up of the mineral halite
41

B. Generally cannot be scratched by an ordinary pocket knife (some
schist is softer)
1. Grains are of about equal size (equigranular)
Granite Quartz and feldspar grains interlock
Pegmatite Large interlocking grains; commonly feldspar,
quartz, mica
Quartzite Rock breaks across the quartz grains
Sandstone Rock breaks through the cement but around the
sand grains
2. Easily seen grains are scattered through a mass of finer grains
Basalt Dark colored, heavy rock
Llanite Rusty pink feldspar and blue quartz grains
embedded in a brownish rock mass
Rhyolite Light to dark colored rock; may show streaks or
porphyry flow structure
3. Grains are arranged in layers
Gneiss Interlocking grains are in straight or wavy bands
Schist Splits in thin layers; some schist is soft enough to
be scratched by a knife
4. FRAGMENTAL appearance (rocks are made up of fragments that
are either loose or cemented together)
Breccia Angular, gravel-size fragments that are cemented
together
Conglomerate Rounded, gravel-size fragments that are
cemented together

Coquina Shells and shell fragments that are cemented
together
Gravel Loose fragments
Pulverulent Loose, powdery fragments; dilute hydrochloric
limestone acid fizzes on them
Sand Loose fragments no larger than a pinhead
Sandstone Sand fragments that are cemented together
Volcanic ash Loose, fine, gritty particles
42

Physiographic outline map of Texas.

43

DESCRIPTIONS OF SOME TEXAS ROCKS
AND MINERALS

The pages that follow contain descriptions of Texas rocks and minerals. The
descriptions are given in alphabetical order, except that related varieties are
described together. For example, agate, amethyst, chert, flint, jasper, onyx,
and chalcedony are discussed under quartz, because they are varieties of
quartz. The descriptions include the properties of the rock or mineral that
will help you identify it and also include information on where the rock or
mineral can be found in Texas, some of its uses, and how it may have
formed. The chart on page 99 lists chemical composition, specific gravity,
and hardness of various Texas minerals.

Agate. See Quartz.

Agatized Wood. See Quartz.

Alabaster. See Gypsum.

Albite. See Feldspar.

Almandite. See Garnet.

Amethyst. See Quartz.

Amphibole Asbestos. See Asbestos.

Anhydrite
Anhydrite, calcium sulfate, is a rather soft mineral that you can scratch with
a pocket knife, although not with a fingernail. It has a glassy or a pearly
luster and is transparent or translucent. Most anhydrite is white, but
impurities cause it to be grayish, bluish, or reddish. When rubbed across a
streak plate, anhydrite gives a white streak. This mineral has an uneven
fracture, and it cleaves in three directions that are at right angles to each
other. It commonly occurs as rectangular cleavage fragments or as sugary
crystalline masses.

Anhydrite resembles dolomite, limestone, or gypsum. You can use a
hardness test to distinguish it from gypsum (anhydrite is harder) and an acid
test to distinguish it from limestone and dolomite. A drop of dilute
hydrochloric acid will fizz when you put it on limestone or powdered
dolomite. On anhydrite, the acid does not fizz.

Massive anhydrite.

Anhydrite occurs at several places in Texas. It is, for example, seen in bluffs
along the Double Mountain Fork and the Salt Fork of the Brazos River in
north-central Texas. Most of the Texas anhydrite, however, occurs
underground. In the Gulf Coastal Plain, the anhydrite is found below the
surface in salt domes. (Salt domes are described with halite on p. 66 and
with sulfur on p. 91.)

Another anhydrite locality is in the subsurface Permian basin of west Texas.
Oil wells drilled in this basin penetrate great, thick deposits of massive
anhydrite. The anhydrite was deposited during the Permian Period from a
sea that covered this area. As the sea gradually evaporated, the mineral
matter that was dissolved in it came out of solution to form anhydrite, halite,
and several other minerals.

Antigorite. See Serpentine.

Argentite. See Silver Minerals.

Asbestos
Asbestos is not really any one particular mineral. It is the name given to
several minerals that occur in masses of slender, delicate fibers. In the 44
more typical kinds of asbestos, these fibers—when pulled apart—
resemble soft, fluffy, silk strings.

Several small deposits of amphibole asbestos have been found in the Llano
uplift area of central Texas. This asbestos is a variety of the mineral
tremolite, a calcium-magnesium silicate. It has fibers that break rather easily,
and it has a silky luster. It is a shade of green or gray and gives a white
streak when rubbed across a streak plate. When you pull its fibers apart, you
actually are breaking the mineral along its two directions of perfect cleavage.
This amphibole asbestos is softer than other varieties of the mineral
tremolite—a copper penny scratches it easily.

Greenish, silky amphibole asbestos from northeastern Gillespie County, Texas.

The asbestos occurs in veins in Precambrian metamorphic rocks in southern
Llano County, northwestern Blanco County, and northeastern Gillespie
County. These deposits are small.

A variety of the mineral serpentine called chrysotile asbestos is the kind
most used by industry. Its fibers are commonly flexible enough and strong

enough to be woven into cloth. This cloth is made into articles, such as
fireproof suits, gloves, and theater curtains. Some chrysotile has been found
in Precambrian metamorphic rocks in northwestern Blanco County, but it
does not break into fibers fine enough or flexible enough to be called
asbestos.

Azurite. See Copper Minerals.

Barite
Barite, barium sulfate, is a fairly common mineral in Texas. It has a glassy or
a pearly luster, and it is transparent to translucent. Barite is colorless, white,
brownish, bluish, yellowish, or reddish. When rubbed across a streak plate, it
gives a white streak. It is not extremely hard—you can scratch it with a
pocket knife, although not with a fingernail.

Barite is distinctive because of its weight and cleavage. It cleaves in three
directions, and some cleavage fragments are flat or platy. For a mineral with
a nonmetallic luster, barite is heavy—it has a specific gravity of 4.5.

Barite cleavage fragment from west Texas.

Barite commonly occurs as prism-shaped and as flat crystals, as granular
masses, as cleavable masses, and as rounded masses called nodules. In
Texas, some of it was deposited in sedimentary rocks by underground
waters. As the waters seeped through these rocks, mineral matter came 45
out of solution to form the barite. Some of the barite in Texas also
formed from solutions that came from hot magmas.

A number of barite deposits have been found in Texas, but many of them are
small. Barite occurs in Precambrian metamorphic rocks in Gillespie and
Llano counties, in Pennsylvanian shale in Brewster County, in Permian
shales in Baylor and Taylor counties, and in Permian limestones in
Culberson County. It is found in Triassic red shales in Howard County and in
Cretaceous sedimentary rocks in Brewster, Brown, Hudspeth, Jeff Davis,
Kinney, and Val Verde counties. In Live Oak County, barite occurs in

Tertiary bentonitic clays. Barite is being mined from a deposit in the Seven
Heart Gap area northeast of Van Horn in Culberson County.

Barite is used in a number of ways. It is a source of barium chemicals, and it
also is powdered and used as an ingredient in paint. The oil industry uses
large amounts of barite. In drilling for oil by the rotary method, water and
muds are pumped down the hole to aid drilling. Barite is added to these
drilling fluids to make them heavy, since high-pressure gases are not as
likely to blow heavy fluids out of the hole.

Basalt
Basalt is a heavy igneous rock that is black, dark gray, or dark brown. This
rock is made up chiefly of a feldspar mineral, such as labradorite, and a
pyroxene mineral, such as augite. Other minerals may be present.

The mineral grains of some basalts are so small that you cannot distinguish
them even with a magnifying glass. Other basalts, however, are porphyritic,
which means that they contain larger, easily seen crystals and grains of
feldspar and pyroxene scattered either through a mass of the small mineral
grains or through glassy material.

Some basalts contain many small holes. These holes, called vesicles, were
formed when bubbles of gas were trapped in the hardening magma. Later,
solutions moving through the rocks may have deposited another mineral—
such as calcite or chalcedony—in some of the vesicles.

Basalt forms from molten rock material that hardens either on or beneath the
surface—it can be extrusive or intrusive. Much of the basalt now found in
the Trans-Pecos country of west Texas formed from lava that flowed out
onto the surface during the Tertiary Period. A few of the places where basalt
occurs in west Texas are the Chinati Mountains of Presidio County, the
Chisos Mountains of Brewster County, the Davis Mountains of Jeff Davis
County, and the Van Horn Mountains of Culberson and Hudspeth counties.

Basalt from Brewster County, Texas.

Several varieties of basalt occur in the Balcones fault region of Bandera,
Comal, Hays, Kinney, Medina, Travis, and Uvalde counties. These basalts
formed from molten magma that forced its way into rocks just below the
earth’s surface.

Some basalt, which is known commercially as trap rock, is produced in
Uvalde County. It is crushed and used for railroad ballast, road building
material, and as concrete aggregate.

Bentonite. See Clay.

Biotite. See Mica.

Braunite. See Manganese Minerals.

46

Calcite
Calcite, calcium carbonate, is one of the most abundant minerals in Texas. It
is the chief mineral in limestone and in some marble. It also serves as the
cementing material in many sandstones. Crystals, grains, and cleavable
masses of calcite, which have been deposited by underground water, occur in
cracks and cavities in many of the igneous, metamorphic, and sedimentary
rocks of Texas. Calcite also occurs as cave, spring, and stream deposits and
as caliche.

Calcite is transparent or translucent, and—depending on the variety—its
luster is glassy to earthy. Most calcite is white or colorless, but it can be a
shade of pink, blue, green, brown, yellow, or gray. It gives a white streak
when you rub it across a streak plate.

Two properties of calcite to notice are the hardness and the cleavage. This
mineral cleaves perfectly in three directions that are not at right angles to
each other, and some of the cleavage fragments are rhombohedrons. Calcite
is rather soft—you can scratch it with a copper penny but not with a
fingernail. A drop or two of dilute hydrochloric acid also will help you to
identify this mineral. The acid will readily fizz and bubble when it is placed
on calcite.

Calcite has perfect rhombohedral cleavage. The three directions of cleavage are not at right
angles to each other.

Calcite occurs in more different kinds of crystal shapes than any other
mineral. Some of these crystals are flat and tabular; some are
rhombohedrons; some are prisms. Pointed crystals, called dog-tooth spar,
and twinned crystals have been found in the Terlingua area of Brewster
County in west Texas. A somewhat unusual occurrence of calcite crystals is
in geodes. Some of these are found in Lower Cretaceous rocks west of
Austin in Travis County.

Transparent crystals and cleavage fragments of calcite show a property
called double refraction (other minerals show it, too). To test this property,
you can mark a single dot on paper. When you look at the dot through a

piece of clear calcite, you will see two dots instead of one. This happens
because a ray of light is bent (refracted) and is split into two rays as it enters
the mineral. These two rays travel through the calcite in slightly different
directions, and each carries an image of the dot through the mineral. The two
images that you see are at the points where the two rays leave the calcite.

Calcite that is deposited at springs, along river and creek banks, and in caves
is known as travertine. Cave forms of travertine, including stalagmites and
stalactites, occur in several caves in Texas. Another kind of travertine is
called calcareous tufa or calcareous sinter. It is a porous and spongy-
looking material deposited from water carrying dissolved limestone and is
found around the openings of some springs and along some creek and river
banks.

A dull, earthy calcite deposit, known as caliche, occurs in areas of Texas that
have scant rainfall, such as the High Plains, west Texas, and the
southwestern part of the Gulf Coastal Plain. Caliche commonly is found
mixed with other materials, such as clay, sand, or gravel. This substance may
be firm and compact or loose and powdery.

It is thought that caliche forms when ground moisture, containing dissolved
calcium bicarbonate, moves upward. In dry areas of the country, this 47
moisture evaporates. As it does, it leaves a crust of calcium carbonate
in the form of caliche on or near the surface of the ground.

Calcite crystals (dog-tooth spar) from the Terlingua area of Brewster County, Texas.

Caliche is quarried in many counties in Texas and is used chiefly as road
material and as an aggregate.

Caliche. See Calcite.

Carnotite. See Uranium Minerals.

Cassiterite

Cassiterite, tin dioxide, is the mineral that serves as the chief source of tin.
Tin does not corrode and tarnish, and one of its main uses is in the making of
tin cans. (Actually, our tin cans are made from thin sheets of steel that have
been coated with a protective layer of tin.)

Cassiterite has either a nonmetallic or a submetallic luster. Some specimens
are brilliant and shiny; others are dull. Cassiterite may be translucent to
transparent. It may be black, brown, gray, reddish brown, or yellowish
brown. When rubbed across a streak plate, this mineral leaves a pale brown,
a pale yellow, or a white streak. Cassiterite is quite heavy—it has a specific
gravity of 6.8 to 7.1. It is too hard to be scratched by an average pocket
knife.

Sometimes, prospectors use a chemical test to help them identify cassiterite.
They put small pieces of metallic zinc into a jar or test tube containing dilute
hydrochloric acid. Then they add a few fragments of the mineral that they
suspect is cassiterite. If the fragments are cassiterite, they become covered
with a pale gray coating of metallic tin.

Cassiterite’s most common crystal shape is a short, 8-sided prism with 48
pyramids at each end, but perfect crystals are not often found. Most
Texas cassiterite does not show a crystal shape. Instead, it occurs as
crystalline masses in igneous rocks and as loose pebbles that have weathered
out of these rocks.

Cassiterite occurs in a number of places in the United States but not in large
quantities. A small amount of cassiterite has been found in quartz veins in
Precambrian granite in both central Texas and west Texas. In El Paso
County, the cassiterite is found on the east side of the Franklin Mountains a
few miles north of El Paso, where some of it has been mined. In central
Texas, cassiterite occurs in the Streeter area of Mason County.

When the granite rocks in these areas were formed, probably not all of the
hot magmas cooled and hardened at the same time. The fluids given off by
the remaining magmas contained tin and several other elements. It is
believed that these fluids moved up into cracks in the granite rocks and
formed the cassiterite.

Celestite
Celestite is a strontium sulfate mineral. It is colorless, white, yellow, or gray.
Light blue specimens of this mineral also are found, and it is because of this
sky-like color that celestite gets its name. The word celestite comes from the
Latin word caelestis, meaning of the sky.

Celestite has a glassy to a pearly luster, and it is either transparent or
translucent. It gives a white streak when rubbed across a streak plate.
Celestite has a specific gravity of 3.95 to 3.97. It is, however, lighter than
barite, a mineral that it resembles. Celestite is not very hard—a knife will
scratch it, although your fingernail will not. It cleaves in three directions,
and some of the fragments are flat and slabby.

Celestite cleavage fragment from Lampasas County, Texas.

Celestite occurs commonly either as prism-shaped or flat crystals and 49
as cleavable, granular, or fibrous crystalline masses. In Texas, it is
found in geodes, as rounded nodules, or as bedded or layer-like deposits in
limestones and other sedimentary rocks. In Real County, celestite occurs on
the walls of a cave in Cretaceous limestone.

Some celestite may be deposited by sea water, but much of the Texas
celestite is believed to have been deposited by underground water that
seeped through cracks and pores in the limestones and other sedimentary
rocks. This water picked up and dissolved strontium compounds that were
scattered in small amounts through the rocks. Then, it re-deposited the
strontium in the rocks as celestite.

In Texas, beds of celestite occur in Permian rocks in Coke, Fisher, and Nolan
counties and in Lower Cretaceous rocks in Brown, Comanche, and Mills
counties. Celestite geodes and nodules are found in Lower Cretaceous
limestone rocks in Lampasas, Travis, and Williamson counties, and in
Permian rocks in Coke, Fisher, Nolan, and Taylor counties.

Celestite is one of two minerals (the other mineral is strontianite, strontium
carbonate) used as a source of strontium. Strontium compounds give a
crimson-red color to a flame, so they are used in fireworks, tracer bullets,
and flares. Perhaps you have seen a red flare set out on the highway at night
to warn motorists that a truck has stalled. The chances are good that the
flare’s red flame was due to a strontium compound. Some of the Texas
celestite has been mined, but most of the strontium minerals now used in the
United States are imported from England and Mexico.

Cerargyrite. See Silver Minerals.

Chalcedony. See Quartz.

Chalcocite. See Copper Minerals.

Chalcopyrite. See Copper Minerals.

Chalk. See Limestone.

Chert (Flint). See Quartz.

Chrysotile. See Asbestos; Serpentine.

Cinnabar
Cinnabar, which is mercuric sulfide, is the most common mercury mineral. It
has a dark red or a bright yellowish-red color and is transparent to
translucent. When rubbed across a streak plate, it leaves a dark red streak. If
pure, cinnabar has a brilliant, shiny, nonmetallic luster. It is, however,
commonly found mixed with impurities, such as clay, calcite, iron oxide, or
bituminous material, and then it looks dull and earthy. Cinnabar is quite
heavy—it has a specific gravity of 8.10. It is rather soft, and you can scratch
it with a copper penny.

Some prospectors use a quick chemical test to identify cinnabar. They rub a
clean, shiny copper coin with a mineral sample that has been moistened with
a drop or two of dilute hydrochloric acid. If the sample is cinnabar, a light
silvery-gray coating appears on the coin.

Cinnabar occurs as small crystals or as fine-grained or compact crystalline
masses. It is found in veins that fill cracks in rocks and also occurs as crusts
and coatings on rocks. It also may be widely scattered through rocks, such as
limestones.

Cinnabar occurs in the Terlingua area of Brewster and Presidio counties in
west Texas. It has been mined there, off and on, since about 1894, and
during this time, mercury worth many millions of dollars has been produced.

Most of this west Texas cinnabar is found in cracks, pores, and breccia-filled
cavities of Cretaceous limestones and clays. If you will look at the Texas

geologic map (pp. 4-5), you will see that igneous rocks occur in this district.
Many millions of years ago during the Tertiary Period, when these igneous
rocks were still hot magma, some of them pushed up under the Cretaceous
rocks and emitted fluids containing mercury. The fluids moved upward
through cracks and pores in the Cretaceous rocks where they deposited 50
the mercury as cinnabar and as other mercury minerals.

Cinnabar crystals (dark) with calcite crystals (white) from the Terlingua area of Brewster
County, Texas.

Mercury is an unusual element. Instead of occurring as a solid metal at
ordinary room temperatures, as do gold, silver, and lead, it remains a liquid
until it is cooled to 38 degrees below zero Fahrenheit. Because the silvery

little drops of liquid mercury roll about as if they were alive, this element
long has been called quicksilver.

Mercury is used in a variety of ways. In some noiseless light-switches, a
glass tube containing a small ball of mercury tilts when the switch is turned
“on.” The mercury then rolls to the end of the tube that contains electrical
contacts and quietly completes the electrical circuit. In other uses, mercury is
added to silver, tin, and other metals to make fillings for teeth. Some
medicines, such as calomel and mercurochrome, contain mercury. Fulminate
of mercury helps to set off dynamite and other explosives. Mercury is used
in many barometers and thermometers, and farmers use mercury poisons to
control insects and fungi.

Mercury also commonly is used to obtain gold from its ores. One 51
method of accomplishing this is to pass wet gold-bearing gravel or
crushed rock over metal plates that are coated with mercury. The gold
particles quickly mix with the mercury to form an amalgam, which later can
be scraped off the plates. The gold is then recovered by heating the amalgam
to drive off the mercury.

Clay
Clay is a smooth, soft, earthy rock made up of mineral particles no bigger
than specks of dust. Some of the particles are clay minerals, which consist of
aluminum, silicon, and other elements. In addition, tiny particles of quartz,
calcite, and other minerals may also be present in the clay.

The clay particles are all that remain of rocks and of minerals, such as
feldspar, that have been broken into fragments or altered into clay minerals
by weathering. Some clay remains at the place where it formed, but some is
carried away and deposited elsewhere.

Clay is white, tan, brown, red, green, blue, gray—almost any color. When
moist, it has an earthy odor. You can moisten a piece of clay enough to
notice this just by breathing on it. Most clays, when wet, can be molded into
many different shapes—that is, they are plastic, but when they are dry, they
are firm and solid.

Clay is abundant in Texas and has a number of uses. Some goes to make
portland cement, and some is baked or burned in a kiln to make brick, tile,
sewer pipes, pottery, and other products. This kind of clay is obtained from
Tertiary formations of the Gulf Coastal Plain, from Upper Cretaceous
formations in central Texas, and from Pennsylvanian formations in north-
central Texas. (You can locate Tertiary, Cretaceous, and Pennsylvanian rocks
on the Texas geologic map, pp. 4-5.)

A special kind of white burning clay that can be used to make chinaware is
called kaolin or china clay. It contains particles of the clay mineral kaolinite
as well as several other clay minerals. Deposits of china clay occur in
southern Jeff Davis County and in Real County near Leakey, but none is
being produced.

Another kind of clay, bentonite, forms from weathered volcanic ash.
Bentonite contains the clay mineral montmorillonite and looks smooth and
soap-like. Fresh samples of this clay are white, pale green, or pale blue, but
dried-out or weathered samples are tan, brown, yellow, or reddish. When
wet, bentonite absorbs water, swells, and then has a jelly-like appearance.

