Chapter III: Introduction (1)

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.

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_.

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.

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)

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.

Aluminum Al
Barium Ba
Beryllium Be
Boron B
Calcium Ca
Carbon C
Cerium Ce
Chlorine Cl
Copper Cu
Fluorine F
Gold Au
Hydrogen H
Iron Fe
Lead Pb
Magnesium Mg
Manganese Mn
Mercury Hg
Molybdenum Mo
Oxygen O
Potassium K
Silicon Si
Silver Ag
Sodium Na
Strontium Sr
Sulfur S
Thorium Th
Tin Sn
Uranium U
Vanadium V
Yttrium Y
Zinc Zn
Zirconium Zr

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

Minerals

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

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_.

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.

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.

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.

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_.

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.

_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_.

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.

_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).

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.

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.

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.

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

Suppose, for example, that a mineral can be scratched by fluorite, which has a hardness of _4_ on Mohs scale, but cannot be scratched by calcite, which has a hardness of _3_. We then know that this mineral is softer than fluorite, but harder than calcite; therefore, it has a hardness of about _3½_. In the same way, if a mineral can be scratched by a pocket knife, which is slightly more than _5_ in hardness, but not by a copper penny, which has a hardness of about _3_, we know then that its hardness is between _3_ and _5_.

Streak or Powder

The streak is the mark, made of fine powder, that a mineral leaves as you rub it across a streak plate. A streak plate is a flat piece of white tile or porcelain that has a dull, unglazed surface. The streak plate is about as hard as quartz, which is _7_ on Mohs scale, and you will not be able to use it for minerals that have a greater hardness. For these, you can obtain the powder by scratching the mineral or by crushing a small piece of it.

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_.

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.

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.

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

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.

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.

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,

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

If the mineral has a _nonmetallic_ luster,

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

Mineral Identification Charts

_Chart _Mineral_ _Streak_ _Remarks_ _Hardness_
No._

1. METALLIC luster, WHITE color

A. Does not readily leave mark on paper but can be scratched by
ordinary pocket knife
Native silver Shiny silver Silver-white 2½-3
white, unless color that
tarnished tarnishes to
gray, black, or
yellowish brown;
heavy; can be
flattened when
hit with hammer

2. METALLIC or SUBMETALLIC luster, GRAY color

A. Will leave mark on paper
Argentite Shiny, blackish Lead-gray color 2-2½
to lead gray that tarnishes to
dull black; knife
cuts it smoothly;
heavy; may occur
as masses and
coatings
Galena Grayish black Shiny lead-gray 2½
color; heavy;
cube-shaped
fragments and
crystals
Graphite Black Steel-gray color; 1-2
greasy feel; very
soft; splits into
thin flakes
B. Does not readily leave mark on paper but can be scratched by
ordinary pocket knife
Chalcocite Grayish black Shiny lead-gray 2½-3
color that
tarnishes to dull
black; knife cuts
it smoothly; may
have black sooty
coating; commonly
occurs as compact
or granular masses
Hollandite Black Silvery-gray 4-6
color; may occur
as rounded masses
C. Cannot be scratched by ordinary pocket knife
Braunite Steel gray or Dark steel-gray 6-6½
black color and
submetallic luster
Hematite Dark reddish brown Steel-gray color; 5½-6½
commonly occurs (may be
as granular or softer)
compact masses;
shiny, scaly
variety is
_specular_
hematite; notice
streak
Hollandite Black Silvery-gray 4-6
color; may occur
as rounded masses

3. METALLIC luster, YELLOW color

A. Does not readily leave mark on paper but can be scratched by
ordinary pocket knife
Chalcopyrite Greenish black Brass-yellow or 3½-4
golden-yellow
color that may
tarnish and show
rainbow-like
colors; commonly
massive; notice
streak
Gold Shiny golden Shiny yellow 2½-3
yellow color; extremely
heavy; flattens
when hit with
hammer; notice
streak
B. Cannot be scratched by ordinary pocket knife
Pyrite Black, greenish Shiny, pale 6-6½
black, or golden-yellow or
brownish black brass-yellow
color that may
tarnish; occurs
as grains, as
masses, or as
cubes or other
crystal shapes;
notice hardness
and streak

4. METALLIC or SUBMETALLIC luster, BROWN color

A. Does not readily leave mark on paper but can be scratched by
ordinary pocket knife
Limonite Rusty yellowish Dark-brown color; 5-5½
brown some specimens
have a shiny
black surface;
notice streak
B. Cannot be scratched by an ordinary pocket knife
Cassiterite Pale brown, pale Brown; 6-7
yellow or white submetallic;
heavy; notice
streak
Hematite Dark reddish brown Dark brown color; 5½-6½
commonly occurs (may be
as granular or softer)
compact masses;
notice streak
Limonite Rusty, yellowish Dark brown color; 5-5½
brown some specimens
have a shiny
black surface;
notice streak

