Texas Rocks And Minerals: An Amateur's Guide - free ebook by Roselle M. Girard

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...

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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

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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 p

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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 ....

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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 t

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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. 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 ....

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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 o

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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 ow

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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 onl

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Occurrence and Properties of Minerals

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 . 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

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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 no

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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

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Key to Mineral Identification Charts

If the mineral has a metallic or submetallic luster, If the mineral has a nonmetallic luster,...

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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. 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: In Rock Chart 3 , the granular rocks also are arranged

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Rock Identification Charts

Physiographic outline map of Texas. 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 o

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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 cle

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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 lust

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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.

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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

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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 transl

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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

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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, l

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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

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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 i

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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 wi

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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 fr

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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-al

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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

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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 Texa

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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

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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

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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 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 in

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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 ceme

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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 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 r

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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

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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 h

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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 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

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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 limesto

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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

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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 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

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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 a

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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 t

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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 caus

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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, resul

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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 volc

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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

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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 dike

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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

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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 p

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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 th

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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 slo

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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 wea

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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) scat

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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

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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 sedimen

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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 c

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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 . W

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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

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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

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Tourmaline

Tourmaline is a complex silicate of boron and aluminum. Other elements , such as magnesium, sodium, lithium, calcium, iron, or fluorine, also 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 e

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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 daugh

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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, gr

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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 . 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....

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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 &amp; 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

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Textbooks and Other Reference Books

Economic Mineral Deposits, by Alan M. Bateman. John Wiley &amp; Sons, Inc., New York, N. Y. (second edition, 1950). A Textbook of Mineralogy, by Edward S. Dana, revised by William E. Ford. John Wiley &amp; 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 Miner

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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 She

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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

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