Surface deposits of bentonite occur chiefly in Eocene Tertiary formations of
the Gulf Coastal Plain, in Cretaceous formations of the Big Bend area of
west Texas, and in Quaternary formations of the High Plains.

Bentonite is used as a drilling-fluid additive in the rotary method of drilling for petroleum and
gas.

Some bentonite is used to absorb unwanted coloring material in petroleum
and in vegetable oils. It is then known as a bleaching clay. Bentonite
bleaching clay is obtained from some of the Tertiary formations along the
Texas Gulf Coastal Plain. It has been produced in Angelina, Fayette, 52
Gonzales, Jasper, Walker, and other counties in this area.

Another important use of bentonite, and of other clay, too, is as drilling mud.
In the rotary method of drilling for oil and gas, mud is pumped down into the
drilled hole. This mud carries the rock cuttings up to the surface, it cools the

drilling tools, and it coats and seals the walls of the hole. Along the Gulf
Coastal Plain, drilling clay is obtained from Tertiary formations.

Common Opal. See Opal.

Copper Minerals (Chalcocite, Chalcopyrite, Malachite,
Azurite)
A number of minerals containing copper, such as chalcocite, chalcopyrite,
malachite, and azurite, occur in small deposits in Texas. They are found
chiefly in the Llano uplift area of central Texas, in the Van Horn area of
Culberson and Hudspeth counties in west Texas, and in a group of counties
in north-central Texas.

Copper is an important element. Because it is an unusually good conductor
of electricity (only silver, which costs much more, is a better one), it is used
for many kinds of wires for switchboards, generators, motors, telephone and
telegraph equipment, and light and power lines.

Manufacturers commonly combine copper with other elements. For
example, some copper is mixed with zinc to make brass and with tin and a
little zinc to make bronze. These mixtures are called alloys. Many products
are made from copper alloys, including tubing, pipes, jewelry, pots, and
pans. Even our coins contain copper.

Sometimes, a prospector uses a chemical test to find out if copper is present
in a mineral. First, he crushes a small sample of what he believes is a copper
mineral (such as chalcocite, chalcopyrite, azurite, or malachite). He then puts
the sample in a glass jar or test tube and pours in a small amount of dilute
nitric acid (this acid, like hydrochloric acid, is poisonous). After the sample
has dissolved in the acid, he adds enough ammonium hydroxide to make the
solution alkaline. If the sample is a copper mineral, the solution turns a deep-
blue color.

One of the copper minerals, chalcocite, copper sulfide, also is known as
copper glance. It is a metallic mineral that commonly tarnishes to a dull

black. By chipping off a fragment to obtain a fresh surface, you will see that
it has a shiny lead-gray color. Chalcocite is rather soft, and it is sectile, that
is, a knife will cut through it as well as scratch it. When you rub chalcocite
across a streak plate, it gives a grayish-black streak. This mineral commonly
occurs as compact masses or as granular masses.

Chalcocite, with its dark color, does not look at all like copper, which is a
bright reddish brown. Chalcocite, however, is the chief copper mineral at the
most important copper mine in Texas, the Hazel mine, which is about 15
miles northwest of Van Horn in Culberson County in west Texas. This mine,
although now idle and almost filled with water, has produced about one and
a half million pounds of copper along with more valuable silver ores. Here,
the chalcocite and other minerals occur in material that fills large cracks in
red sandstone of the Precambrian Hazel Formation. It is thought that long
ago, molten igneous rock material far below the surface sent out hot
solutions containing copper and other elements. These solutions moved
upward and deposited minerals in the fracture zone in the sandstone.

Chalcocite occurs also in north-central Texas. It is found in Archer, Baylor,
Clay, Foard, Hardeman, King, Knox, Stonewall, and several other counties
of this area. Here, it occurs in Permian sedimentary rocks (called “red beds”)
as rounded masses, as scattered grains, and as petrified wood. Because these
deposits are far from any igneous rocks, they apparently did not form in the
same way as those at the Hazel mine. These north-central Texas deposits
have never really been commercially developed. During the Civil War,
however, some copper from this area was made into percussion caps for the
Confederacy.

53

The Hazel copper-silver mine, Culberson County, Texas, as it appeared in 1951. Photograph by
P. T. Flawn.

Another copper mineral, chalcopyrite, is a copper-iron sulfide. It also is
known as copper pyrites and yellow copper ore. This mineral has a metallic
luster and a brass-yellow or a golden-yellow color. When rubbed across a
streak plate, it gives a greenish-black streak. Chalcopyrite will tarnish and
then has bronze, blue, purple, and other rainbow-like colors. This mineral is
fairly soft—you can scratch it with a pocket knife. Because of chalcopyrite’s
yellow color, it has often been mistaken for gold. For this reason, it, like iron
pyrite, is often called fool’s gold. (See Gold, p. 60, for ways to tell them
apart.)

Chalcopyrite commonly is found in compact masses that show no crystal
shapes. These masses either are scattered through rocks or occur in material
that fills cracks in rocks.

Some chalcopyrite is found in Precambrian sandstone at the Hazel mine and
in other deposits in the Van Horn area of Culberson and Hudspeth counties.

It also occurs in Precambrian rocks at the Sheridan and Pavitte prospects in
Burnet County. These chalcopyrite localities are in districts where igneous
rocks occur.

It is likely that, long ago, hot solutions containing copper moved upward, out
of deeply buried molten magma. While still far below the surface, the
solutions deposited the chalcopyrite in cracks and other openings in the
nearby rocks.

Two copper minerals of Texas, azurite and malachite, are copper carbonates.
Azurite is commonly called chessylite and blue copper; malachite is called
green copper carbonate. Because these minerals are carbonates, a drop of
dilute hydrochloric acid will fizz and bubble when placed on either of them.

Azurite has a bright, intense blue color and leaves a blue streak when rubbed
across a streak plate. Malachite has a bright green color and leaves a green
streak. These minerals have a nonmetallic luster and a glassy to dull
appearance. Commonly, they are translucent, although some specimens of
azurite are transparent. Both azurite and malachite are fairly soft—a pocket
knife will scratch them, but a copper penny will not.

Azurite and malachite occur as individual crystals, but you are more 54
likely to find them as crusts on rocks and on other minerals. Malachite
is also found in rounded fibrous masses that resemble bunches of grapes
(described then as botryoidal).

Both azurite and malachite are formed in the same way. Underground waters
seep through rocks that contain deposits of copper minerals (such as
chalcocite and chalcopyrite) and cause chemical reactions which change
these minerals into malachite and azurite.

Malachite is more plentiful than azurite, but both minerals can be found
together. You can expect to find at least one of them at the same localities
where chalcocite, chalcopyrite, and other copper minerals occur.

Coquina. See Limestone.

Diatomite. See Opal.

Dolomite
Dolomite is the name given both to a rock and to a mineral. The mineral is a
calcium-magnesium carbonate and has a glassy or a pearly luster. It is any of
a number of colors, such as white, pink, brown, or gray, or it can be
colorless. Dolomite leaves a white streak on a streak plate and is transparent
to translucent. It is not particularly hard and can be scratched with a pocket
knife, although not with a copper penny. Dolomite cleaves perfectly in three
directions, and some of the cleavage fragments are rhombohedrons.
However, the cleavages of the individual mineral grains in specimens of
fine-grained massive dolomite are not readily distinguishable.

Dolomite rock from the vicinity of Fairland, Burnet County, Texas.

Most Texas dolomite occurs as coarse-, medium-, and fine-grained
crystalline masses as the chief mineral in dolomite rock and in dolomitic
marble. It is also found as 6-sided crystals that are rhomb-shaped; when the
faces are curved, they have a saddle-like appearance.

Crystals of the mineral dolomite commonly occur in cavities in the dolomite
rocks. It is believed that they were deposited there by seeping underground
waters. The waters dissolved some of the dolomite in the rocks and then re-
deposited it as crystals.

Dolomite rock is made up mostly of crystalline grains of the mineral
dolomite. In addition, quartz grains, calcite, and other minerals may be
present. Dolomite rock is almost any color—white, buff, pink brown, gray. It
resembles some limestone, and these two rocks actually are closely related.

To help tell them apart, dilute hydrochloric acid often is used. A few drops of
this acid will readily fizz and bubble if the rock you put them on is a
limestone. If the rock is dolomite, the acid will effervesce only very little or
not at all. (If, however, the acid is put on powdered dolomite, it then will fizz
readily.) Dolomite is slightly harder than limestone, and it also is slightly
heavier.

Some dolomite rocks formed directly from materials that were dissolved in
sea water, and others are altered limestone rocks. Some limestones altered
into dolomite on the sea floor by the addition of magnesium from the sea
water. Others changed into dolomite much later after the sea had withdrawn
and the limestones had become a part of the land; underground waters
containing magnesium seeped through these limestones and altered 55
them into dolomite.

Many of the dolomite rocks are found with limestones. In Texas they occur
mostly in Cambrian, Ordovician, Mississippian, Pennsylvanian, Permian,
and Cretaceous formations. The geologic map (pp. 4-5) indicates where
these strata appear at the surface in Texas.

Dolomite is abundant in the Llano uplift area of central Texas—particularly
in the Cambrian and Ordovician rocks. A number of these central Texas
dolomites have been quarried for use as building stones. Some of them also

have been crushed and used as a road-building material and as a stone
aggregate that is mixed with cement to make concrete. This dolomite is also
used as terrazzo chips (terrazzo floors are described with serpentine on p.
88). In addition, Ellenburger (Ordovician) dolomite from Burnet County was
used during World War II as a source of the lightweight metal magnesium.

Dravite. See Tourmaline.

Feldspar
Feldspar is the name given to a group of nonmetallic minerals that are much
alike. Several of them are so similar that a petrographic microscope must be
used to tell them apart. Each of the feldspar minerals is an aluminum silicate.
Each of them contains, in addition, at least one of the following elements:
potassium, sodium, calcium, and barium. The feldspar minerals that are
found in Texas include albite, a sodium-aluminum silicate, and orthoclase
and microcline, which are both potassium-aluminum silicates.

The feldspar minerals are transparent to translucent and have either glassy or
pearly lusters. They can be white, cream, or a shade of red, brown, yellow,
blue, gray, or green. When you rub a feldspar across a streak plate, it leaves a
white streak. The feldspars are rather hard—a pocket knife will not scratch
them, although a piece of quartz or a steel file will. These minerals have
good cleavage in two directions. The cleavages meet at an angle of about
90°, so that the cleavage fragments have square corners.

Feldspar cleavage fragment from Llano County, Texas. The two directions of good cleavage meet
at an angle of about 90°.

The feldspars are important rock-forming minerals. You can find them in
igneous rocks, such as granite or pegmatite, and in metamorphic rocks, such
as gneiss. They also occur as fragments in sedimentary rocks, such as some
sandstone and conglomerate.

Although the feldspars can originate in other ways, they form mostly from
hot magmas that cool and crystallize into igneous rocks. These minerals
occur in the rocks as grains, as cleavable masses, and as individual crystals.
The crystals may be shaped like prisms, or they may be flat and slabby.

Good places to look for feldspars are in areas where granites, pegmatites,
and other intrusive igneous rocks appear at the surface. The pegmatite rocks

of Burnet, Gillespie, Llano, and Mason counties in the Llano uplift area of
central Texas, and those of the Van Horn Mountains in Hudspeth and
Culberson counties in west Texas, are especially good sources of feldspar.
Large cleavable masses and crystals that are more than a foot long are found
in some of these rocks.

The feldspars have a number of uses. Some of the pegmatite feldspars from
Llano County in central Texas have been crushed and used as granules 56
for built-up and composition roofs. In addition, some have been
shipped to Mexico for glass-making. Some of the other uses of feldspar are
in making porcelain, ceramic glazes, and scouring compounds. A few of the
feldspar minerals, such as the variety of microcline known as amazonstone,
are used as gemstones.

Microcline feldspar crystals from near Granite Shoals Lake, Llano County, Texas.

Fibrous Gypsum. See Gypsum.

Flint. See Quartz.

Fluorite
Fluorite is calcium fluoride. The fluorite that is mined and sent to market,
however, commonly is found mixed with quartz, calcite, limestone, or other
rocks and minerals. Industry calls this mixture fluorspar.

Fluorite is a transparent to translucent mineral that has a glassy luster. It may
be colorless, or it may be white, pink, green, purple, brown, or blue. Some
specimens show more than one color. When you rub fluorite across a streak
plate, it leaves a white streak. This mineral is not particularly hard—a pocket
knife will scratch it, although a copper penny will not. Fluorite has perfect
cleavage in four directions. By carefully breaking a specimen, you can
obtain cleavage fragments that are shaped like octahedrons.

Fluorite occurs as cleavable masses, as fine or coarse grains, and as crystals.
Most of the crystals are cubes, but some may be octahedrons,
dodecahedrons, or combinations of these.

Fluorite has been found both in west Texas and in central Texas. In the Llano
uplift area of central Texas, it occurs in a number of Precambrian granite,
pegmatite, schist, and gneiss rocks. The most important, although small,
deposit in this area is near Spring Creek a few miles west of Burnet in
Burnet County. Here, prospectors have dug holes and pits in gneiss and
schist rocks and found layers of fluorite in them.

The largest known fluorite deposits in Texas (they are not particularly large
when you compare them with the deposits in Illinois and Kentucky) are
those in the Eagle Mountains of Hudspeth County. This fluorite occurs in
both igneous and sedimentary rocks. Many years ago, probably during the
late part of the Tertiary Period, hot magma far below the surface gave off
liquids and gases containing fluorine. These fluids moved up through large
cracks (called faults) in Cretaceous limestones and Tertiary igneous 57
rocks and deposited fluorite in them. In places, beds of limestone have
been replaced by fluorite. Some of this west Texas fluorite has been mined
and shipped to market.

Fluorite has octahedral cleavage. The four directions of perfect cleavage can result in cleavage
fragments that are octahedrons.

Fluorite is extremely important as a flux in steel-making to help the
ingredients of the molten steel blend together. In addition, it combines with
sulfur, phosphorus, and other unwanted substances so that they can be

removed from the steel. Other important uses of fluorite are in glass-making
and in the manufacture of hydrofluoric acid. This acid is used in the
aluminum industry as well as in industries that make high-octane gasoline,
insecticides, and refrigerants for refrigerators and freezers.

Galena
Galena, lead sulfide, is a shiny, lead-gray, metallic mineral that has a specific
gravity of 7.4 to 7.6. It is soft enough to mark paper, and it leaves a grayish-
black streak on a streak plate. This mineral cleaves perfectly in three
directions, and the cleavage fragments have square corners—some are
cubes.

Galena occurs as cleavable masses, as fine or coarse grains, and as crystals,
most of which are cubes. Galena commonly is associated with other
minerals; for example, some of the west Texas galena either contains some
silver (then called argentiferous galena) or occurs with it. Sphalerite, a zinc
mineral, is commonly found with galena.

Galena is an important mineral because it is the chief source of lead.
Compounds of lead, called white lead, red lead, and litharge, are used as
paint pigments. Automobile batteries contain lead plates, and tetraethyl lead
is added to gasoline to keep the car’s motor from knocking. Some other uses
of lead are in bullets, type metal, solder, and cable coverings.

Galena has perfect cubic cleavage. The three directions of cleavage are at right angles to each
other resulting in cubic cleavage fragments.

Galena has been found in several areas of Texas and has been mined in
central and west Texas. None, however, has been produced in recent years.
Most of the galena mined in west Texas was obtained from silver mines,
where the galena was a by-product. Some of the west Texas galena deposits
are at Altuda Mountain east of Alpine in Brewster County, in the Eagle
Mountains and the Quitman Mountains in Hudspeth County, and in the
Chinati Mountains and the Shafter area in Presidio County. Most of the
mining has been from the Shafter area (this area is described with 58
silver minerals on p. 90).

In central Texas, several small galena deposits have been found in Blanco,
Burnet, and other counties of the Llano uplift area. Some galena has been
mined at Silver Creek in northwestern Burnet County. Here, galena occurs in
cracks and as scattered grains in Cambrian limestones and sandstones.

It is probable that much of the galena in west Texas and in central Texas was
formed when hot magma forced out solutions containing lead. These
solutions moved up through cracks and other openings in the subsurface
rocks and deposited the galena in them.

Small amounts of galena, which likely had a different origin, have been
found in Fisher, Foard, Hardeman, and Young counties. A little occurs also in
rocks associated with salt in a number of the Gulf Coastal Plain salt domes.

Garnet
Garnet is not one mineral but is the name given to a group of several
minerals that are very much alike. In fact, it often is impossible to tell some
of them apart without using special laboratory tests.

Garnet crystal forms include: A, trapezohedron; B, dodecahedron; C and D, combination
trapezohedron and dodecahedron.

The garnet minerals have glassy to resinous lusters and are transparent or
translucent. A pocket knife will not scratch them, and some specimens are
too hard even for quartz to scratch. Two of the garnet minerals most
commonly found in Texas are almandite, an iron-aluminum silicate, and
grossularite, a calcium-aluminium silicate. Almandite has a deep-red or a

brownish-red color. Grossularite is pale green, brownish yellow, cinnamon
brown, or rose red.

Garnet minerals occur as crystals and as masses that are scattered through
some of the metamorphic and igneous rocks. After they have weathered out
of these rocks, the garnets make up a part of many sands and sandstones.
Because these minerals so commonly occur as crystals, it is helpful to learn
to recognize the crystal shapes.

Garnet minerals are found in the igneous and metamorphic rocks of both
central Texas and west Texas. In central Texas, they occur in ancient
Precambrian schist and pegmatite rocks of the Llano uplift area. Some of
these central Texas garnet localities are in northeastern Mason County,
central and northwestern Llano County, west-central Burnet County, and
northeastern Gillespie County.

In west Texas, garnets occur in metamorphic rocks in the Quitman
Mountains, which are southwest of Sierra Blanca in Hudspeth County, and
in the Mica Mine area, which is south of Van Horn near the Hudspeth-
Culberson County line. Garnets also have been found in igneous rocks in the
Franklin Mountains a few miles north of El Paso in El Paso County.

Garnets that are found in metamorphic rocks such as schists were formed
when great forces squeezed and heated rocks far below the earth’s surface.
This heat and pressure caused elements in the rocks to join together into
different combinations to form new minerals, such as garnets. Garnets that
occur scattered through igneous rocks, such as some pegmatites and granites,
cooled and crystallized from hot, igneous fluids when the rocks themselves
formed.

Most Texas garnets are not transparent. A few, however, are clear enough to
be used as gemstones. These can be cut, polished, and mounted in 59
rings, brooches, bracelets, and earrings. Although some garnet is
widely used as an abrasive, none from Texas has been produced for this
purpose.

Gneiss

Gneiss is a metamorphic rock that has parallel layers or bands. Some gneiss
is made up of the same minerals (chiefly feldspar and quartz) as granite, and
it is then called granite gneiss. Several of the other kinds of gneiss are
known as mica gneiss, conglomerate gneiss, gabbro gneiss, and hornblende
gneiss. In order to be a gneiss, a metamorphic rock has to have bands or
layers. These bands may be either straight or wavy and either wide or
narrow. In most gneisses, you will find a layer made up of long or flat
mineral grains next to a layer made up of the grains of an entirely different
mineral. The bands may show color differences, too. For example, a pink
layer made up of feldspar grains may be found next to a black layer made up
of hornblende grains. The mineral grains interlock as they do in igneous
rocks, and they are generally large enough to be seen without a magnifying
glass.

Gneiss from Blanco County, Texas, showing light and dark bands.

Gneiss can form from an igneous rock, such as granite, or from a
sedimentary rock, such as sandstone. Heat, fluids, and pressures below the
earth’s surface change these rocks into gneiss.

Gneiss that formed during Precambrian time is now seen at the surface in
both west Texas and central Texas. In west Texas, it occurs principally in the
Van Horn area of Culberson and Hudspeth counties. In central Texas, it is
found in Blanco, Burnet, Gillespie, Llano, and Mason counties of the Llano
uplift area.

One of the Llano uplift rocks is called the Valley Spring Gneiss. It generally
has a light color (much of it is pinkish), and it is believed to have once been
a sandstone. Another gneiss of this area, the Big Branch Gneiss, which has a
medium to dark gray color, occurs in northern Gillespie and Blanco counties
and is an altered igneous rock. Some of the Texas gneiss rocks are suitable
for use as building stones.

Gold
Gold commonly occurs in nature as a single element—gold—but much
native gold has a small amount of some other element, such as silver, copper,
or iron, mixed with it.

Native gold is a shiny, yellow, metallic mineral that does not tarnish, and it
leaves a shiny, golden-yellow streak when you rub it across a streak 60
plate. If silver is present, the color and streak have a lighter shade. Pure
gold is extremely heavy—its specific gravity is 19.3. Because it is malleable,
this mineral will flatten into a thin sheet when hammered. It is ductile
enough to be drawn out into wires. Gold is also soft—a pocket knife will
scratch it easily. When it is to be used for ornaments and jewelry, gold is
usually mixed with other metal, such as silver, copper, nickel, or palladium,
to make it harder. The amount of gold that is present is then indicated by
carats (or karats). Pure gold is 24 carats. If you have a gold ring that has 14
K stamped inside it, you know that it is made of a mixture of 14 parts gold
and 10 parts of other metal.

