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

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

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.

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].

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.

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)

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 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.

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.

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 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.

[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 [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 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 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, 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

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 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.

[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 common objects with their hardnesses.

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, 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 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 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 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]).

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 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.

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.

Key to Mineral Identification Charts

If the mineral has a metallic or submetallic luster,

If the mineral has a nonmetallic luster,

Mineral Identification Charts

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.

Rock Identification Charts

[Physiographic outline map] of Texas.

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 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 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].

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 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 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 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].