Gold commonly occurs in nature as plates, scales, or grains. Some of the
grains are large enough to be called nuggets. It also is found in a wire-like
shape described as filiform, it occurs in a network, called reticulate, and it
can have a branching and fern-like shape, described as dendritic. Gold is not
often found as individual crystals.

Several other minerals, such as pyrite, chalcopyrite, and mica, are sometimes
mistaken for gold. None of these, however, is malleable and ductile, and
none is nearly as heavy as gold. Pyrite and chalcopyrite have dark-colored
streaks unlike that of gold. Mica cleaves so perfectly that it can be split into
thin, flat sheets, but gold has no cleavage at all.

The best places to look for gold are in areas near igneous rocks and along the
creeks and rivers that drain these areas. It is thought that most gold originally
was carried up from molten igneous rock by hot solutions. The solutions
moved into cracks and other openings in nearby rocks and deposited the
gold, commonly along with quartz. Later, some of these gold-bearing rocks
weathered away. The gold that the rocks contained either remained at the
spot or was washed into creeks and rivers. These transported accumulations
of loose gold are called placer deposits.

Placer gold in very small quantities has been found in some of the stream gravels of Texas.

No really important gold deposit has ever been found in Texas, although
traces and small amounts have been reported in several areas. A little gold
has been found in the Llano uplift area of central Texas. It occurs in quartz
veinlets that cut through some of the Precambrian metamorphic rocks of
Llano, Mason, northeastern Gillespie, and west-central Burnet counties.
Many years ago, a small amount of gold was mined northeast of Llano in
Llano County from the Heath mine. Some gold also has been found in sands
and gravels along streams, such as along Sandy Creek and its tributaries, in
parts of this Llano uplift area.

In the Trans-Pecos country of west Texas, small amounts of gold have been
found in the Van Horn area of Culberson and Hudspeth counties, in the
Quitman Mountains district of Hudspeth County, and in the country around
Shafter in Presidio County. Most of the small quantity of gold that was
mined in west Texas was obtained as a by-product from the Presidio mine in
the Shafter district (described with silver minerals on p. 90).

Small amounts of gold have been reported from other parts of Texas. Some
of these localities are in Eocene Tertiary sandstones in the Gulf Coastal
Plain, in Cretaceous limestones in Irion, Uvalde, and Williamson counties,
and in sand and gravel in Howard and Taylor counties. None of these
deposits has been found to have any commercial value.

61

Granite
Granite is an intrusive igneous rock that is made up chiefly of crystalline
grains or crystals of quartz and a feldspar mineral, such as orthoclase or
microcline. Several other minerals, including mica and hornblende, may also
be present.

All of the mineral grains in granite are about the same size, and you can
distinguish them without using a magnifying glass. A granite may be coarse
grained, medium grained, or fine grained. When you examine this rock, you
will see that its grains are not cemented but interlocked like the pieces of a
jigsaw puzzle. The color of granite, which is pink, red, gray, or brownish,
depends chiefly on the color of its feldspar grains.

Most granites formed from hot, molten magma that slowly cooled and
hardened far below the earth’s surface. Because of this slow cooling, fairly
large mineral grains were formed.

Polished section of pink granite from Gillespie County, Texas.

Granites are now seen at the surface in several areas of Texas. They were
gradually uncovered as the areas became higher and the overlying rocks
slowly weathered away. One of these areas is the Llano uplift of central
Texas where the granites occur in Blanco, Burnet, Gillespie, Llano, and
Mason counties. These granites formed during Precambrian time and are
believed to be about a billion years old. (Scientists are now able to 62
determine the age of some rocks accurately by very precisely

measuring the relative amounts of isotopes produced by decay of radioactive
minerals.)

Texas State Capitol building at Austin is made of Burnet County granite obtained from Granite
Mountain near Marble Falls, Texas.

Granites also appear at the surface in the Trans-Pecos country of west Texas.
Some of these areas include the Franklin Mountains of El Paso County, the
Quitman Mountains of Hudspeth County, the Chisos Mountains of Brewster
County, and the Chinati Mountains of Presidio County.

Red, pink, and gray granites from quarries in the Llano uplift area are widely
used as building stones and monument stones. A large quarry at Granite
Mountain just west of Marble Falls in Burnet County has supplied pink
granite for buildings in many parts of the United States. The Texas Capitol
building and several other State buildings in Austin are made of this granite.

Graphite
Graphite is a mineral that is made up of a single element—carbon.
(Diamond, although it does not look at all like graphite, is a crystalline 63
form of carbon.) Graphite is a steel-gray or black mineral that
commonly has a metallic luster. It is not heavy and is extremely soft.
Graphite will soil your fingers and leave a black mark on paper. This mineral
cleaves perfectly in one direction and splits into thin flakes that feel greasy.

To help distinguish graphite from molybdenite, a mineral it resembles, you
can use a shiny, glazed surface, such as is found on a saucer or a plate, to test
its streak. When rubbed across this kind of surface, graphite will leave a
black streak, but molybdenite will leave a greenish one.

Graphite commonly occurs as scales, as sheet-like layers, or as compact
masses. It may be found mixed with clay or other impurities, and it then
looks dull and earthy. Crystals of graphite, which are seldom found, are 6-
sided and flat.

Graphite occurs in Llano, Burnet, and other counties in the Llano uplift area
of central Texas. One of the Nation’s most important graphite mines is
located in the Clear Creek area several miles northwest of Burnet in Burnet
County. Some graphite has also been mined near Lone Grove in Llano
County. In addition, a graphite schist, obtained south of Llano in Llano
County, has been used as a filtering material.

Graphite is used in pencil lead, generator brushes, and lubricants.

All of this graphite occurs in extremely old Precambrian graphite schist
rocks that we now see at the surface in this part of Texas. It is believed that
the schists were once ancient sedimentary rocks, such as shales, which
contained organic matter. Long ago, great forces below the earth’s surface
altered these rocks. When this happened, the organic material that they
contained changed into the mineral we know as graphite.

Graphite has a number of uses. It is mixed with clay to make the pencil lead
that we use for writing. It serves as a lubricant, either alone or mixed with
oil, grease, or water. In addition, graphite is used to make generator brushes,
stove and shoe polish, and special paints. Because it can stand great heat
without melting, some graphite is mixed with clay to make the pots or
crucibles that hold molten metals.

Grossularite. See Garnet.

Gypsite. See Gypsum.

Gypsum
Gypsum is a hydrous calcium sulfate. This mineral is normally colorless or
white, but impurities cause it to appear gray, brownish, yellowish, or
reddish. It is transparent or translucent and is not heavy. When you rub
gypsum across a streak plate, it leaves a white streak. This mineral is so soft
that a fingernail scratches it easily. Gypsum occurs in several varieties.

The colorless, glassy, and transparent variety of gypsum is called selenite. It
is found as cleavable masses and as crystals that are prism-shaped or flat
and diamond-shaped. It is not uncommon for two crystals to be joined
together so that they have a swallow-tail shape—these crystals are twinned.
Groups of flat selenite crystals arranged together so that they resemble
flowers are called rosettes. Many of these have been found in Nolan
County.

Gypsum has four directions of cleavage. One of these directions is so
perfect that some selenite splits into thin, clear sheets that may be mistaken
for mica; other selenite cleavage fragments may be mistaken for 64
calcite. You can distinguish selenite sheets from calcite by testing
their hardness (selenite is softer) and by putting a drop or two of dilute
hydrochloric acid on them. The acid will fizz and bubble on calcite but not
on the selenite gypsum. There is also a quick way to distinguish the thin
selenite cleavage fragments from mica. After you carefully bend a thin
sheet of mica, it will snap back to its original shape without breaking.
Selenite gypsum, however, is not elastic. It will bend, but it will break if
you try to straighten it again.

Selenite gypsum crystal from Bastrop County, Texas.

Selenite is found in cracks and cavities in rocks. Good crystals have been
collected at Gyp Hill, a salt dome southeast of Falfurrias in Brooks County,
and some selenite has been mined there. Selenite crystals also occur
scattered through clays, particularly along creek banks, in Lee, Fayette,
Bastrop, and several other counties.

Another variety of gypsum is known as fibrous gypsum. It is made up of
slender, brittle, needle-like fibers that fill the cracks in some rocks. If
fibrous gypsum has a silky or pearly luster, it is called satin spar. One of
the places where satin spar occurs is in Permian rocks in Hardeman County.

Selenite gypsum rosettes from Nolan County, Texas.

Most of the fibrous gypsum and selenite is formed by solutions. Some of
these solutions develop when underground waters, seeping through rocks,
pick up and dissolve minerals that contain sulfur (such as pyrite). This
dissolved material changes the water into very weak sulfuric acid. 65
When the sulfuric acid meets calcium carbonate (as in limestone or
calcite), it combines with the calcium to form the gypsum.

Fibrous gypsum from Terlingua area, Brewster County, Texas.

A massive, fine-grained, and translucent variety of gypsum, known as
alabaster, is used for articles such as lamp bases, statuettes, vases, and
book-ends.

A loose, earthy, crumbly variety of gypsum, called gypsite, is ordinarily
found mixed with other materials, such as clay, sand, and soil. It occurs
either at or near the surface of the ground. Gypsite is found in Culberson,
Reeves, and other counties in west Texas.

A massive, granular variety of gypsum, called rock gypsum, may occur in
large deposits. This is the gypsum that is used for making products such as
plaster, wallboard, and some cements.

Deposits of rock gypsum are found both underground and at the surface in
Texas. Surface deposits occur in Permian rocks in several counties to the
east of the Texas High Plains. They also occur in the area between the Pecos
River and the Delaware and Apache Mountains in Culberson and Reeves
counties. Some of the other surface deposits are found near the Malone
Mountains in Hudspeth County and in Lower Cretaceous rocks in Gillespie
and Menard counties. Rock gypsum has been mined from the deposits in
Fisher, Gillespie, Hardeman, Hudspeth, and Nolan counties. It also has been
produced from the cap-rock at Hockley salt dome in Harris County.

Gypsum and another mineral, anhydrite, have very nearly the same
composition. Both are calcium sulfates. Gypsum, however, contains water
of crystallization, and anhydrite does not. It is likely that most of the rock-
gypsum deposits of Texas originally were beds of anhydrite. By absorbing
water that seeped through it, the anhydrite changed into gypsum.

Halite
Halite, sodium chloride, is the table salt you sprinkle on food for seasoning.
This mineral ordinarily is white or colorless, but other materials cause it to
be tinted red, blue, gray, brown, or green. When you rub halite across a
streak plate, it leaves a white streak.

Because halite cleaves in three directions, all at right angles to each other,
the cleavage fragments are shaped like cubes. You can see some of them by
looking at a few grains of table salt through a magnifying glass.

Halite has a salty taste and dissolves easily in water. It also is transparent to
translucent and has a glassy luster. This mineral is soft enough for a copper
penny to scratch it. Halite commonly occurs as cubic crystals and as
granular or compact masses.

In addition to its use as table salt, much halite goes to make soda ash,
chlorine, and other chemicals. A few of its other uses are in leather making,
meat packing, and food canning.

Texas has large underground deposits of halite. These deposits, known as
rock salt, occur in the Permian subsurface basin of west Texas and in the
salt domes of the Gulf Coastal Plain. The Permian basin, which extends
under parts of west Texas, New Mexico, Oklahoma, Colorado, and Kansas,
is now completely filled with sediments. It appears level and flat when you
travel across it and does not look at all like a basin or a valley. During
Permian time, however, this area was covered by a salty sea. As the 66
sea gradually dried up, the dissolved material that it contained was
deposited as thick beds of halite, anhydrite, and other minerals. Later, these
minerals were covered by sedimentary rocks which were deposited on top
of them. Now, the minerals are found many hundreds of feet below the
surface. In Hutchinson, Mitchell, Ward, and Yoakum counties, some of this
Permian basin salt has been produced (as brine) from wells that have been
drilled into it.

Salt domes, which are huge, underground columns of halite, occur on the Gulf Coastal Plain.

The Gulf Coastal Plain salt domes are huge and almost circular columns of
halite, some of which are more than 2 miles wide. Some are less than 300
feet below the surface, but most of them are much deeper. These salt
columns pushed upward many thousands of feet from great, deeply buried
salt deposits. The halite is mined from shafts dug into the Hockley salt
dome in Harris County and into the Grand Saline salt dome in Van Zandt
County. Salt brines are produced from wells drilled into several salt domes
of this area.

At the surface in Texas, halite occurs in salt lakes in Crane and Hudspeth
counties and in alkali lakes on the High Plains. It is found also on the shores
of bays and lagoons in Cameron, Kenedy, Kleberg, and Willacy counties,
and it occurs at springs and seepages in various places in the State.

Hematite
Hematite, iron oxide, the chief ore of iron, is found in many places in Texas
but not in large deposits. This mineral may have a metallic luster and 67
appear reddish brown, dark brown, steel gray, or black or it may occur
as a soft, red, earth-like material called red ocher.

Specular hematite from Carrizo Mountains, Hudspeth County, Texas.

Most metallic hematite is too hard for a pocket knife to scratch, but quartz
or a steel file will scratch it. Hematite is fairly heavy, for it has a specific
gravity of 5.26. This mineral has no cleavage, but some specimens show
three directions of parting that are almost at right angles to each other. A
great help in identifying hematite is the dark reddish-brown streak it leaves
when you rub it across a streak plate.

Some hematite occurs as rounded masses that resemble kidneys or bunches
of grapes (then called kidney ore); it also is found as flat crystals. Most of

the Texas hematite occurs as granular or compact masses. One of these
massive varieties is composed of shiny scales or plates and is called
micaceous or specular hematite. This variety has been found in Hudspeth
County and in northeastern Mason County. Hematite also commonly occurs
as cementing material in many Texas sandstones.

Some hematite is formed by the alteration of magnetite, another iron
mineral. This hematite is known as martite, and some of it still has the
crystal shape (an octahedron or a dodecahedron) that belonged to the
magnetite. Most of the hematite found in the Llano uplift area of central
Texas is believed to be altered magnetite. In this central Texas area, some
massive, granular martite has been mined at the Gamble prospect, a few
miles southeast of Fredonia in northeastern Mason County, where it occurs
as layers in Precambrian gneiss.

Small deposits of hematite occur in other parts of Texas, too. Some of the
west Texas localities include Sierra Blanca, the Quitman Mountains, and the
Carrizo Mountains of Hudspeth County and the area around Shafter in
Presidio County.

Hollandite. See Manganese Minerals.

Hyalite. See Opal.

Jasper. See Quartz.

Kaolin. See Clay.

68

Limestone
Limestone is a sedimentary rock made up chiefly of calcite, a calcium-
carbonate mineral. This rock also commonly contains grains of quartz, clay
minerals, the mineral dolomite, or other materials. If a large amount of

dolomite is present, the rock is called dolomitic limestone. In some
limestones, the mineral grains are too small to be distinguished from each
other without a magnifying glass or a microscope, but in other limestones,
the individual mineral grains are easily seen.

Pure limestone is white, but if it contains clay or plant or animal matter it is
light gray, dark gray, or black. Limestone also may be some shade of
yellow, brown, or red. It is fairly soft and can be scratched with a knife.
Because this rock contains calcite, an easy chemical test will help identify
it: a drop or two of dilute hydrochloric acid will quickly fizz and bubble
when placed on the limestone.

Limestones form in fresh water, such as in lakes, but most of them form in
the seas. As some earlier-formed rocks are weathered, the calcium minerals
that they contain are dissolved. Creeks and rivers carry this dissolved
material to the sea. There, small animals, such as corals, crinoids, sponges,
and foraminifers, take the dissolved material out of the water to build their
calcium carbonate shells. Plants, such as algae, can take calcium carbonate
out of solution too, and it collects on them. Shells, shell fragments, and
plant remains accumulate on the sea floor, forming limy deposits that later
become limestone.

Limestones also originate in a slightly different way. When the temperature
and chemical composition of the water permit, calcium carbonate
precipitates as millions of tiny grains of calcite and forms a limy mud that is
converted to limestone. Many limestones contain shell or plant fragments in
addition to these tiny grains of calcite.

Polished section of Lower Cretaceous Edwards Limestone from Travis County, Texas,
containing fossil gastropods.

There are several special kinds of limestone. If the rock is made up of many
little rounded calcite grains that resemble fish eggs, it is called öolitic 69
limestone. Another limestone, chalk, is soft, white, and fine grained. It
consists mostly of tiny shell fragments and fine-grained calcite. Coquina is
a porous limestone made up of loosely cemented shells and shell fragments.
Another special kind of limestone, known as lithographic limestone,
because it can be used in printing, is smooth, firm, and hard. Its mineral
grains are too small to be recognized without a microscope. This kind of
limestone breaks with a smooth, sometimes curved, fracture. Still another
variety, pulverulent limestone, is loose, soft, powdery, and white. It occurs
in the Lower Cretaceous Edwards Limestone in Williamson and Bell
counties of central Texas. Some of this limestone is used to polish rice
grains, and it is added to livestock feeds to provide calcium for the animals.

Limestone quarry in Lower Cretaceous Edwards Limestone at Georgetown, Williamson
County, Texas.

Much limestone is found at the surface in Texas in Cambrian, Ordovician,
Mississippian, Pennsylvanian, Permian, and Cretaceous formations. If you
will look at numbers 5, 6, 9, 10, and 11 on the Texas geologic map (pp. 4-
5), you will see that these strata appear at the surface in central, north-
central, and Trans-Pecos Texas.

Limestone has many important uses. Much Texas limestone is crushed and
used as a road-building material and as an aggregate that is mixed with
cement to make concrete. Farmers in some areas improve their crops by
adding limestone to the soil. Limestone also is sent to the iron furnaces in
east Texas to be used in the production of pig iron and steel.

Some of the Texas limestones are heated to a fairly high temperature in
order to change them into lime (calcium oxide). Industry uses a large
amount of lime in making chemicals, steel, glass, paper, and other products.

Builders use it to make plasters, mortars, and stuccos. At plants in Comal,
Johnson, Travis, and Williamson counties, lime is made from Cretaceous
limestones.

Another important use of limestone is in making portland cement. The
limestone is mixed with clay or shale, and the mixture is burned in a 70
kiln until it just begins to melt. Then it is allowed to cool. Next, it is
finely ground and in order to keep the finished cement from hardening or
setting too quickly when it is used, a retarder, such as gypsum, is added. A
number of cement-manufacturing plants in Texas use Cretaceous
limestones, shales, and clays.

Many of the Texas limestones make excellent building stones. Some of
them are quarried from Pennsylvanian and Cretaceous formations in north-
central Texas and from Lower Cretaceous formations in counties near the
Llano uplift of central Texas. A large quarry on the Williamson-Travis
County line near Cedar Park in central Texas has supplied Cretaceous
limestone for many buildings and monuments in the United States and
Canada.

Limonite
Limonite is not really a definite mineral but is a mixture of iron oxides
containing water. It is believed to be closely related to an iron mineral
called goethite. Some limonite may be dull and earthy with the appearance
of brownish-yellow or rusty brown clay. This variety is so soft that a
fingernail will scratch it easily.

Other limonite has a dark brown or black color and a metallic or almost
metallic luster. A copper penny will not scratch it, but a steel file will. This
kind of limonite may have a shiny black surface that resembles glossy
lacquer. The property that will help you most in identifying limonite is the
rusty, yellowish-brown streak it leaves when rubbed across a streak plate.

Limonite has no cleavage and no crystal shape of its own. But crystals of
other iron minerals, such as pyrite and magnetite, alter to form limonite. It
then occurs with a crystal shape that originally belonged to one of these

other minerals. (Such false forms of minerals are called pseudomorphs.)
Limonite also occurs as layers in rocks, as hollow or solid concretions, or as
coatings on other minerals. It is found mixed with minerals such as clays
and serves as the cementing material in some sandstones.

Limonite is found in many localities in Texas including Blanco, Brewster,
Burnet, Llano, and San Saba counties. The most important limonite deposits
in Texas, however, are in the eastern part of the State, particularly in
Anderson, Cass, Cherokee, Henderson, Marion, Morris, Nacogdoches,
Smith, and Upshur counties.

The east Texas limonite deposits occur mainly in Weches sedimentary
rocks. These rocks, which were deposited in the sea during Eocene Tertiary
time, contain clay along with greensands. (Greensands are small, soft grains
that contain glauconite, a mineral composed of iron, silicon, and several
other elements.) Later, as the sea retreated, these sediments became a part of
the land. Waters seeping through the sediments changed into weak solutions
of carbonic and sulfuric acid that dissolved the iron out of some of the
greensands. When conditions were favorable, this iron was re-deposited as
an iron-carbonate mineral called siderite. Siderite was changed to limonite
by weathering. Some siderite is still found in east Texas, and it is also
mined along with the limonite as an iron ore.

East Texas iron ore has been mined from time to time ever since about
1855, and records show that a number of local iron furnaces once operated.
The brown iron ore (as the limonite is also called) now is mined from open
pits in Cass, Cherokee, and Morris counties.

This ore, after being washed, goes into blast furnaces at Lone Star (near
Daingerfield) and at Houston. In the blast furnaces the ore is changed into
metallic iron by mixing it with coke (made from coal) and limestone and
blowing in blasts of hot air.

To make steel, the iron from the blast furnace (called pig iron) is put into
open-hearth furnaces together with scrap iron, limestone, and other
materials. This mixture is heated and melted together to get rid of unwanted
substances. Then other elements, such as molybdenum, manganese, or 71
nickel, are added to make steel with the right strength and toughness.

Limonite ore is changed to metallic iron in a blast furnace.

Skip car

Hot Gases
Blast Furnace
Iron Ore
Limestone
Coke
Fire Brick lining
Steel Plate covering
3550°F
Hot Air Blast
Slag
Slag Ladle
Molten Iron
Iron Ladle

Steel mills alongside the furnaces in Texas turn out many products, such as
steel plates for oil tanks, ships, and tank cars and steel beams for framework
in buildings and bridges. Some of their other products include pipes for the
oil and chemical industries and wire for nails and fencing material.

Lithographic Limestone. See Limestone.

Llanite
Llanite is a unique rock that is found only in Llano County in central Texas.
This intrusive igneous rock is made up of easily seen crystals and 72
grains of quartz and feldspar that are scattered through a brown-
colored mass of extremely small mineral grains. The quartz is beautiful,
sky-blue, and opal-like; the feldspar has a rusty pink color. (Because the
quartz looks like opal, this rock often is called opaline granite.) The mineral
grains that make up the brown-colored mass are so tiny that they can be
identified only with a microscope. They are quartz, feldspar, mica, fluorite,
and apatite.

Llanite formed during Precambrian time. Molten rock material forced its
way upward into cracks that cut across granite and schist rocks while the
rocks were still far underground. This hot magma remained in the cracks

where it cooled and hardened to form long, narrow, wall-like masses (called
dikes) of llanite. We can see some of the llanite dikes exposed at the earth’s
surface to the north and northeast of Llano in Llano County because the
overlying rocks have weathered away.

Llanite has been quarried from one of the dikes west of Babyhead in
northern Llano County. Because llanite is both attractive and strong, it has
been used as an ornamental stone and as a monument stone.

Magnetite
Magnetite, iron oxide, is a black, metallic mineral with an outstanding
physical property: it is magnetic—fragments of magnetite readily cling to a
magnet. It also leaves a black streak when rubbed across a streak plate.
Although this mineral is too hard to be scratched by the average pocket
knife, a steel file will scratch it. Magnetite is fairly heavy—it has a specific
gravity of 5.18.

Magnetite occurs as compact or granular masses, as scattered grains, and as
crystals. Most of the crystals are octahedrons, but some dodecahedrons are
found. Magnetite helps make up a part of many metamorphic and igneous
rocks, and it also occurs as tiny crystals and grains in some sands,
sandstones, and other sedimentary rocks.

Metallic iron, after leaving the blast furnace, is made into steel in an open-hearth furnace.

Open-Hearth Furnace
Scrap metal
Alloying Elements
Limestone
Furnace Interior
Live Fuel Burner
Air pre-heated

Most of the magnetite that has been found in Texas occurs in Precambrian
gneiss and schist rocks of the Llano uplift area of central Texas, particularly
in Llano County and in eastern Mason County. It occurs as thin layers, as
thick lens-shaped deposits, and as scattered grains in the rocks. Probably at
least a billion years ago these gneisses and schists were sedimentary rocks,
such as shales and sandstones. Some geologists believe that these 73
rocks could have contained iron sediments (perhaps in the form of
glauconite). Great forces below the earth’s surface crumpled and squeezed
the sedimentary rocks and changed them into the metamorphic schist and
gneiss rocks we see today. As this happened, the iron sediments in the rocks
were changed into magnetite.

Granular magnetite fragments from northwest of Llano, Llano County, Texas, are attracted to
a magnet.

At least some of the magnetite in this area (such as the deposit at Iron
Mountain in Llano County) probably had a different sort of origin. Molten
igneous rock material containing iron could have moved up into cracks in
the ancient sedimentary rocks. Then the magnetite formed from this iron
material when the igneous and sedimentary rocks were changed into the
schists and gneisses of today.

None of the Llano and Mason County magnetite deposits is really very
large. Nevertheless, prospecting and a little mining have been carried on
from time to time at several deposits in this area. At Iron Mountain, which
is about 12 miles northwest of Llano in Llano County, magnetite has been
mined from open pits. Although magnetite is commonly used as a source of
iron, the magnetite from this deposit was used as a heavy concrete
aggregate.

Malachite. See Copper Minerals.

Manganese Minerals (Braunite, Hollandite, Pyrolusite)
Although manganese does not occur alone in nature as a native element, it
makes up a part of many minerals and compounds. This element has an
important use in steel making, where it helps rid the steel of unwanted
substances, such as oxygen and sulfur, and, in addition, it is used to make
tough, hard, manganese steel for armor plate, railroad tracks, safes, and
steam shovels. Manganese has various uses outside the steel industry. It is
added to copper and nickel to make alloys, it is used in the manufacture of
dry-cell batteries, and (as manganese sulfate) it is used as a fertilizer.

Manganese minerals and compounds, such as braunite, hollandite,
pyrolusite, and wad, occur in several counties in Texas. No large,
commercial deposits have been found here.

Some manganese compounds and minerals are covered with a soft, sooty
black material that will soil your fingers. This can help you recognize these
minerals; however, a few non-manganese minerals, such as some
chalcocite, also have a black coating that soils your fingers in a similar way.

One of the manganese minerals, braunite, is a complex oxide of manganese
that contains silica. It has a submetallic luster and is dark steel-gray or
black. When rubbed across a streak plate, it leaves a steel-gray or a black
streak. This mineral is too hard to be scratched by a pocket knife, but a
piece of quartz or a steel file will scratch it. Braunite has a specific gravity
of 4.75 to 4.82. It has four directions of cleavage that are parallel to 74
the faces of a pyramid.

In the Spiller mine, about 15 miles northeast of Mason in Mason County,
masses of braunite occur as lens-shaped layers in Precambrian gneiss and
quartzite rocks. This braunite may have formed from another manganese
mineral (possibly manganese garnet) that was exposed at the earth’s surface
after the overlying rocks eroded away. As this other mineral weathered, it
may have altered into braunite, or the braunite could have been deposited
from solutions emanating from hot magmas before the great thickness of
overlying rock was removed.

The mineral variety hollandite is a rare manganate of manganese and
barium. It has a metallic luster, and its color is silvery gray or black. When
you rub it across a streak plate, hollandite leaves a black streak. It has a
specific gravity of 4.7 to 5. Hollandite is rather hard, but a steel file will
scratch it.

Hollandite from Jeff Davis County, Texas.

Hollandite occurs in western Jeff Davis County in west Texas at what is
called the Mayfield prospect. Here, it is found as rounded masses that occur
in a vein near a large fault in Lower Cretaceous limestone rocks.

Other manganese compounds, pyrolusite and wad, are found in several
important deposits near the Pecos River in western Val Verde County.
Pyrolusite is a manganese dioxide mineral. It is black, opaque, and so soft
that it rubs off on your fingers like soot. Pyrolusite may be granular and
massive or may be powdery. It also occurs as a fern-like coating on rocks.
Wad is not really a mineral but is an impure, dull-black or brownish-black
mixture of manganese oxide, water, and other substances. It can be soft
enough to soil your fingers, or it can be too hard to scratch with a 75
pocket knife. Wad occurs in earthy or compact masses or in crusts or
stains on rocks.

In Val Verde County, the wad and pyrolusite are found mixed with soil,
clay, gravel, sand, and plant remains. This material fills cracks in Lower
Cretaceous limestones, it is scattered through gravels, and it is deposited in
low places at the surface. The manganese in these deposits came from
limestone rocks that have since weathered away. Rainwater trickled into
these rocks and dissolved the manganese minerals they contained. This
manganese was washed down toward the Pecos River and was deposited as
wad and pyrolusite.

Marble
Marble is a metamorphic rock made up chiefly of sparkling grains of calcite
or dolomite, but other minerals may be present. The marble may be fine
grained, medium grained, or coarse grained; commonly, all the mineral
grains are about the same size.

Marble may be of uniform color, banded, spotted, or streaked. If it is made
up only of pure calcite or dolomite, the marble is white. If, however, it
contains carbonaceous material, such as graphite, it is grayish or black.
Limonite impurities cause the marble to be yellowish brown, and
manganese oxides and hematite give it a brownish, pinkish, or reddish
color.

Marble is a rather soft rock, and you can scratch it easily with a pocket
knife. A few drops of dilute hydrochloric acid will bubble and fizz readily

on calcite marble; on dolomite marble, it may fizz slightly.

Marble forms from limestone or from dolomite rock. Heat and pressure
below the earth’s surface cause the calcite and dolomite mineral grains in
these rocks to recrystallize. A fine-grained limestone can be changed into a
coarse-grained calcite marble. The marble is not made up of new and
different minerals, but it has a new texture unlike that of the limestone. (To
a builder, the word marble has another meaning. He considers rocks such as
unaltered limestone, unaltered dolomite, or even serpentine to be marble, if
they will take a high polish.)

Polished section of Precambrian metamorphic marble from Llano County, Texas.

Metamorphic marbles occur at the surface in central Texas and in west
Texas. Some of the west Texas occurrences are in the Van Horn area of
Culberson and Hudspeth counties and in the Big Bend area of 76
Brewster County. In central Texas, Precambrian marbles are found in
Burnet, Gillespie, Llano, and Mason counties of the Llano uplift area. Many
of them are suitable for use as monument and building stones. Some of the

Llano County marble is quarried and used as granules for roofs and as
terrazzo chips for making colorful floors (described with serpentine on p.
88).

Martite. See Hematite.

Mica
Mica is not just one mineral but is the name given to a group of similar
minerals. The mica minerals are easy to recognize. Because they have
perfect cleavage in one direction, they split into thin, flat sheets. You can
see through some mica sheets, and they are elastic enough to be bent back
and forth. (Another mineral, selenite gypsum, also will split into thin, flat,
transparent sheets, but selenite sheets break when you bend them.)

Mica minerals have perfect cleavage in one direction, resulting in thin, sheet-like cleavage
fragments.

Basal Cleavage

Two of the mica minerals that you are most likely to find in Texas are
muscovite and biotite. Both these minerals are potassium-aluminum
silicates, and biotite, in addition, contains magnesium and iron. In general,
muscovite is light colored, that is, it has a light brown, yellow, or green tint,
or is colorless, and biotite is dark colored, commonly dark green, brown, or

black. These minerals have glassy or pearly lusters and are rather soft—a
copper penny scratches them. The specific gravity of biotite is 2.8 to 3.2,
and that of muscovite is 2.76 to 3.1.

Mica minerals occur in igneous rocks, such as granite and pegmatite, and in
metamorphic rocks, such as schist and gneiss. They also are found as tiny
flakes in some sandstones, limestones, and other sedimentary rocks. Most
of the Texas mica is found in the Llano uplift area (particularly in Llano
County) and in the Mica Mine area. (The Mica Mine area is in the Van Horn
Mountains about 15 miles south of Van Horn in west Texas.) In both these
areas, the mica minerals occur mostly in Precambrian pegmatites and mica
schists.

The gleaming mica schists were once igneous rocks or sedimentary rocks
such as sandstones and shales. Long ago, great forces beneath the earth’s
surface changed the rocks into mica schists. The mica that is found in
pegmatites formed from hot fluids of igneous origin when the pegmatite
rock itself was formed.

Clusters of mica in the pegmatites are called books, because the thin sheets
into which the mica splits look like pages. Some muscovite books up to 8
inches across are found in the Mica Mine area of the Van Horn Mountains.

The books or sheets of muscovite mica that occur in pegmatites are
especially valuable to industry. Muscovite can stand great heat without
melting, it is tough, it splits into thin sheets, and it lets very little heat and
electricity pass through. Because of these properties, muscovite is used in
fuses and as insulators in heating elements of electric irons and toasters.
(Biotite is not used, because the iron it contains makes it a conductor of
electricity.) Sheet muscovite also is widely used by the electronics industry
as a non-conducting material in the manufacture of tubes and other
products.

Both muscovite and biotite from mica schist rocks, as well as scrap pieces
of sheet mica from pegmatites, are ground into flakes or powder. This
ground-up mica has many uses, ranging from a powder coating for 77
automobile inner-tubes to Christmas tree “snow.”

Only a small amount of mica has been mined in Texas. A fair grade of sheet
mica occurs in the pegmatites at Mica Mine in west Texas, but the deposit is
not large. In the pegmatites of the Llano uplift area of central Texas, no
sheet mica has been found that is considered good enough for the
requirements of industry. Mica suitable for grinding, however, is found in
both these Texas areas.

Micaceous Hematite. See Hematite.

Microcline. See Feldspar.

Milky Quartz. See Quartz.

Muscovite. See Mica.

Native Silver. See Silver Minerals.

Obsidian and Vitrophyre
Obsidian is a dark, glassy-looking igneous rock. Most obsidian contains the
same chemical elements as granite and rhyolite, since all three of these
rocks can form from the same type of molten rock material. Obsidian,
however, has no separate minerals, because its chemical elements are not
combined in an orderly way. It is a natural glass.

Because it is a glass, we know that obsidian forms very quickly. One way
for it to form is from the sudden cooling of hot, molten lava that flows out
of volcanoes. If the lava cools and hardens before the separate minerals can
crystallize, it becomes a natural glass, such as obsidian.

This rock is smooth and shiny. Most of it is black, but some can be dark
green or dark brown. Obsidian allows light to pass through it, and it breaks
with a curved, conchoidal fracture. The broken edges are very sharp.

Another glassy igneous rock that forms from fast-cooling lava is vitrophyre.
It looks like obsidian except that it has crystals or crystalline mineral grains
(which may be light colored) scattered through the dark glassy material.

Obsidian was used by the Indians to make arrowheads.

Obsidian and vitrophyre are found in the Big Bend area of Brewster and
Presidio counties in west Texas. They occur with other igneous rocks that
formed there during Tertiary time.

The Indians who long ago roamed this area used the smooth, shiny
vitrophyre and obsidian to make some of their arrowheads and scrapers.
Today, rock collectors pick up these attractive rocks for their collections,
and some of them cut and polish obsidian and vitrophyre for use as
gemstones.

Onyx. See Quartz.

Öolitic Limestone. See Limestone.

78

Opal
Opal is like hardened jelly or gelatin. It has no crystalline inner structure
and no crystal shape of its own—it is amorphous. This mineral has almost
the same chemical composition as quartz. Both are silicon dioxides (silica),
but opal, in addition, contains water.

Opal can be almost any color—red, yellow, blue, brown, gray, white—or it
can be colorless. It is transparent or translucent and appears glassy,
resinous, greasy, or dull. Opal has a specific gravity of 1.9 to 2.2—this
mineral is a little lighter than quartz. It also is softer than quartz. A copper
penny will not scratch opal, but quartz will. Opal has a white streak and a
curved, conchoidal fracture but no cleavage.

Opal occurs in a number of places in Texas. In the Trans-Pecos country of
west Texas, it fills cracks and cavities in some of the extrusive igneous
rocks. It occurs on the High Plains of northwest Texas, and it is found in
Tertiary formations of the Gulf Coastal Plain where it occurs as masses that
fill cracks and cavities in sedimentary rocks, as the cementing material in
some sandstones (such as in the Catahoula sandstone), and as opalized
wood.

Much opal forms from underground waters that contain silicon. These
solutions move through the rocks and deposit the opal in them.

Opal is found in a number of varieties. Some show a beautiful, lustrous play
of colors that comes from inside the specimens. These varieties are known
as precious opal and are prized as gemstones. In Texas, some precious opal
is found near Alpine in Brewster County. It has a milky white to bluish-
white color, is translucent, and shows a fiery orange, red, blue, and green
play of colors.

The variety known as common opal shows no play of colors. It may be
white, gray, bluish, reddish, greenish, or yellowish, and it is only slightly
translucent. It is found in Brewster, Jeff Davis, Presidio, and other counties
of the Trans-Pecos country of west Texas. It occurs also around some of the
wet-weather (playa) lakes on the Texas High Plains. In the Gulf Coastal
Plain, common opal is found with chalcedony (a variety of quartz) in
Tertiary formations. A south Texas locality sometimes visited by collectors
is near Freer in Duval County.

A clear, commonly rounded, variety of opal that looks like ice is called
hyalite. Two areas in which it has been found are in Presidio County in west
Texas and in Llano County in central Texas.

Opalized wood from Washington County, Texas.

A variety of petrified wood, called opalized wood, is opal that replaced the
fibers of a piece of wood. Wood opal is found at a number of places in the
Gulf Coastal Plain. It occurs there in Tertiary formations within about 20
miles of the boundary line between areas 2 and 3 shown on the geologic
map (pp. 4-5).

A soft opaline material called diatomite, or diatomaceous earth, is made up
chiefly of the skeletons of diatoms—tiny, one-celled plants that live in fresh
or salt water. These little plants are able to take silica from the water to
make opal skeletons for themselves. When the diatom skeletons collect at
the bottom of a lake or sea, they form the light, crumbly, white, gray, or
cream-colored deposit of impure opal known as diatomite. Industry uses
this material as a filter, as insulation, as an abrasive, and as a filler.

Diatomite formed in ancient lakes on the Texas High Plains during 79
late Tertiary (Pliocene) and early Quaternary (Pleistocene) times. It is
found in Armstrong, Crosby, Dickens, Ector, Hartley, and Lamb counties.

Opaline Granite. See Llanite.

Orthoclase. See Feldspar.

Pegmatite
Pegmatites occur in igneous rock areas, and most geologists consider them
intrusive igneous rocks. They are made up of crystals and crystalline
mineral grains that fit together—the grains are interlocked. The crystals and
grains in pegmatites are larger than those of surrounding rocks, and some
are huge, even larger than a man. However, there is a wide range of grain
sizes in pegmatite.

Some pegmatites cut through igneous or metamorphic rocks in such a way
that they resemble walls (called dikes). Others are found as veins, as flat
masses, or as odd-shaped bodies in rocks. Many pegmatites occur in
granites and contain feldspar, quartz, mica, and other minerals, as granite
does. Some pegmatites occur with other kinds of igneous rocks and contain
the same minerals as these rocks. A few pegmatites contain rare and unusual
minerals.

Many geologists believe that pegmatites form from hot fluids of igneous
origin that are left after other igneous rocks, such as granite, have already
formed. These left-over fluids contain large amounts of aluminum,
potassium, silicon, sodium, and several other elements. While the granite or
other rocks are still far underground, this material pushes up into them, and
may even partly dissolve them. Then it slowly cools and hardens into
pegmatite. It is believed that, later, more fluids move into cracks in some
pegmatites. This new material adds other minerals to the pegmatites and
alters some of those minerals already there.

Quartz-feldspar pegmatite from Burnet County, Texas.

Some of the pegmatites we now see at the surface in Texas are probably
about a billion years old. They formed during Precambrian time and occur
with other extremely old rocks. One well-known Texas pegmatite area is the
Mica Mine district of west Texas. It is about 15 miles south of Van Horn in
the Van Horn Mountains of Culberson and Hudspeth counties. Another
pegmatite area is in the Llano uplift of central Texas. These central Texas
pegmatites occur in Burnet, Gillespie, Llano, and Mason counties. 80

Large crystals and grains of feldspar, mica, and quartz are found in the
pegmatites of both these areas. A small amount of mica has been mined
from the west Texas pegmatites, and feldspar has been produced from the
central Texas pegmatites.

An extremely rare and unusual pegmatite occurs in the Llano uplift area at
Baringer Hill, which is west of Burnet in Llano County. This pegmatite was
once on the bank of the Colorado River, but when Buchanan Dam was
built, the area was flooded. The Baringer Hill pegmatite now lies beneath
the water of Lake Buchanan. Many rare minerals, which contain beryllium,
cerium, thorium, uranium, yttrium, zirconium, and a number of other

elements, occur in this pegmatite. Some of these minerals, such as those
containing yttrium and zirconium, glow or incandesce when they are
heated. During the early part of this century, before the area was flooded,
several of the yttrium minerals were mined and used in making lamp
mantles.

Pitchblende. See Uranium Minerals.

Precious Opal. See Opal.

Pulverulent Limestone. See Limestone.

Pumicite. See Volcanic Ash.

Pyrite
Pyrite is a shiny, pale golden-yellow or brassy-yellow metallic mineral. This
mineral, an iron disulfide, is so often mistaken for gold that it is widely
known by the nickname fool’s gold.

Except for their similar color and luster, pyrite and gold are really very
different. When you rub pyrite across a streak plate, it leaves a black, a
greenish-black, or a brownish-black streak, but the streak of gold is gold-
colored. Pyrite is too hard for the average pocket knife to scratch, but a
knife will scratch gold easily. Pyrite is brittle and readily breaks, but gold is
malleable and flattens out when hit with a hammer. Pyrite is only about 5
times as heavy as an equal volume of water, but pure gold is over 19 times
as heavy. And pyrite may have a brown or a multicolored tarnish on it, but
gold never tarnishes.

Pyrite veins in white marble from Llano County, Texas.

Cubic crystals of pyrite.

Pyrite is a common mineral and is found in many of the igneous,
metamorphic, and sedimentary rocks of Texas. It may be scattered through
the rocks, or it may fill cracks and cavities in them. This mineral occurs as
granular and compact masses, as rounded masses, or as crystals. The
crystals are commonly cubes, pyritohedrons, or octahedrons. In some 81
crystals, the shapes are combined (such as a cube with an octahedron
or two pyritohedrons grown through each other). You may notice that the
sides of some cubes and pyritohedrons have fine, parallel grooves (called
striae or striations) on them.

Pyrite originates in a number of different ways. Some of it forms, along
with other minerals in igneous rocks, from hot magmas. It also forms in
metamorphic rocks by the same processes that produce these rocks. Some
of the pyrite in limestone and other sedimentary rocks is formed when the

rocks themselves are deposited by seas or streams. Pyrite also is deposited
by the hot fluids that are given off by magmas. These fluids travel up into
cracks and other openings in rocks and then form pyrite as well as other
minerals. Much pyrite forms in still another way. As water seeps through
rocks, it dissolves some of the iron minerals that they contain. When, under
certain conditions, these iron solutions mix with hydrogen sulfide (this is
the gas that makes some water smell like rotten eggs), pyrite is formed.

Pyrite alters easily. Because of this, most builders carefully check the
limestone, granite, marble, or whatever other building stone they plan to use
to be sure that it does not contain large amounts of pyrite. When exposed to
the weather, pyrite changes to limonite and causes an unsightly rust stain.

Pyrite is used as a source of sulfur, and it is produced for this purpose in
several states. In Texas, however, no pyrite deposits have been found that
are large enough to be mined.

Pyrolusite. See Manganese Minerals.

Quartz
Quartz, silicon dioxide, is one of the most common minerals. It is glassy,
waxy, greasy, or dull and is transparent or translucent. Pure quartz is
colorless, but impurities make some varieties white, black, or a shade of
red, yellow, blue, violet, or brown. Quartz is a hard mineral. It scratches
window glass and cannot be scratched by a pocket knife or even by a steel
file. It has a specific gravity of 2.65. The curved, conchoidal fracture shown
by many specimens helps identify it.

Quartz is plentiful in Texas. It occurs in igneous rocks, such as granite,
llanite, and pegmatite; in metamorphic rocks, such as quartzite, schist, and
gneiss; and in sedimentary rocks, such as some sandstone, conglomerate,
and breccia.

Quartz crystal, with inclusions, from Burnet County, Texas.

Quartz is found as crystals and as masses. Some of the masses are coarsely
crystalline, but some are made up of extremely small crystalline particles
called cryptocrystalline quartz. Some of the cryptocrystalline varieties of
quartz found in Texas are chalcedony, chert, and jasper. Some of the
coarsely crystalline varieties found here are amethyst, milky quartz, 82
rose quartz, smoky quartz, and rock crystal.

Amethyst geode from the Alpine area of Brewster County, Texas.

A colorless, glassy variety of quartz, called rock crystal, is clear enough to
see through. It is found as crystals that are 6-sided prisms with pyramid-like
faces on the ends. This variety is commonly associated with igneous rocks,
such as those of the Llano uplift area of central Texas and of the Trans-
Pecos country of west Texas. It is commonly used as a gemstone and is
made into necklaces, earrings, and other jewelry. Some specimens of rock
crystal have slender, needle-like crystals of other minerals, such as
tourmaline, actinolite, or rutile, enclosed in them.

A clear, glassy variety of quartz, amethyst, has a purple or violet color. It,
like rock crystal, is commonly found in 6-sided prisms with pyramid-
shaped ends and is also prized as a gemstone. Amethyst has been found in
Precambrian rocks in the Llano uplift area of central Texas. (Amethyst Hill,
a locality well known to collectors for many years, is in northeastern
Gillespie County.) In west Texas, amethyst has been found in Cenozoic

igneous rocks in the Sierra Blanca and Quitman Mountains of Hudspeth
County and in the Alpine area of Brewster County.

Milky quartz from Burnet County, Texas.

A variety of quartz with a milk-white color and a glassy to greasy luster is
called milky quartz. It occurs either as crystals or as crystalline masses. Very
little light will pass through it. In central Texas, milky quartz occurs
abundantly in the Precambrian rocks of the Llano uplift area in 83
Blanco, Burnet, Gillespie, Llano, and Mason counties. It also is found
in some of the rocks of the Trans-Pecos country of west Texas, such as in
the Carrizo Mountains of Culberson and Hudspeth counties. Other good
places to look for this variety of quartz are in the sands and gravels along
many streams in Texas.

Some quartz has a glassy to a greasy luster and a rose or pink color. Rose
quartz, as this variety is called, commonly occurs as masses rather than as
individual crystals. It can be found along some of the streams in Texas and
also in igneous rocks, such as those of the Llano uplift area of central Texas.

A kind of quartz with a smoky brown, a smoky yellow, or a dark brownish-
black color is called smoky quartz. Its luster is glassy, and it may be either
translucent or transparent. Smoky quartz is commonly found as crystals that
are shaped like 6-sided prisms with pyramid-like ends. It is commonly
associated with igneous rocks, and beautiful specimens have been found in
the Lake Buchanan area of Llano and Burnet counties in central Texas.

Smoky-quartz crystals from Burnet County, Texas.

A cryptocrystalline variety of quartz, chalcedony, has a waxy to dull luster
and a tan, white, gray, or light-blue color. It is translucent but not
transparent. Chalcedony does not have its own crystal shape but instead is
found in masses that line or fill cracks, pores, and other cavities in rocks. It
is formed when water containing silicon slowly seeps into these openings in
the rocks and deposits the silicon dioxide there as chalcedony.

Chalcedony commonly occurs in some of the Tertiary rocks of the Gulf
Coastal Plain. For example, chalcedony associated with opal is found near
Freer in northern Duval County. In the High Plains of west Texas, it is
found in alkali-lake deposits, such as at Shafter Lake in Andrews County
and at Cedar Lake in Gaines County. In the Trans-Pecos country of west
Texas, it can be found filling small cavities in extrusive igneous rocks.

Polished agate from Rio Grande gravels of Zapata County, Texas.

A variety of chalcedony that generally is made up of more than one color is
called agate (although agates consisting of several shades of a single 84
color are also found). The colors may be spread out unevenly so that

the agate has a cloudy appearance, or they can be arranged in wavy, in
straight, or in concentric lines or bands. If the bands are straight and
parallel, the specimen is called onyx. Agate that has a moss-like or tree-like
design in it is called moss agate. Some agates make attractive gemstones
when cut and polished.

Jasper from Uvalde County, Texas. Dark areas are brownish red; light areas are a yellowish-
tan.

Much agate has been found filling cavities in Cenozoic igneous rocks in
Brewster, Presidio, and other counties in the Trans-Pecos country of west
Texas. It has been found also in an area about 10 to 15 miles wide along the
Rio Grande, mostly in southern Webb County and in Zapata and Starr
counties.

Trees and other plants have been replaced by agate. Many specimens of
agatized wood have been collected from Tertiary formations in Fayette,
Gonzales, Lee, Washington, and other counties of the Gulf Coastal Plain.
(The agatized wood, along with opalized wood, occurs within about 20
miles of the boundary between no. 2 and no. 3 on the geologic map, pp. 4-
5.)

A hard, compact, slightly translucent variety of cryptocrystalline quartz is
called jasper. It commonly has a red, brown, or yellow color due to the
presence of an iron oxide, such as hematite. Some jasper is made up of
irregular bands of more than one of these colors. This variety of quartz
often is polished to make attractive gem or ornamental stones. It has been
collected at several localities in Texas, particularly from creek and river
gravels. Starr and other nearby counties along the Rio Grande have
furnished a number of good specimens.

A hard, smooth, compact, translucent rock that is made up mostly of
cryptocrystalline quartz is called chert or flint. It is white, black, or some
shade of gray, brown, or pink, and its luster is waxy, slightly glassy, or dull.
Chert is found in many creek and river gravels in Texas. It also occurs with
limestone, such as in the Lower Cretaceous Edwards Limestone of central
Texas and in the Ordovician Ellenburger strata in the Llano uplift area.
Chert also is found with the Ordovician rocks of the Marathon area of
Brewster County.

Geologists do not agree on whether chert and flint are two names for one
variety of rock, or whether each is a separate variety. Some, however, now
give chert a geological meaning and flint an archaeological meaning. They
use the word chert to describe geological formations or rock specimens.
They give the name flint to the same rock when it has been used by Indians
in making arrowheads, scribers, scrapers, and spearheads.

Quartzite
Quartzite is either a metamorphic rock or a sedimentary rock. (The
sedimentary kind of quartzite is described with sand and sandstone on p.

86.) Metamorphic quartzite is made up mostly of quartz. It forms when heat
and fluids below the earth’s surface cause the grains and cement of a quartz
sandstone to recrystallize. When this happens, the grains interlock and are
no longer held together by cement. Metamorphic quartzite, like sedimentary
quartzite, is a hard, firm rock that breaks through the quartz grains 85
instead of between them.

Ancient Precambrian metamorphic quartzite occurs at the surface in the
Llano uplift area of central Texas, in the Van Horn area of west Texas, and
in the Franklin Mountains north of El Paso in extreme west Texas.

Rhyolite
Rhyolite is a fine-grained or glassy igneous rock that commonly is extrusive
or volcanic. It has a pink, red, tan, white, gray, purple, or black color. This
rock, like granite, is made up chiefly of feldspar and a silica mineral, such
as quartz, but other minerals may be present. Both rhyolite and granite form
from the same kind of molten rock material. Nevertheless, even though
their compositions are the same, these two rocks do not look alike. Their
textures differ because granite forms slowly and rhyolite forms quickly.

Much of the Texas rhyolite formed from hot, molten lava. This lava flowed
out onto the surface either through volcanic cones or cracks in the ground.
Some of the lava cooled and hardened too quickly for mineral grains to
develop. This rapidly cooled lava formed a rhyolite rock that is made up, at
least partly, of glass. In many of the rhyolites, crystalline mineral grains
were able to form, but these grains are extremely small, and you may not be
able to distinguish them even with a magnifying glass. Some rhyolite,
because it hardened from moving, flowing lava, has streaks and bands of
different colors and textures. This rhyolite has flow structure.

One variety of rhyolite has easily seen crystals and grains of minerals, such
as feldspar, quartz, and mica, scattered through a mass of the tiny
crystalline grains (in much the same way that raisins are scattered through a
cake). The easily seen crystals and grains are called phenocrysts, and the
rock itself is called a rhyolite porphyry.

Many rhyolites and rhyolite porphyries occur in the Tertiary igneous rocks
of the Trans-Pecos country of west Texas. Just a few of these localities
include the Barrilla Mountains of Jeff Davis and Reeves counties, the
Chisos Mountains of Brewster County, the Chinati Mountains of Presidio
County, and the Davis Mountains of Jeff Davis County.

Rock Crystal. See Quartz.

Rock Gypsum. See Gypsum.

Rock Salt. See Halite.

Rose Quartz. See Quartz.

Salt. See Halite.

Sand and Sandstone
Sand is a loose, uncemented sedimentary deposit made up of fragments of
weathered rocks and minerals. These fragments must be of a certain size
(between ¹/₁₆ millimeter and 2 millimeters in diameter) in order to be called
sand grains. The largest sand grains are about the size of a pinhead. Sand
grains are smaller than the fragments known as granules; they are larger
than those known as silt.

Many sands are made up chiefly of grains of quartz. This mineral is
plentiful and does not easily weather away. In addition, rock fragments and
many other minerals, such as feldspar, mica, gypsum, magnetite, and
garnet, are found as sand grains.

Rains wash many of the sand grains and other weathered rock and mineral
fragments into creeks and rivers. These streams may carry the sand and
other sediments long distances before depositing them. Today, we find

sands along the banks of many creeks and rivers in Texas and along the
beaches of the Gulf of Mexico. The sand in the rivers is in transit to the
Gulf. In addition, sand occurs at the surface in other Cenozoic formations
and in some of the Paleozoic and Mesozoic formations of Texas.

Sand has many uses. Much building sand, which is used in mortar and
concrete, is produced from numerous sand and gravel pits in Texas. Pure
quartz sand that can be used to make glass is known as glass sand. 86
Some of it is found in north-central Texas in Lower Cretaceous
formations. A large glass sand quarry is located at Santa Anna in Coleman
County. Along the Gulf Coastal Plain, sand that is used in glassmaking
occurs in Eocene Tertiary strata.

A coarse-grained sand, blast sand, is used with compressed air to clean the
walls of brick and stone buildings and to carve designs on monument
stones, such as marbles. Some coarse sand is also used as a filtering sand in
purifying water. These types of sand have been produced from the Gulf
Coastal Plain as well as from other areas of Texas.

Sand grains, when nature cements them together, make up the sedimentary
rock sandstone. Some sandstones form when underground water carrying
dissolved mineral matter moves through loose sand. As the dissolved
mineral matter comes out of solution, it forms a cement that binds the sand
grains together.

The cement may be material such as calcite (calcium carbonate), quartz,
chalcedony, or opal, which are silica minerals, and limonite and hematite,
which are iron oxides. Clay also may serve as a cement. It is either
deposited along with the sand or is formed from weathered feldspar sand
grains.

The color of the cementing material helps determine the color of the rock.
Iron oxide cement, for example, causes the sandstone to have a reddish,
yellowish, or brownish color. Sandstones also are white, black, gray, green,
or cream colored.

Ordinarily, sandstones break through the cementing material, not through
the sand grains. Thus, the broken surface of the rock feels rough and gritty.

Some quartz sand grains, however, are tightly cemented with silica to form
an extremely hard and compact rock. If this rock breaks smoothly through
the grains instead of between them, it is known as quartzite. Some of this
sedimentary quartzite occurs in the Texas Gulf Coastal Plain in the Tertiary
Catahoula strata. (Another kind of quartzite is described on pp. 84-85.)

Sandstone from the Eocene Wilcox Group of strata of northwestern Zavala County, Texas.

Ordinary sandstones are seen at the surface in many localities in Texas, and
a number of them have been used as building stones. Some of the places
where sandstones occur are in the Cambrian and Pennsylvanian formations
of the Llano uplift area of central Texas and in the Pennsylvanian, Permian,
and Lower Cretaceous formations of north-central Texas. Tertiary
sandstones occur in the Texas Gulf Coastal Plain, and Triassic 87
sandstones are found along the edges of the Texas High Plains.
Sandstone is also found in many formations of the Trans-Pecos country of
west Texas.

Sandstone. See Sand and Sandstone.

Satin Spar. See Gypsum.

Schist

Schist is a metamorphic rock that splits easily along thin, generally parallel
layers, called folia. These layers may be either straight or curved, and they
are made up of crystalline grains of one or more than one mineral. This
structure is called schistosity or foliation. When you examine schist, you
will see that many of the mineral grains are flat or long, and that they are
lined up in one direction to form the layers. Some schists have fairly large
crystals (many with perfect shapes) scattered through them. For example,
mica schists may contain beautiful crystals of garnet.

Each kind of schist is named for an outstanding mineral that it contains.
Mica schist contains a large amount of mica. We also find hornblende
schist, actinolite schist, chlorite schist, talc schist, and graphite schist.
(Graphite schist is discussed with graphite on p. 63.)

Schists form from other rocks, such as granite, gabbro, or shale. The rocks
are changed into schists by fluids and by heat and pressure below the earth’s
surface.

Extremely ancient schists that formed during Precambrian time are exposed
at the surface in the Allamoore—Van Horn area of west Texas and in the
Llano uplift area of central Texas. Geologists believe that the Packsaddle
Schist of the Llano uplift area was once shale. Good exposures of this schist
are seen in the Honey Creek area near Packsaddle Mountain in Llano
County.

Schorl. See Tourmaline.

Sedimentary Quartzite. See Sand and Sandstone.

Selenite. See Gypsum.

Serpentine

Serpentine is the name given both to a rock and to a mineral. The mineral
serpentine (a hydrous magnesium silicate) is found in two different forms.
If it is fibrous, it is called chrysotile; if it is layered and platy, it is known as
antigorite. Antigorite is brownish green and smooth and waxy looking.
Some of it can be split into thin sheets. Chrysotile is made up of greenish,
silky fibers, which may be brittle and break apart in large pieces. If,
however, the fibers can be pulled apart into soft flexible, little threads, the
mineral is called chrysotile asbestos.

Light will pass through both these varieties of serpentine, and both are soft
enough to be scratched by a pocket knife. When rubbed across a streak
plate, they leave white streaks. Antigorite and chrysotile have no crystal
shapes of their own, but several other minerals can alter to form these two
varieties of serpentine. Thus antigorite and chrysotile may be found as
pseudomorphs in a crystal shape that originally belonged to another
mineral.

Antigorite and chrysotile are commonly found closely mixed with dolomite,
talc, magnetite, calcite, pyrite, and several other minerals. These minerals
make up serpentine rock (also called serpentinite). This rock ordinarily is
some shade of green (such as whitish, yellowish, brownish, bluish, or dark
blackish green), and it may be mottled. It is brittle or tough and generally is
massive. Serpentine rock, like the serpentine minerals, is fairly soft—you
can scratch it with a pocket knife.

In the Llano uplift area of central Texas, serpentine rock is found among
Precambrian metamorphic rocks, such as gneiss and schist. An especially
large deposit in this area is known as the Coal Creek serpentine mass. It is
over 3½ miles long, and at one place, it is almost 1½ miles wide. This mass
of serpentine extends across the Blanco-Gillespie County line in the
extreme northern parts of these two counties. (A little fibrous 88
chrysotile is found here, but it will not break into flexible enough
threads to be called chrysotile asbestos.) Several other deposits of
serpentine occur in northeastern Gillespie County and in southern Llano
County.

It is believed that the Coal Creek serpentine was formed from an igneous
rock such as peridotite, which is made up chiefly of grains of the mineral

olivine. The peridotite may have been altered into serpentine by
underground waters that seeped through it. It is possible, however, that
other serpentines in the area were formed when rocks were altered by hot
fluids and great pressures far below the earth’s surface.

The Llano area serpentine has been widely used in terrazzo floors. To make
these floors, small pieces of serpentine and other colored rocks are put into
cement that is spread over a concrete slab. Then, after the cement has
hardened, it is ground to a flat, smooth surface and polished. The resulting
terrazzo floor is both colorful and durable.

Serpentine rock also is cut into slabs, polished, and used as indoor building
stones. Verde antique, a variety often seen in the lobbies of office buildings,
consists of green serpentine rock with streaks of white calcite or dolomite in
it.

In the Balcones fault zone area (shown on the Texas physiographic outline
map, p. 42) from Uvalde County to Williamson County, serpentine occurs
with Upper Cretaceous rocks. The serpentine rock is seen at the surface in a
few places (such as in Travis and Uvalde counties), but much of it is
underground. In several oil fields of this area (as at Thrall field in
Williamson County and at Lytton Springs field in Caldwell County), the
serpentine rocks contain oil.

Serpentinite. See Serpentine.

Shale
Shale is a sedimentary rock made up of tightly packed clay and mud
particles. It has a smooth appearance because it is so fine grained. In fact,
most of the particles in it are too small to be distinguished with a
magnifying glass. These particles are the weathered remains of earlier
rocks. They were carried by creeks and rivers to other parts of the land or to
the sea, where they formed layers of clay and mud. Later, other sediments
were deposited on top of them. The weight of these new sediments
squeezed the clays and muds together to form firm, compact shale.

Shale looks very much like some clays. It, like clay, can be almost any
color. If the shale contains animal or plant matter, it is black, gray, or blue.
If it contains iron oxide (many minerals containing iron alter to this
material), it is a shade of red, yellow, or brown. Shale is soft and can be
easily scratched by a knife. It also is brittle and crumbles easily. This rock
has a property that will help you to distinguish it from clay: the particles
that make up the shale were deposited in layers, and the shale splits into
flat, thin flakes along these layers, which clay will not do.

Shale is fairly abundant in Texas, especially in Mississippian,
Pennsylvanian, and Cretaceous formations. For example, Pennsylvanian
shales are found at the surface in north-central Texas, in the area around the
Llano uplift of central Texas, and in the Marathon and Solitario uplifts of
west Texas.

Many of shale’s uses are the same as those of clay. Some of it can be used
to make brick, tile, and other products, and some is often used instead of
clay in making portland cement. Cement plants at Dallas, El Paso, Fort
Worth, and Waco are located at places where Cretaceous limestones, which
also are used in cement making, and Cretaceous shales are found near each
other at the surface.

Oil shale, from which petroleum can be obtained by heating, has been
found in central Texas. It occurs in Mississippian formations in Lampasas,
McCulloch, and San Saba counties. Because oil is much less expensive to
obtain from wells, it is not produced from these shales.

89

Silver Minerals (Argentite, Cerargyrite, Native Silver)
Silver has many uses. Like gold, it is a beautiful metal that long has been
used for coins and ornaments. A large amount of silver goes to make articles
such as spoons, forks, platters, and trays. The photographic industry uses
silver—much of the film for cameras is coated with a silver halide. Doctors
and dentists use silver, too. The mixture that a dentist uses to fill teeth
contains silver along with several other metals. Doctors sometimes use

silver wire to fasten broken bones, and silver compounds and solutions,
such as silver nitrate, are used in some kinds of medical treatment.

Perhaps more people have heard of legendary, lost silver mines of Texas
than of the actual and important silver deposits found in the Trans-Pecos
country of west Texas. Some of the west Texas silver minerals include
argentite, cerargyrite, and native silver. Although the argentite and native
silver commonly found there are mixed with galena, a lead mineral, or with
chalcocite, a copper mineral, they also occur separately.

The element silver is found alone as native silver. When pure, it is rather
easy to recognize. It is metallic and has a silver-white color that may tarnish
to gray, black, or yellowish brown. Native silver is heavy (it has a specific
gravity of 10.5) and soft (a pocket knife scratches it easily). When you rub
it across a streak plate, native silver, unless it is tarnished, leaves a shiny,
silver-white streak. This metal is so ductile that it can be drawn into a wire.
It is also malleable and flattens when hit with a hammer.

Silver occurs as crystals, which are poorly shaped cubes and octahedrons,
or as irregular masses. It may have a net-like appearance (called reticulate),
or it may be shaped like little needles (described then as acicular). It occurs
in wires (then called filiform) or as scales or plates.

Prospector.

Two of the Texas silver minerals, argentite and cerargyrite, do not resemble
silver at all. Argentite, a silver sulfide, is also called silver glance. It is 90
a dark, lead-gray mineral with a metallic luster that weathers to a dull
black. When you rub it across a streak plate, argentite gives a shiny,
blackish to lead-gray streak. This mineral is soft enough to leave a mark on
paper. It has a specific gravity of 7.3, and it is sectile enough to be cut
smoothly (like soap) with a knife. In some places argentite is found as
irregular masses or as a coating on rocks and other minerals.

Another silver mineral, cerargyrite (or horn silver) is a silver chloride. This
mineral has a nonmetallic luster and is transparent to translucent. It
resembles pearl-gray, white, greenish, or colorless wax. When exposed to
the light it turns violet brown or black. Cerargyrite is soft—you can scratch

it with a fingernail. Like argentite, it is sectile. This mineral has a specific
gravity of 5.5, and it commonly occurs as irregular masses and as crusts.

These silver minerals have been mined at a number of places in Trans-
Pecos Texas. The largest silver mine in Texas, the Presidio mine, is located
near Shafter in south-central Presidio County. It contains argentite,
cerargyrite, and native silver, along with galena and several other minerals.
This mine is not open now, but in the years between 1885 and 1942, it
produced a large amount of silver along with some lead and gold. There are
several other lead-silver mines in this Shafter area, but none has produced
as much as the Presidio mine.

In this mine, the silver minerals occur mostly in large, flat deposits in
Permian limestone and other sedimentary rocks. The minerals are believed
to have been deposited there—probably during Tertiary time—by solutions
that came from hot magma far below the rocks. As they moved in along the
layers of limestone, the solutions replaced portions of this rock with
minerals containing silver, lead, and other elements. Later, water seeped
into these deposits and dissolved some of the minerals. This dissolved
material was then re-deposited, and it formed most of the minerals we now
find there.

No silver is being mined in Texas at present, but it has, in the past, been
produced from other Trans-Pecos mines. Galena that contains silver (called
argentiferous galena) has been mined at the Bird mine at Altuda Mountain
(about 14 miles east of Alpine) in northern Brewster County. It also has
been obtained from mines in the Quitman Mountains and in the Eagle
Mountains of Hudspeth County. Some cerargyrite has been mined at the
Plata Verde mine near the Culberson-Hudspeth County line.

Several mines in the Van Horn area of Culberson and Hudspeth counties
have produced silver along with copper. An important silver mine in this
area is the now idle and flooded Hazel mine. (This mine is described with
copper minerals on p. 52.)

Smoky Quartz. See Quartz.

Soapstone. See Talc and Soapstone.

Specular Hematite. See Hematite.

Sulfur
Sulfur is one of Texas’ most valuable minerals. It consists of only a single
element, sulfur. This mineral has a resinous luster and is transparent to
translucent. Sulfur ordinarily is yellow, but impurities cause it to look
greenish, brownish, reddish, or grayish. When you rub it across a streak
plate, it leaves a white or a pale-yellow streak. Sulfur has a specific gravity
of 2.04 to 2.09 and is soft enough to be scratched by a copper penny. It
breaks with a conchoidal to uneven fracture. When it gets hot enough (478°
Fahrenheit), sulfur will burn. For this reason, it often is called brimstone.

Sulfur does not conduct electricity and is a poor conductor of heat. You can
test how poorly heat passes through it by holding a fragment of sulfur up to
your ear. You may be able to hear a crackling sound. The sound results
when the outer part of the fragment expands (due to the heat from your
hand) while the inner part (which has received no heat) remains 91
unchanged.

Crystals of sulfur are sometimes found, and most of them have either a
double-pyramid shape or a flat, tabular shape. Sulfur also occurs as compact
masses, as crusts, and as scattered grains.

Native sulfur deposits are found in two widely separated areas of Texas—
one in west Texas and the other along the Gulf Coast in southeast Texas,
extending over into Louisiana. In the Gulf Coast area, native sulfur is found
on some of the salt domes.

The salt domes are huge (from about half a mile to more than 2 miles
across), column-shaped masses made up of halite and some anhydrite.
These masses have pushed up toward the surface through thousands of feet
of sand, clay, and other sedimentary rocks. On top of many of the salt

columns is a covering of limestone (calcite), anhydrite, and gypsum known
as the cap-rock. It is in this cap-rock that the sulfur is found.

It is thought that when the masses of halite and anhydrite pushed toward the
earth’s surface, some of the upper part of the halite dissolved. The
anhydrite, however, did not dissolve, and it remained on top of the salt
column. Then, a part of this anhydrite was altered into the gypsum,
limestone, and sulfur that now are found in some of the cap-rocks.
Laboratory experiments have shown that the sulfur in the cap-rocks likely
formed through the action of sulfate-reducing bacteria. These bacteria, in
the presence of petroleum, converted the sulfate in some of the anhydrite
into hydrogen sulfide. Later, hydrogen sulfide was oxidized—perhaps by
reaction with more of the anhydrite—to form the sulfur.

Most of the large cap-rock sulfur deposits are about 1,500 to 2,400 feet
underground. At first, an attempt was made to get this sulfur out of the
ground by digging shafts down to it, but loose, wet, caving sands and
poisonous gases, such as hydrogen sulfide, made this mining method almost
impossible. Finally, a chemist, Herman Frasch, found a way to obtain the
sulfur by making use of sulfur’s low melting point. When sulfur gets
slightly hotter than boiling water (235° to 247° Fahrenheit), it melts and
becomes a dark, yellowish-brown liquid.

In the Frasch method of sulfur mining, a well is drilled into the salt-dome
cap-rock, and three pipes, one inside the other, are put into the well.
Superheated water under pressure (hotter than 212° Fahrenheit, the
temperature at which water ordinarily turns into steam) is sent down one of
the pipes to melt the sulfur in the cap-rock around the bottom of the well.
Then, compressed air is sent down another of the pipes. This air presses
against the liquid sulfur and forces it up to the surface through the third
pipe. At the surface, the sulfur is poured into bins, where it cools and
becomes a solid again, or it is transported molten, in pipelines and tankers.

Sulfur has been obtained from a number of the Texas Gulf Coast salt domes
including Bryan Mound, Clemens dome, Damon Mound, and Hoskins
Mound in Brazoria County; Palangana dome in Duval County; Long Point
dome, Nash dome, and Orchard dome in Fort Bend County; High Island
dome in Galveston County; Fannett dome and Spindletop dome in Jefferson

County; Moss Bluff dome in Liberty County; Gulf dome in Matagorda
County; and Boling dome in Wharton County.

In west Texas, sulfur occurs in Permian rocks both at the surface and
underground. A small amount of sulfur has been mined in the Rustler
Springs area of northeastern Culberson County and northwestern Reeves
County, about 50 miles northwest of Pecos. There, scattered grains, crystals,
and irregular masses of sulfur occur in cracks and in dissolved-out openings
in the Castile Gypsum and in the surface gravel, gypsum, sand, and clay
that cover most of this formation.

Sulfur has many uses. It is used as an insect-killer, thus helping our food
crops to grow. It is used in pulp and paper manufacturing and in the
vulcanizing of rubber. Some other uses are in the making of paints, dyes,
and explosives. A large amount of sulfur goes to make sulfuric acid, which
itself has numerous uses in the chemical, steel, oil refining, and other
industries.

92

Sulfur is obtained from the cap-rock of Gulf Coastal Plain salt domes by the Frasch process.

Sulfur
Uncemented Sediments
Limestone
Sulfur-Bearing Limestone
Hot Water
Melted Sulfur
Anhydrite

93

Talc and Soapstone
Talc, a hydrous magnesium silicate, is an extremely soft mineral—your
fingernail scratches it easily. It has a greasy or a pearly luster, and its color
is white, light green, or gray. When rubbed across a streak plate, it leaves a
white streak.

Talc cleaves perfectly in one direction, and the cleavage fragments are thin,
flat, and sheet-like. Its fracture is uneven. This mineral has a soaplike or
greasy feel, and it is sectile—a knife will cut through it. Talc is not
particularly heavy—it has a specific gravity of 2.7 to 2.8. This mineral
seldom occurs with a crystal shape. More commonly it is massive and is
granular or layered.

Talc is not always found as a single, pure mineral. In nature, it commonly
occurs mixed with one or more other minerals, such as tremolite,
anthophyllite, chlorite, and magnetite. This combination of talc with other
minerals forms a soft, greasy or soapy-feeling metamorphic rock called
soapstone. The talc in this rock may be difficult to identify without special
laboratory tests.

In Texas, talc and soapstone are found in Precambrian metamorphic rocks.
In west Texas, talc occurs in an area about 20 miles long (just north of U. S.
Highway 80 in the vicinity of Allamoore, Eagle Flat siding, and Talc Rock
siding) in Hudspeth County. Some of this talc is mined from open pits and

used by the ceramic industry to make wall tile. Some of it is finely ground,
mixed with insect poison, and used as insect powders and dusts.

Talc schist from the Allamoore area of Hudspeth County, Texas.

Deposits of soapstone, containing talc, occur in the Llano uplift area of
central Texas with schist, gneiss, and serpentine rocks in northeastern 94
Gillespie, northwestern Blanco, and southern Llano counties. Smaller
deposits occur in northeastern Mason County and in northwestern and
southeastern Llano County.

The Llano uplift area soapstones are light green to light buff. It is thought
that some of them were once igneous rocks that contained magnesium
minerals. Fluids, along with great heat and pressures below the earth’s
surface, changed these igneous rocks into soapstone.

Some of this Llano uplift area soapstone is mined from open pits near
Willow City in Gillespie County. It is used mostly in making insect powders

and roofing granules. In addition, some of the central Texas soapstones
have been used for hearths and for fireplace linings.

Topaz
Topaz, an aluminum fluorosilicate, is a mineral especially prized by
collectors because many specimens are gemstones. Topaz is transparent, has
a glassy luster, and is quite hard (neither quartz nor a steel file will scratch
it). The topaz that has been found in Texas is either colorless, pale blue, or
sky blue. This mineral is fairly heavy—its specific gravity is 3.4 to 3.6. It
cleaves perfectly in one direction (called basal cleavage), and some of the
cleavage fragments have a flat, slabby appearance.

Topaz is commonly found as prism-shaped crystals, as cleavage fragments,
and as irregular grains. Some fragments of topaz look like quartz. Topaz,
however, is harder and heavier than quartz, and it has perfect basal
cleavage, which quartz does not have.

In Texas, crystals, grains, and cleavage fragments of topaz occur in the
Llano uplift area of central Texas. They are found near Streeter and Grit in
west-central Mason County and near Katemcy in northern Mason County.
Here, some of the topaz occurs in Precambrian pegmatite veins that cut
through granite rocks. Most of the topaz, however, is found as pebbles in
the gravels of nearby creeks, where it has washed after weathering out of
the rocks.

Topaz crystal from near Streeter, Mason County, Texas.

Topaz probably originates when hot fluids move up out of molten magma
into cracks and cavities in the surrounding rocks. There, the fluids react
with elements in the rocks to form the topaz.

Topaz is a good gemstone because, in addition to its beauty, it is hard and is
not easily marred by scratches. The Mason County topaz makes excellent
gemstones. Most of it is beautiful and clear and is either colorless or of a
pleasing blue color. These stones are cut, polished, and mounted in rings

and other jewelry. A number of specimens of this Mason County topaz are
displayed in museums.

Tourmaline
Tourmaline is a complex silicate of boron and aluminum. Other elements,
such as magnesium, sodium, lithium, calcium, iron, or fluorine, also 95
may be present. This mineral has a glassy to resinous luster. Only the
dark-colored varieties of tourmaline have been found in Texas. One is a
black variety called schorl, and another is a brown variety called dravite.
Other kinds of tourmaline, although not found in Texas, are colorless or
some shade of blue, yellow, red, pink, or green. Some crystals even show
more than one color.

Tourmaline is too hard to scratch with a steel file, it has a specific gravity of
3 to 3.25, and it has a conchoidal to uneven fracture. Very little light passes
through the dark varieties, and some fragments of schorl look like shiny,
black coal.

Tourmaline occurs as masses without crystal shapes, but crystals are
commonly found. The crystals are prism-shaped and have small vertical
grooves, called striations, on the prism faces. When you look at some
crystals from an end, you will see that the cross section is a triangle with the
sides bowed outward.

Black tourmaline crystals with milky quartz from north of Llano, Llano County, Texas.

Both the black and the brown varieties of tourmaline have been found at
several places in the Llano uplift of central Texas. One well-known locality
is at Town Mountain north of Llano in Llano County. Here, the tourmaline
occurs in milky quartz that is associated with Precambrian granite rocks. In
west Texas, in Culberson and Hudspeth counties, black tourmaline occurs in
pegmatite rocks in the Van Horn Mountains, the Carrizo Mountains, and the
Wylie Mountains. In the Eagle Mountains of Hudspeth County, it is found
in metamorphic rocks as well as in pegmatites.

Some tourmaline formed from hot fluids containing boron that were given
off by magmas far below the earth’s surface. These fluids traveled up
through cracks and other openings in overlying rocks. As the fluids reacted
with other elements and compounds, the tourmaline formed.

The clear, light-colored varieties of tourmaline are much admired, and they
are more widely used as gemstones than are the dark-colored varieties.
Some collectors, however, find that the dark-colored Texas tourmalines,
when cut and polished, make shiny, attractive gemstones.

Some tourmaline is used as grinding material, but no Texas tourmaline is
produced for this purpose.

Travertine. See Calcite.

Uranium Minerals (Carnotite, Uranophane, Pitchblende)
In 1945, the world suddenly became aware of the awesome power of atomic
energy when the element uranium was used to produce some of the first
atomic bombs. Uranium does not occur alone in nature but is found
combined with other elements in a number of minerals.

All of the uranium minerals are radioactive. The uranium they contain is
gradually breaking down and changing into a series of 13 other elements,
called daughter elements. Each daughter element breaks down and changes
into the next daughter element of the series. While breaking down, these
elements give off particles and rays of energy.

This energy or radioactivity is made up of what are called alpha particles,
beta particles, and gamma rays. You cannot see, hear, taste, smell, or feel
them. The alpha and beta particles are weak and do not travel far. The 96
gamma rays, however, can travel farther and can pass through
seemingly solid material. Scientists have found that these rays can move
through about 1 foot of rock, 2½ feet of water, and several hundred feet of
air.

Prospectors searching for uranium minerals carry instruments that are able
to detect this radioactivity. The uranium itself gives off only alpha particles,
but some of its daughter elements give off gamma rays. These daughter
elements are normally found with the uranium, and it is their strong gamma
rays that the instruments are most apt to detect.

A Geiger counter is used to detect radioactivity.

One of the instruments used is the Geiger counter. It indicates radioactivity
by means of a meter, a flashing light, or a clicking sound, which can be

heard through earphones. Another instrument for detecting radioactivity is
the scintillation counter. It is more sensitive than the Geiger counter and it
can detect radioactivity from a greater distance. The scintillation counter
can be used from an automobile or an airplane, but the Geiger counter must
be quite close to the source of radioactivity to be of use. 97

Various uranium minerals have been found, mostly in small amounts, in a
number of places in Texas. Some of these minerals, such as uraninite or
pitchblende, are heavy and dark colored. Others, including carnotite,
tyuyamunite, autunite, and uranophane, are a shade of yellow or green.
They are quite soft. Deposits of the light-colored uranium minerals have
been mined from two areas of Texas. One of these areas is in Garza County
on the Texas High Plains, and the other is in Karnes and Live Oak counties
in the Gulf Coastal Plain.

One of the light-colored uranium minerals, carnotite, is a potassium-
uranium vanadate, which has a bright canary-yellow or lemon-yellow color.
This mineral is transparent to translucent and has an earthy or a pearly
luster. Carnotite usually is found as crusts and as powdery masses. It is
quite soft and can be scratched with a fingernail.

Carnotite, along with tyuyamunite, autunite, and several other soft,
yellowish or greenish uranium minerals, is found in the Texas Gulf Coastal
Plain. These minerals occur in the Jackson, Catahoula, and Oakville strata
(which are Tertiary in age) in an area extending from Gonzales County to
the Rio Grande (in parts of the area indicated by no. 2 and no. 3 on the
geologic map, pp. 4-5). The largest deposits in this district have been found
in the Karnes County area.

The Gulf Coastal Plain uranium minerals occur mostly with sandstones and
clays in a sequence of strata that contains volcanic ash. It is believed that
small scattered amounts of uranium compounds that were present in the
volcanic ash sediments were dissolved by seeping underground water.
These waters then moved into the sandstones and clays where they
deposited the uranium as carnotite and as other uranium minerals.

Another uranium mineral, uranophane (calcium-uranium silicate), also
occurs in Texas. Uranophane has a yellow to yellow-orange color and a

pearly to greasy luster. When rubbed across a streak plate, it leaves a light
yellow to a light yellow-orange streak. It is soft enough to be scratched by a
copper penny. Uranophane has been found in extrusive igneous rocks in
northwestern Presidio County in west Texas.

A dark-colored uranium mineral, pitchblende, is a variety of the mineral
uraninite, uranium dioxide. Pitchblende does not occur with a crystal shape
but rather as rounded and irregular-shaped masses. It is brownish black,
greenish black, or black. If you rub it across a streak plate, pitchblende
leaves a brownish-black streak. This mineral is heavy (it has a specific
gravity of 6.5 to 8.5) and hard (a pocket knife will not scratch it, although a
steel file will). Pitchblende has a submetallic luster and looks dull, greasy,
or like pitch or tar.

Small amounts of pitchblende have been found at several places in Texas.
One of these localities is a few miles west of Burnet in Burnet County in
central Texas. Here, the pitchblende occurs in Precambrian igneous rocks
that are associated with gneiss. In south Texas, some fine, scattered particles
of pitchblende have been found about 325 feet below the surface in Tertiary
(Pliocene) sediments that cover the Palangana salt dome in Duval County.
No pitchblende is mined in Texas.

Uranophane. See Uranium Minerals.

Vitrophyre. See Obsidian and Vitrophyre.

Volcanic Ash (Pumicite)
Volcanic ash deposits, which also are known as pumicite, are loose and
powdery. They are made up mostly of material that is thrown into the air
when volcanoes erupt. If a volcano erupts with a violent explosion, the
nearby rocks are blown into powder. Molten lava also is hurled into the air,
where some of it immediately cools to become tiny bubbles and particles of
glass. The winds may carry some of this fine material far away before
depositing it.

Deposits of volcanic ash are white, bluish, greenish, yellowish, or 98
grayish, and some of them glisten like snow in the sunlight. They feel
rough and gritty. When examined under a microscope, this material shows
the tiny curved and sharp-cornered particles of the broken volcanic glass.
Deposits of volcanic ash may also contain clay, silt, sand, or other
impurities.

Volcanoes, which may have been located in the Davis Mountains and in
other areas of west Texas and in northern Mexico, erupted during Tertiary
time. The volcanic ash that we find at the surface today in some of the
Tertiary formations in Texas could have come from these volcanoes.
Tertiary volcanic ash deposits occur in the Texas Gulf Coastal Plain (such
as in Brazos, Fayette, Karnes, Polk, Starr, Trinity, and other counties) and in
the Trans-Pecos country of west Texas.

Volcanic ash deposits of Quaternary (Pleistocene) age, which are less than a
million years old, are found in a number of counties on the Texas High
Plains. Farther to the east, ash deposits occur in Baylor, Dickens, Kent, and
Wilbarger counties. This volcanic ash may have come from a volcano that
erupted in northern New Mexico during Quaternary time.

Volcanic ash or pumicite has several commercial uses. Some is used to
make pozzolan cement, and some is used in sweeping compounds,
cleansing and scouring powders, and abrasive soaps. Pumicite has been
mined in Dickens, Scurry, Starr, and several other counties of Texas.

Wad. See Manganese Minerals.

Wood Opal. See Opal.

99

COMPOSITION, HARDNESS, AND SPECIFIC
GRAVITY OF SOME TEXAS MINERALS

For convenient reference, the Texas minerals described in this book are
listed below, together with their chemical compositions, specific gravities,
and hardness. You will be able to find similar information about additional
minerals in mineralogy textbooks such as those noted on page 24.

Mineral Composition Specific Hardness
Gravity
Albite NaAlSi₃O₈ 2.62 6
Almandite Fe₃Al₂ (SiO₄)₂ 4.2 7
Amphibole Ca₂Mg₅Si₈O₂₂(OH)₂ 3.0-3.3 1-2½
asbestos
Anhydrite CaSO₄ 2.9 3-3½
Argentite Ag₂S 7.3 2-2½
Azurite Cu₃(CO₃)₂(OH)₂ 3.77 3½-4
Barite BaSO₄ 4.5 3-3½
Biotite K(Mg, Fe)₃AlSi₃O₁₀(OH)₂ 2.8-3.2 2½-3
Braunite 3MnMnO₃MnSiO₃ 4.75-4.82 6-6½
Calcite CaCO₃ 2.72 3
Carnotite K₂O·2UO₃·V₂O₅·nH₂O 5.03 2
Cassiterite SnO₂ 6.8-7.1 6-7
Celestite SrSO₄ 3.95-3.97 3-3½
Cerargyrite AgCl 5.5 1-1½

Chalcocite Cu₂S 5.5-5.8 2½-3
Chalcopyrite CuFeS₂ 4.1-4.3 3½-4
Cinnabar HgS 8.10 2½
Dolomite CaMg(CO₃)₂ 2.85 3½-4
Feldspar (see Albite, Microcline, Orthoclase)
Fluorite CaF₂ 3.18 4
Galena PbS 7.4-7.6 2½
Garnet (see Almandite, Grossularite)
Gold Au 15.0-19.3 2½-3
Graphite C 2.2 1-2
Grossularite Ca₃Al₂(SiO₄)₃ 3.53 6½
Gypsum CaSO₄·2H₂O 2.32 2
Halite NaCl 2.16 2½
Hematite Fe₂O₃ 5.26 1-6½
Hollandite MnBaMn₁₆O₁₄ 4.7-5 4-6
Limonite FeO(OH)·nH₂O 3.6-4.0 1-5½
Magnetite Fe₃O₄ 5.18 6
Malachite Cu₂CO₃(OH)₂ 3.9-4.03 3½-4
Mica (see Muscovite, Biotite)
Microcline KAlSi₃O₈ 2.54-2.57 6
Muscovite KAl₃Si₃O₁₀(OH)₂ 2.76-3.1 2-2½
Opal SiO₂·nH₂O 1.9-2.2 5-6
Orthoclase KAlSi₃O₈ 2.57 6
Pitchblende UO₂ 6.5-8.5 5½
Pyrite FeS₂ 5.02 6-6½
Pyrolusite MnO₂ 4.75 1-2
Quartz SiO₂ 2.65 7
Serpentine Mg₃Si₂O₅(OH)₄ 2.48 3-4
Silver Ag 10.5 2½-3
Sulfur S 2.05-2.09 1½-2½
Talc Mg₃Si₄O₁₀(OH)₂ 2.7-2.8 1
Topaz Al₂SiO₄(F,OH)₂ 3.4-3.6 8

Tourmaline Complex silicate of boron and 3.0-3.25 7-7½
aluminum
Uranophane CaO·2UO₃·2SiO₂·7H₂O 3.8-3.9 2-3
100

BOOKS ABOUT ROCKS AND MINERALS

Many books have been written about rocks and minerals. Some are listed
below, and it is likely that your librarian will be able to suggest others.

Nontechnical Books for Beginners
Getting Acquainted With Minerals, by George L. English and David E.
Jensen. McGraw-Hill Book Company, Inc., New York, N. Y. (second
edition, 1958).

The Rock Book, by Carroll L. Fenton and Mildred A. Fenton. Doubleday &
Company, Inc., Garden City, N. Y. (1940).

Mineral Collector’s Guide, by David E. Jensen. Ward’s Natural Science
Establishment, Inc., Rochester, N. Y. (1953).

My Hobby is Collecting Rocks and Minerals, by David E. Jensen. Hart
Book Company, New York, N. Y. (1955).

Rocks and Minerals, by Richard M. Pearl. Barnes & Noble, New York, N.
Y. (1956).

1001 Questions Answered About the Mineral Kingdom, by Richard M.
Pearl. Dodd, Mead & Company, New York, N. Y. (1959).

Rocks and Minerals, by Herbert S. Zim and Paul R. Schaffer. Simon and
Schuster, Inc., New York, N.Y. (1957).

Textbooks and Other Reference Books
Economic Mineral Deposits, by Alan M. Bateman. John Wiley & Sons,
Inc., New York, N. Y. (second edition, 1950).

A Textbook of Mineralogy, by Edward S. Dana, revised by William E. Ford.
John Wiley & Sons, Inc., New York, N. Y. (fourth edition, 1932).

Industrial Minerals and Rocks (Nonmetallics Other Than Fuels), Joseph L.
Gillson, Editor-in-Chief. The American Institute of Mining,
Metallurgical, and Petroleum Engineers, New York, N. Y. (third
edition, 1960).

Dana’s Manual of Mineralogy, revised by Cornelius S. Hurlbut, Jr. John
Wiley & Sons, Inc., New York, N. Y. (seventeenth edition, 1959).

Mineralogy, by Edward H. Kraus, Walter F. Hunt, and Lewis S. Ramsdell.
McGraw-Hill Book Company, Inc., New York, N. Y. (fifth edition,
1959).

Nonmetallic Minerals, by Raymond B. Ladoo and W. M. Meyers. McGraw-
Hill Book Company, Inc., New York, N. Y. (second edition, 1951).

Rocks and Rock Minerals, by Louis V. Pirsson, revised by Adolph Knopf.
John Wiley and Sons, Inc., New York, N. Y. (third edition, 1947).

A Field Guide to Rocks and Minerals, by Frederick H. Pough. Houghton
Mifflin Company, Boston, Mass. (third edition, 1960).

Mineral Facts and Problems, by the Staff of the Bureau of Mines. U. S.
Bureau of Mines Bulletin 585. U. S. Government Printing Office,
Washington, D. C. (1960).

Selected References on Texas Rocks and Minerals
Entries marked with an asterisk are published by the Bureau of
Economic Geology, The University of Texas, Austin. Those not out

of print are distributed at nominal sale price, and a list of
publications will be sent on request. These publications can be
consulted at many public libraries and Chamber of Commerce
offices.

*Report on the Pavitte Silver-Copper Prospect in Burnet County, Texas, by
V. E. Barnes. Univ. Texas, Bureau Econ. Geol. Mineral Resource
Survey Circ. 5 (1936).

*Report on the Sheridan Copper Prospect in Burnet County, Texas, by V. E.
Barnes. Univ. Texas, Bur. Econ. Geol. Mineral Resource Survey Circ.
9 (1936).

*Building Stones of Central Texas, by V. E. Barnes, R. F. Dawson, and G.
A. Parkinson. Univ. Texas Pub. 4246 (1947).

*Iron Ore in the Llano Region, Central Texas, by V. E. Barnes. Univ. Texas,
Bur. Econ. Geol. Rept. Inves. No. 5 (1949).

*Utilization of Texas Serpentine, by V. E. Barnes, D. A. Shock, and W. A.
Cunningham. Univ. Texas Pub. 5020 (1950).

*Lead Deposits in the Upper Cambrian of Central Texas, by V. E. Barnes.
Univ. Texas, Bureau Econ. Geol. Rept. Inves. No. 26 (1956).

*Mineral Resources of the Colorado River Industrial Development
Association Area, by J. W. Dietrich and J. T. Lonsdale. Univ. Texas,
Bureau Econ. Geol. Rept. Inves. No. 37 (1958).

*Some Uranium Occurrences in West Texas, by D. H. Eargle. Univ. Texas,
Bureau Econ. Geol. Rept. Inves. No. 27 (1956).

*A Preliminary Report on the Stratigraphy of the Uranium-Bearing Rocks
of the Karnes County Area, South-Central Texas, by D. H. Eargle and
J. L. Snider. Univ. Texas, Bureau Econ. Geol. Rept. Inves. No. 30
(1957).

The Brown Iron Ores of Eastern Texas, by E. B. Eckel. U. S. Geol. Survey
Bull. 902 (1938).

*The Rustler Springs Sulphur Deposits as a Source of Fertilizer, by G. L.
Evans. Univ. Texas, Bureau Econ. Geol. Rept. Inves. No. 1 (1946).

Origin of the Gulf Coast Salt-Dome Sulphur Deposits, by Herbert W. Feely
and J. Lawrence Kulp. Bull. Amer. Assoc. Petrol. Geol., vol. 41, pp.
1802-1853 (1957).

*Pegmatites of the Van Horn Mountains, Texas, by P. T. Flawn. Univ.
Texas, Bureau Econ. Geol. Rept. Inves. No. 9 (1951).

*The Hazel Copper-Silver Mine, Culberson County, Texas, by P. T. Flawn.
Univ. Texas, Bureau Econ. Geol. Rept. Inves. No. 16 (1952).

*Basement Rocks of Texas and Southeast New Mexico, by P. T. Flawn.
Univ. Texas Pub. 5605 (1956).

*Texas Miners Boost Talc Output, by P. T. Flawn. Univ. Texas, 101
Bureau Econ. Geol. Rept. Inves. No. 35 (1958).

*Geology and Mineral Deposits of Pre-Cambrian Rocks of the Van Horn
Area, Texas, by P. B. King and P. T. Flawn. Univ. Texas Pub. 5301
(1953).

*Igneous Rocks of the Balcones Fault Region of Texas, by J. T. Lonsdale.
Univ. Texas Bull. 2744 (1927).

Mineral Resources of the Llano-Burnet Region, Texas, with an Account of
the Pre-Cambrian Geology, by Sidney Paige. U. S. Geol. Survey Bull.
450 (1911).

*Mineral Resources of the Texas Coastal Plain (Preliminary Report), by J.
M. Perkins and J. T. Lonsdale. Univ. Texas, Bureau Econ. Geol.
Mineral Resource Circ. 38 (1955).

Geology and Ore Deposits of the Shafter Mining District, Presidio County,
Texas, by C. P. Ross. U. S. Geol. Survey Bull. 928-B (1943).

*The Geology of Texas, Vol. II, Structural and Economic Geology, by E. H.
Sellards, C. L. Baker, and others. Univ. Texas Bull. 3401 (1935).

*Texas Mineral Resources, by E. H. Sellards and others. Univ. Texas Pub.
4301 (1946).

*Geological Resources of the Trinity River Tributary Area in Texas and
Oklahoma, by H. B. Stenzel, A. E. Weissenborn, and others. Univ.
Texas Pub. 4824 (1948).

Uranium at Palangana Salt Dome, Duval County, Texas, by A. D. Weeks
and D. H. Eargle. In U. S. Geol. Survey Prof. Paper 400-B (1960).

Geology of the Quicksilver Deposits of the Terlingua District, Texas, by R.
G. Yates and G. A. Thompson. U. S. Geol. Survey Prof. Paper 312
(1959).

102

GLOSSARY

Amorphous—without crystalline structure and therefore without regular
form.

Balcones fault zone—a system of faults extending from north of Waco in
McLennan County, through Travis and Bexar counties, to near Del Rio
in Val Verde County (see p. 42).

Boulder—a large rock or mineral fragment that has a diameter greater than
256 millimeters (about 10 inches).

Breccia—a rock made up of sharp-cornered, cemented fragments with
diameters greater than 2 millimeters (about ⁸/₁₀₀ of an inch).

Cambrian—the earliest period of the Paleozoic Era (see p. 3).

Cenozoic—the present era, one of the great divisions of geologic time (see
p. 3). This era began about 63 million years ago.

Clastic—made up of broken fragments of rocks or minerals.

Cleavage—occurs when minerals split along smooth flat surfaces that are
parallel to possible crystal faces. These planes as well as crystal faces
are controlled by the crystal lattice or atomic structures of the
minerals.

Cleavage fragment—a mineral specimen that has been broken along its
planes of cleavage.

Cobble—a rock or mineral fragment that has a diameter between 64 and
256 millimeters (about 2½ and 10 inches).

Conchoidal—a curved fracture surface shaped like the inside of a shell or
spoon.

Conglomerate—a rock composed of cemented, rounded rock or mineral
fragments, most of which are of gravel size.

Cretaceous—the third and latest period of the Mesozoic Era (see p. 3).

Cryptocrystalline—made up of tiny crystalline particles that are too small
to be distinguished even under high magnification.

Crystalline—having a definite, orderly internal structure.

Cube—a solid that has six equal, square sides.

Dodecahedron—a solid that has twelve plane, four-sided faces.

Element—a basic building block of all matter, which cannot be separated
into different substances by ordinary chemical means.

Eocene—the second epoch of the Tertiary Period (see p. 3).

Epoch—a unit of geologic time that is a subdivision of a period.

Era—a major division of geologic time, which consists of several periods.

Extrusive rocks—igneous rocks formed from magma that was extruded on
the earth’s surface.

Fault—a break in the rocks or strata of the earth’s crust along which
movement or slippage has taken place.

Fluid—a substance made up of particles that can move freely about; it can
be a liquid or a gas.

Formation—rocks or strata that are recognized and mapped as a unit.

Fracture—the kind of surface obtained if a mineral is broken in a different
direction from that of the cleavage or parting. Commonly, fracture
surfaces are rough, uneven, or curved, whereas cleavage surfaces are
smooth.

Geologic map (areal)—shows the extent and distribution of formations
exposed at the earth’s surface.

Granular—the texture of a rock or mineral that is made up of visible
grains. If all the grains are about the same size, the term equigranular
is used.

Granule—a rock or mineral fragment that has a diameter of from 2 to 4
millimeters (about ⁸/₁₀₀ to ¹⁵/₁₀₀ of an inch).

Gravel—uncemented rock or mineral fragments that have diameters greater
than 2 millimeters (about ⁸/₁₀₀ of an inch).

Gulf Coastal Plain—an area that extends, in Texas, from the Gulf of
Mexico to the Balcones fault zone and in which Quaternary, Tertiary,
and Upper Cretaceous strata crop out at the surface (see p. 42).

High Plains—an area in northwest Texas extending from the Pecos River
valley north to the Oklahoma-Texas boundary (see p. 42).

Igneous rocks—rocks formed by the cooling and hardening of hot, molten
rock material.

Intrusive rocks—igneous rocks that have formed below the surface of the
earth.

Lava—molten rock material that has poured out onto the earth’s surface
from volcanoes; also the rock that is formed after the molten material
has cooled and hardened.

Llano uplift—an area in central Texas where Precambrian and early
Paleozoic rocks occur at the earth’s surface (see p. 42).

Magma—hot, molten rock material from which igneous rocks form.

Massive—in a mass, without a regular or complete form.

Mesozoic—an era, one of the great divisions of geologic time (see p. 3).
This era began about 230 million years ago and lasted until about 63
million years ago.

Metamorphic rock—rock formed from igneous or sedimentary rocks that
are altered by heat, pressure, and fluids below the earth’s surface.

Miocene—the fourth epoch of the Tertiary Period (see p. 3).

Mississippian—the fifth period of the Paleozoic Era (see p. 3).

Nodule—a small, rounded mass or lump.

Octahedron—a solid that has eight triangular faces.

Opaque—no light can pass through.

Ordovician—the second period of the Paleozoic Era (see p. 3). 103

Paleozoic—an era, one of the great divisions of geologic time (see p. 3).
This era began at the end of Precambrian time and lasted until about
230 million years ago.

Parting—occurs when a mineral breaks along a flat surface that is not a
true cleavage plane.

Pebble—a rock or mineral fragment that has a diameter between 4 and 64
millimeters (about ¹⁵/₁₀₀ and 2½ inches).

Pennsylvanian—the sixth period of the Paleozoic Era (see p. 3).

Period—a unit of geologic time, a subdivision of an era.

Permian—the last period of the Paleozoic Era (see p. 3).

Physiographic outline map—shows location of natural regions (p. 42).

Playa lake—a temporary shallow lake in a nearly level, closed basin, which
has no drainage outlet.

Pleistocene—the first epoch of the Quaternary Period (see p. 3).

Pliocene—the last epoch of the Tertiary Period (see p. 3).

Precambrian—comprises the Early and the Late Precambrian Eras, the
earliest great divisions of geologic time. Rocks that formed more than
600 million years ago are known as Precambrian rocks.

Pyritohedron—a solid that has twelve 5-sided faces.

Quaternary—the present period of geologic time; the second period of the
Cenozoic Era (see p. 3).

Recent—the present epoch of geologic time; the second epoch of the
Quaternary Period (see p. 3).

Sectile—describes material, such as soap, that can be cut smoothly with a
knife.

Sediments—material deposited by water, wind, or ice on the earth’s
surface.

Sedimentary rocks—rocks made up of sediments.

Series—a subdivision of a system that includes all rocks formed during an
epoch.

Specific gravity—the ratio of the weight of a substance to the weight of an
equal volume of water.

Streak—the color of the powder of a mineral.

System—all rocks formed during a period.

Tertiary—the first period of the Cenozoic Era (see p. 3).

Translucent—light will pass through, but objects cannot be seen.

Transparent—light will pass through, and objects can be seen.

Trans-Pecos—area of Texas located west of the Pecos River (see p. 42).

Volcanic rocks—igneous rocks that have formed on the earth’s surface;
extrusive rocks.

104

Index

A B C D E F G H I J K L M N O P Q R S T U V
W X Y Z

A
acid: 18, 22
acid tests (see also chemical tests): 18
actinolite: 82
schist: 87
agate: 21, 83
agatized wood: 20, 84
age, earth’s crust: 2
alabaster: 65
albite: 55
alkali lakes: 66, 83
Allamoore: 87, 93
alloys: 52
almandite: 58
Alpine: 57, 78, 82, 90
Altuda Mountain: 57, 90
amalgam: 51
amazonstone: 56
amethyst: 37, 82
Amethyst Hill: 82
amorphous minerals, definition of: 15
amphibole asbestos: 29, 33, 34, 44
Anderson County: 70

Andrews County: 83
Angelina County: 52
anhydrite: 28, 29, 30, 35, 36, 43, 65, 66, 91
anthophyllite: 93
antigorite: 87
Apache Mountains: 65
apatite: 72
Archer County: 52
argentite: 26, 27, 89, 90
argentiferous galena: 57, 90
Armstrong County: 79
asbestos: 43
amphibole: 29, 33, 34, 44
chrysotile: 44, 87
ash, volcanic: 41, 97
augite: 45
Austin: 46, 62
autunite: 97
azurite: 35, 52, 53, 54

B
Babyhead: 72
Balcones fault zone: 10, 42, 45, 88
Bandera County: 45
Baringer Hill pegmatite: 80
barite: 15, 28, 31, 32, 35, 36, 38, 44, 48
barium: 45
Barrilla Mountains: 85
Baylor County: 45, 52, 98
basal cleavage: 76, 93, 94
basalt: 40, 41, 45
basement: 2, 9
Bastrop County: 64
Bell County: 69
bentonite: 51, 52
beryllium: 80
Big Bend area: 9, 42, 51, 75, 77

Big Branch Gneiss: 59
biotite: 32, 33, 34, 76
Bird mine: 90
Blanco County: 44, 58, 59, 61, 70, 83, 87, 94
black mica (see biotite)
blast furnace: 70, 71
blast sand: 86
bleaching clay: 51
blue copper: 53
Boerne: 18
Boling salt dome: 91
books about rocks and minerals: 24, 39, 100
brass: 52
braunite: 26, 27, 73
Brazoria County: 91
Brazos County: 98
Brazos River, Double Mountain Fork and Salt Fork of: 43
breccia: 12, 41, 81
Brewster County: 45, 46, 47, 49, 50, 57, 65, 70, 76, 77, 78, 82, 84, 85, 90
brimstone: 90
bronze: 52
Brooks County: 64
Brown, Thomas E.: vii
Brown County: 45, 49
brown iron ore (see also limonite): 70
Bryan Mound salt dome: 91
building sand: 85
Burnet: 56, 80, 97
Burnet County: 18, 53, 54, 55, 56, 58, 59, 60, 61, 62, 63, 70, 76, 79, 80, 81,
82, 83, 97

C
calcareous sinter and tufa: 46
calcite: 7, 14, 18, 19, 20, 21, 28, 30, 31, 32, 34, 35, 36, 38, 45, 46, 49, 50,
51, 54, 56, 64, 68, 75, 86, 87, 88, 91
dog-tooth spar: 46, 47
Caldwell County: 88

caliche: 46, 47
Cambrian: 2, 3, 55, 58, 69, 86
Cameron County: 66
Capitan reef limestone: 1
Capitol building, Texas State: 62
cap-rock: 91, 92
carats: 60
carbon: 62, 63
carbonized wood: 20
carnotite: 30, 95, 97
Carrizo Mountains: 67, 83, 95
Casey, Miss Josephine: vii
Cass County: 70
cassiterite: 27, 32, 47
Castile Gypsum: 91
Catahoula strata: 78, 86, 97
cave deposits: 18
caves: 18, 19, 49
Caverns of Sonora: 19
Cedar Lake: 83
Cedar Park: 70
celestite: 20, 28, 30, 35, 38, 48
cementing materials: 12, 46, 67, 70, 86
cerargyrite: 28, 29, 34, 37, 89, 90
cerium: 80
chalcedony (see also agate): 16, 21, 45, 78, 81, 83, 86
chalcocite: 21, 26, 27, 52, 54, 73, 89
chalcopyrite: 26, 52, 53, 54, 60
chalk: 40, 69
chemical elements: 7
chemical sediments: 12 105
chemical tests: 18, 43, 46, 47, 49, 52, 53, 54, 64, 68, 75
Cherokee County: 70
chert: 19, 40, 81, 84
chessylite: 53
china clay: 51
Chinati Mountains: 45, 57, 62, 85

Chisos Mountains: 45, 62, 85
chlorite: 93
schist: 87
chrysotile: 44, 87, 88
asbestos: 44, 87
cinnabar: 36, 49
clastic rocks: 12
clay: 40, 45, 49, 51, 70, 86, 88, 91
bentonite: 45, 51, 52
bleaching clay: 51
china clay: 51
kaolin: 51
clay minerals: 51
Clay County: 52
Clear Creek area: 63
cleavage: 17
basal: 76, 93, 94
cubic: 17, 57, 65
octahedral: 56, 57
pyramidal: 74
rhombohedral: 46, 54
Clemens salt dome: 91
Coal Creek serpentine: 87, 88
Coke County: 20, 49
Coleman County: 86
color of minerals: 16
columns: 18
Comal County; 45, 69
Comanche County: 49
common opal: 78
composition of minerals: 99
concretions: 19
conglomerate: 11, 12, 41, 55, 81
gneiss: 59
contact metamorphism: 13
copper: 52, 59, 90
blue copper: 53

glance: 52
green copper carbonate: 53
minerals: 52
pyrites: 53
yellow copper ore: 53
coquina: 41, 69
Crane County: 66
Cretaceous: 2, 3, 20, 21, 45, 46, 49, 51, 55, 57, 60, 65, 68, 69, 70, 74, 75,
84, 86, 88
cryptocrystalline quartz: 81, 84
crystal, definition of: 14
crystals, twinned: 46, 63
crystalline minerals, definition of: 14
Crosby County: 79
crust, earth’s: 2
cubic cleavage: 17, 57, 65
Culberson County: 1, 45, 52, 53, 58, 59, 60, 65, 75, 79, 83, 90, 91, 95

D
Daingerfield: 70
Dallas: 88
Damon Mound salt dome: 91
daughter elements: 95, 96
Davis Mountains: 45, 85, 98
Delaware Mountains: 65
diamond: 62
diatomaceous earth: 78
diatomite: 40, 78
Dickens County: 79, 98
Dietrich, John W.: vii
dikes: 72, 79
dog-tooth spar: 46, 47
dolomite: 29, 30, 32, 36, 38, 40, 43, 54, 68, 75, 87, 88
dolomitic limestone: 68
Double Mountain Fork of the Brazos River: 43
double refraction: 46
dravite: 95

Duval County: 78, 83, 91, 97
dynamic metamorphism: 14

E
Eagle Flat: 93
Eagle Mountains: 56, 57, 90, 95
earth’s crust: 2
east Texas: 70
Ector County: 79
Edwards Limestone: 68, 69, 84
effervescence in acid: 18
El Capitan Peak: 1
elements: 7, 8, 95, 96
Ellenburger strata: 55, 84
El Paso: 48, 58, 85, 88
El Paso County: 48, 58, 62
epoch: 2
era: 2
erosion: 11
extrusive igneous rocks, definition of: 9

F
Fairland: 54
Falfurrias: 64
Fannett salt dome: 91
Fayette County: 52, 64, 84, 98
feldspar: 8, 29, 30, 31, 33, 34, 35, 37, 45, 51, 55, 59, 61, 72, 79, 80, 85, 86
albite: 55
amazonstone: 56
microline: 55, 56, 61
orthoclase: 55, 61
fibrous gypsum: 64, 65
filtering sand: 86
Fisher County: 20, 49, 58, 65
Flawn, Peter T.: vii
flint: 84
flowstone: 18

flow structure: 85
fluorite: 29, 32, 34, 35, 37, 38, 56, 72
Foard County: 52, 58
foliation: 87
fool’s gold: 53, 80
formation: 6
Fort Bend County: 91
Fort Worth: 88
fracture of minerals: 17
fragmental rocks: 12
Franklin Mountains: 48, 58, 62, 85
Frasch method of sulfur mining: 91, 92
Fredonia: 67
Freer: 78, 83
furnace (see blast furnace; open-hearth furnace)

G
gabbro: 87
gneiss: 59
Gaines County: 83
galena: 17, 26, 57, 89, 90
argentiferous: 57, 90
Galveston County: 91 106
Gamble prospect: 67
gamma rays: 96
garnet: 31, 33, 34, 37, 58, 85, 87
Garza County: 97
Geiger counter: 96
gemstones: 56, 58, 77, 82, 94, 95
geode: 19, 46, 49, 82
geologic map: 4, 5
geologic time scale: 2, 3
geologists: 2
geology: 2
Gillespie County: 44, 45, 55, 58, 59, 60, 61, 65, 76, 80, 82, 83, 87, 88, 94
glass, natural: 9, 77, 85, 97
glass sand: 85-86

glauconite: 70, 73
glossary: 102-103
gneiss: 13, 41, 55, 56, 59, 67, 72, 73, 74, 76, 81, 87, 93, 97
conglomerate: 59
gabbro: 59
granite: 59
hornblende: 59
mica: 59
goethite: 70
gold: 26, 51, 59, 80, 90
fool’s: 53, 80
Gonzales County: 52, 84, 97
Grand Saline salt dome: 66
granite: 2, 8, 9, 13, 41, 48, 55, 56, 59, 61, 72, 76, 79, 81, 85, 87, 94, 95
gneiss: 59
opaline: 72
Granite Mountain: 62
Granite Shoals Lake: 56
granules: 85
graphite: 26, 27, 62, 75
schist: 63, 87
gravel: 12, 41, 91
green copper carbonate: 53
greensands: 70
Grit: 94
grossularite: 58
group: 6
Guadalupe Mountains, Peak: 1
Gulf Coast: 91
Gulf Coastal Plain: 20, 42, 43, 46, 51, 52, 58, 60, 65, 66, 78, 83, 84, 86, 87,
92, 97, 98
Gulf salt dome: 91
Gyp Hill salt dome: 64
gypsite: 65
gypsum: 28, 29, 30, 32, 36, 37, 43, 63, 70, 85, 91
alabaster: 65
fibrous gypsum: 64, 65

rock gypsum: 40, 65
rosettes: 63, 64
satin spar: 64
selenite: 63, 64, 76

H
halite: 28, 32, 34, 35, 36, 38, 40, 43, 65, 91
hand lens: 22
Hardeman County: 52, 58, 64, 65
hardness of minerals: 16, 99
Harris, Bill M.: vii
Harris, John S.: vii
Harris County: 65, 66
Hartley County: 79
Hays County: 45
Hazel Formation: 52
Hazel mine: 52, 53, 90
Heath mine: 60
hematite: 26, 27, 36, 66, 75, 84, 86
specular: 67
Henderson County: 70
High Island salt dome: 91
High Plains: 42, 46, 51, 65, 66, 78, 79, 83, 87, 97, 98
Hockley salt dome: 65, 66
hollandite: 26, 27, 28, 73, 74
Honey Creek area: 87
hornblende: 8, 59, 61
gneiss: 59
schist: 87
horn silver: 90
Hoskins Mound salt dome: 91
Houston: 70
Howard County: 45, 60
Hudspeth County: 45, 52, 53, 55, 56, 57, 58, 59, 60, 62, 65, 66, 67, 75, 79,
82, 83, 90, 93, 95
Humphreys, Alan: vii
Hutchinson County: 66

hyalite: 78
hydrochloric acid (see also chemical tests): 18, 22

I
identification charts: 24
igneous rocks, definition of: 9
intrusive igneous rocks, definition of: 9
Irion County: 60
iron: 59, 66, 67, 69, 70, 72, 73
Iron Mountain: 73

J
Jackson strata: 97
jasper: 81, 84
Jasper County: 52
Jeff Davis County: 45, 51, 74, 78, 85
Jefferson County: 91
Johnson County: 69

K
kaolin: 51
kaolinite: 51
karats: 60
Karnes County: 97, 98
Katemcy: 94
Kendall County: 18
Kenedy County: 66
Kent County: 98
kidney ore: 67
King, Elbert A., Jr.: vii
King County: 52
Kinney County: 45
Kleberg County: 66
Knox County: 52

L
labeling rock and mineral specimens: 23
labradorite: 45

Lake Buchanan: 80, 83
Lamb County: 79
Lampasas County: 20, 48, 49, 88
lava: 9
lead: 57, 90
Leakey: 51
Lee County: 64, 84
lens: 22
Liberty County: 91
lime: 69
limestone: 12, 13, 18, 19, 40, 43, 45, 46, 49, 54, 55, 56, 57, 58, 60, 68, 74,
75, 76, 84, 88, 90, 91
chalk: 69
dolomitic: 68
lithographic: 69 107
öolitic: 69
pulverulent: 41, 69
limonite: 19, 27, 28, 30, 32, 70, 75, 81, 86
lithographic limestone: 69
Live Oak County: 45, 97
llanite: 41, 71, 81
Llano: 60, 63, 72, 73, 95
Llano County: 44, 45, 55, 56, 58, 59, 60, 61, 63, 70, 71, 72, 73, 76, 78, 80,
83, 87, 88, 94, 95
Llano uplift: 10, 18, 42, 44, 52, 55, 56, 58, 59, 60, 61, 62, 63, 67, 70, 72, 76,
77, 79, 80, 82, 83, 84, 85, 86, 87, 88, 93, 94, 95
Lone Grove: 63
Lone Star: 70
Longhorn Cavern: 18
Long Point salt dome: 91
Lonsdale, John T.: vii
luster of minerals: 16
Lytton Springs oil field: 88

M
Macon, James W.: vii
magma: 9

magnesium, source of: 55
magnetite: 28, 67, 70, 72, 85, 87, 93
malachite: 34, 52, 53, 54
Malone Mountains: 65
manganese: 73, 75
map—
geologic: 4-5
physiographic outline: 42
Marathon area: 42, 84, 88
marble: 13, 14, 40, 46, 54, 75
Marble Falls: 62
Marion County: 70
martite: 67
Mason: 74
Mason County: 48, 55, 58, 59, 60, 61, 67, 72, 73, 74, 76, 80, 83, 94
masses, crystalline, definition of: 14
Matagorda County: 91
Mayfield prospect: 74
McCulloch County: 88
Medina County: 45
Menard County: 65
mercury: 8, 49, 50, 51
metamorphic rocks, definition of: 12
metamorphism, definition of: 13, 14
mica (see also biotite; muscovite): 8, 60, 61, 63, 64, 72, 76, 79, 80, 85, 87
mica gneiss and/or schist: 59, 76, 87
Mica Mine area: 58, 76, 77, 79
microcline: 55, 56, 61
milky quartz: 82
Mills County: 49
mineral identification charts: 24
mineralogists: 2
minerals, definition of: 7, 14, 15
Mississippian: 3, 55, 69, 88
Mitchell County: 66
Mohs scale of hardness: 16, 17
molybdenite: 63

montmorillonite: 51
Morris County: 70
moss agate: 84
Moss Bluff salt dome: 91
muriatic acid: 22
muscovite: 31, 32, 34, 37, 38, 76

N
Nacogdoches County: 70
Nash salt dome: 91
native elements: 8
native mercury: 8
native silver: 26, 89
nodule: 19, 44, 49
Nolan County: 20, 49, 63, 64, 65
north-central Texas: 43, 51, 52, 69, 70, 86, 88

O
Oakville strata: 97
obsidian: 40, 77
occurrence of minerals: 14
ocher, red: 67
octahedral cleavage: 56, 57
oil shale: 88
olivine: 88
onyx: 84
öolitic limestone: 69
opal: 21, 29, 30, 31, 35, 37, 38, 78, 86
opaline granite: 72
opalized wood: 20, 78, 84
open-hearth furnace: 70, 72
Orchard salt dome: 91
Ordovician: 3, 55, 69, 84
orthoclase: 55, 61

P
Packsaddle Mountain: 87

Packsaddle Schist: 87
Palangana salt dome: 91, 97
paleontologists: 2
parting of minerals: 17
Pavitte prospect: 53
Pecos County: 9
Pecos River: 65, 74, 75
pegmatite: 41, 55, 56, 58, 76, 77, 79, 81, 94, 95
Baringer Hill: 80
Pennsylvanian: 3, 45, 51, 55, 69, 70, 86, 88
peridotite: 88
period: 2
Permian: 1, 3, 20, 21, 43, 45, 49, 52, 55, 64, 65, 69, 86, 90
Permian basin: 42, 43, 65, 66
petrified wood: 20, 52, 78, 84
petrologists: 2
phenocrysts: 85
physiographic outline map: 42
pitchblende: 28, 95, 97
placer deposits: 60
Plata Verde mine: 90
Polk County: 98
porphyritic rocks: 45, 85
portland cement: 51, 69, 88
Precambrian: 2, 3, 44, 45, 48, 52, 53, 56, 58, 59, 60, 61, 63, 67, 72, 74, 75,
76, 79, 82, 85, 87, 93, 94, 95, 97
precious opal: 78
Presidio County: 45, 49, 57, 60, 62, 67, 77, 78, 84, 85, 90, 97
Presidio mine: 60, 90
properties of minerals: 14
pseudomorphs: 70, 87
pulverulent limestone: 41, 69
pumicite: 97, 98
pyramidal cleavage: 74
pyrite: 19, 26, 60, 70, 80, 87
pyrolusite: 27, 73, 74
pyroxene: 45

108

Q
quartz: 8, 20, 29, 30, 31, 33, 36, 37, 38, 48, 51, 54, 56, 59, 60, 61, 68, 72,
78, 79, 80, 81, 84, 85, 86, 94
agate: 21, 83, 84
amethyst: 37, 82
chalcedony: 16, 21, 45, 78, 81, 83, 86
chert: 19, 40, 81, 84
cryptocrystalline: 81, 84
jasper: 81, 84
milky: 82
onyx: 84
rock crystal: 82
rose: 82, 83
smoky: 82, 83
quartzite: 41, 74, 84, 86
Quaternary: 2, 3, 51, 79, 98
quicksilver (see also mercury): 50
Quitman Mountains: 57, 58, 60, 62, 67, 82, 90

R
radioactivity: 95, 96
Real County: 49, 51
Recent: 2, 3
“red beds”: 52
red ocher: 67
reef limestone: 1, 12
Reeves County: 65, 85, 91
reference books: 24, 39, 100
refraction, double: 46
rhombohedral cleavage: 46, 54
rhyolite: 40, 85
porphyry: 41, 85
Rio Grande: 83, 84, 97
rock—
crystal: 82
gypsum: 40, 65

identification charts: 39
salt: 40, 65
units: 2
rocks, definition of: 8
Rodda, Peter U.: vii
rose quartz: 82, 83
rosettes: 63, 64
Rustler Springs area: 91
rutile: 82

S
salt (see also halite): 40, 65, 66
domes: 43, 58, 64, 66, 91, 97
lakes: 66
rock: 40, 65
Salt Fork of the Brazos River: 43
sand: 41, 58, 72, 85, 91
blast: 86
building: 85
filtering: 86
glass: 85-86
sandstone: 41, 52, 55, 58, 59, 72, 76, 81, 84, 86
Sandy Creek: 60
San Saba County: 70, 88
Santa Anna: 86
satin spar: 64
Satorsky, Cyril: vii
scale, geologic time: 2, 3
scintillation counter: 96
schist: 41, 56, 58, 72, 73, 76, 81, 87, 93
actinolite: 87
graphite: 63, 87
hornblende: 87
mica: 76, 87
talc: 87, 93
schistosity: 87
schorl: 95

Scurry County: 98
sedimentary rocks, definition of: 10
sediments: 8, 11
chemical: 12
selenite gypsum: 63, 64, 76
series: 2
serpentine: 34, 40, 44, 75, 87, 93
serpentinite: 87
Seven Heart Gap: 45
Shafter: 57, 58, 60, 67, 90
Shafter Lake: 83
shale: 12, 13, 19, 40, 45, 70, 87, 88
oil: 88
Shelby, Cader A.: vii
Sheridan prospect: 53
siderite: 70
Sierra Blanca: 58, 67, 82
silicified wood (see also petrified wood): 20
silt: 12, 85
siltstones: 12
silver: 26, 52, 57, 59, 89
glance: 90
horn: 90
minerals: 89
native: 26, 89
Silver Creek: 58
sinter, calcareous: 46
slate: 13
Smith County: 70
smoky quartz: 82, 83
soapstone: 40, 93
soils: 10
Solitario uplift: 42, 88
Sonora: 18
Caverns of: 19
specific gravity: 18, 99
specular hematite: 67

sphalerite: 57
Spiller mine: 74
Spindletop salt dome: 91
Spring Creek: 56
stalactites: 18, 19
stalagmites: 18, 19
Starr County: 84, 98
static metamorphism: 13
Stonewall County: 52
streak or powder of minerals: 17
streak plate: 17, 22
Streeter: 48, 94
striations: 81, 95
strontianite: 49
strontium: 49
sulfur: 29, 30, 31, 32, 34, 36, 81, 90
Sutton County: 18, 19
system: 2

T
talc: 28, 29, 34, 87, 93
schist: 87, 93
Talc Rock: 93
Taylor County: 45, 49, 60
Terlingua area: 46, 47, 49, 50, 65
Tertiary: 2, 3, 9, 21, 45, 49, 51, 52, 56, 57, 60, 70, 77, 78, 79, 83, 84, 85, 86,
90, 97, 98
terrazzo—
chips: 55, 76
floors: 88
tests, chemical (see chemical tests)
thorium: 80
Thrall oil field: 88
time, geologic: 2, 3 109
tin: 47
topaz: 36, 38, 94
tourmaline: 33, 82, 94

Town Mountain: 95
Trans-Pecos area: 9, 10, 45, 60, 62, 69, 78, 82, 83, 84, 85, 87, 89, 90, 98
trap rock: 45
travertine: 46
Travis County: 20, 45, 46, 49, 68, 69, 70, 88
tremolite: 44, 93
Triassic: 3, 45, 87
Trinity County: 98
tufa, calcareous: 46
twinned crystals: 46, 63
tyuyamunite: 97

U
Upshur County: 70
uranium: 80, 95, 96
minerals: 95
uraninite: 97
uranophane: 31, 95, 97
Uvalde County: 45, 60, 84, 88

V
Valley Spring Gneiss: 59
Val Verde County: 45, 74, 75
Van Horn area: 42, 52, 53, 58, 59, 60, 75, 76, 79, 85, 87, 90
Van Horn Mountains: 45, 55, 76, 79, 95
Van Zandt County: 66
verde antique: 88
vesicles: 45
vitrophyre: 40, 77
volcanic ash: 41, 97
volcanic igneous rocks, definition of: 9
volcanoes: 9, 97, 98

W
Waco: 88
wad: 73, 74
Walker County: 52

Ward County: 66
Washington County: 78, 84
weathering: 11
Webb County: 11, 84
Weches strata: 70
Wharton County: 91
white mica (see muscovite)
Wilbarger County: 98
Willacy County: 66
Williamson County: 20, 60, 69, 70, 88
Willow City: 94
wood opal: 78
wood, petrified: 20, 52, 78, 84
Wylie Mountains: 95

Y
yellow copper ore: 53
Yoakum County: 66
Young County: 58
yttrium: 80

Z
Zapata County: 83, 84
Zavala County: 86
zirconium: 80

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Added a label to one figure (to match the Table of Illustrations).

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