5. METALLIC or SUBMETALLIC luster, BLACK color

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

6. NONMETALLIC luster, WHITE color

A. Can be scratched by a fingernail
Cerargyrite Shiny white or Appears waxy; 1-1½
gray knife cuts it
smoothly; turns
violet brown to
black when
exposed to light
Gypsum White Soft; occurs as 2
crystals or as
fibrous,
granular,
compact, or
earthy masses
Talc White Knife cuts it 1
smoothly; feels
soapy or greasy;
splits into thin
flakes
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Anhydrite White Commonly occurs 3-3½
as sugary-looking
masses
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Calcite White Dilute 3
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Celestite White Not quite as 3-3½
heavy as barite;
crystals commonly
prism-shaped or
flat-looking;
some cleavage
fragments are
flat and slab-like
Halite White Salty taste; 2½
dissolves in
water;
cube-shaped
cleavage fragments
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Anhydrite White Commonly occurs 3-3½
as sugary-looking
masses
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Celestite White Not quite as 3-3½
heavy as barite;
crystals commonly
prism-shaped or
flat-looking;
some cleavage
fragments are
flat and slab-like
Dolomite White Commonly occurs 3½-4
as granular
masses and as
rhomb-shaped
crystals; dilute
hydrochloric acid
may fizz slightly
on dolomite
Fluorite White Cleavage in 4 4
directions can
give fragments
that are shaped
like octahedrons;
crystals commonly
cubes
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull; milky
white and
bluish-white
precious opal
shows plays of
colors
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near
90°; common in
granite and
pegmatite rocks
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull; milky
white and
bluish-white
precious opal
shows plays of
colors
Quartz White Curved conchoidal 7
fracture; occurs
as milky quartz,
chert, and
chalcedony;
crystals commonly
6-sided prisms
with pyramid-like
ends

7. NONMETALLIC luster, GRAY color

A. Can be scratched by a fingernail
Amphibole asbestos White Made up of 1-2½
slender, flexible
fibers that can
be pulled apart
Cerargyrite Shiny white or Appears waxy; 1-1½
gray knife cuts it
smoothly; turns
violet brown to
black when
exposed to light
Gypsum White Soft; occurs as 2
crystals or as
fibrous,
granular,
compact, or
earthy masses
Sulfur White or pale Will burn with a 1½-2½
yellow blue flame;
commonly found as
crystals, crusts,
or grains
Talc White Knife cuts it 1
smoothly; feels
soapy or greasy;
splits into thin
flakes
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Amphibole asbestos White Made up of 1-2½
slender, flexible
fibers that can
be pulled apart
Anhydrite White Commonly occurs 3-3½
as sugary-looking
masses
Calcite White Dilute 3
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Celestite White Crystals commonly 3-3½
prism-shaped or
flat-looking;
some cleavage
fragments are
flat and slab-like
Halite White Salty taste; 2½
dissolves in
water;
cube-shaped
cleavage fragments
Sulfur White or pale Will burn with a 1½-2½
yellow blue flame;
commonly found as
crystals, crusts,
or grains
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Anhydrite White Commonly occurs 3-3½
as sugary-looking
masses
Celestite White Crystals commonly 3-3½
prism-shaped or
flat-looking;
some cleavage
fragments are
flat and slab-like
Dolomite White Commonly occurs 3½-4
as granular
masses and as
rhomb-shaped
crystals; dilute
hydrochloric acid
may fizz slightly
on dolomite
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near
90°; common in
granite and
pegmatite rocks
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull
Quartz White Curved conchoidal 7
fracture; occurs
as chert and
chalcedony

8. NONMETALLIC luster, YELLOW color

A. Can be scratched by a fingernail
Carnotite Yellow Bright canary 2
yellow or lemon
yellow;
radioactive;
occurs as crusts
and powdery masses
Gypsum White Yellowish; soft; 2
occurs as
crystals or as
fibrous,
granular, compact
or earthy masses
Limonite Rusty yellowish Brownish-yellow 1+
brown color; may be
soft and earthy
Muscovite (white White Light colored; 2-2½
mica) splits into thin,
flat, transparent
sheets that will
bend without
breaking
Sulfur White or pale Will burn with a 1½-2½
yellow blue flame;
commonly found as
crystals, crusts,
or grains
Uranophane Light yellow to Yellow to 2-3
light yellow yellow-orange
orange color; radioactive
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Calcite White Yellowish; dilute 3
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Muscovite (white White Light colored; 2-2½
mica) splits into thin,
flat, transparent
sheets that will
bend without
breaking
Sulfur White or pale Will burn with a 1½-2½
yellow blue flame;
commonly found as
crystals, crusts,
or grains
Uranophane Light yellow to Yellow to 2-3
light yellow yellow-orange
orange color; radioactive
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near 90°
Garnet White Commonly occurs 6½-7
as crystals
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull
Quartz White Curved conchoidal 7
fracture;
brownish-yellow
_smoky_ quartz
crystals commonly
6-sided prisms
with pyramid-like
ends;
_chalcedony_ and
_jasper_ may be a
shade of yellow,
too

9. NONMETALLIC luster, BROWN color

A. Can be scratched by a fingernail
Gypsum White Brownish; soft; 2
occurs as
crystals or as
fibrous,
granular, compact
or earthy masses
Limonite Rusty yellowish May be soft and 1+
brown earthy
Muscovite (white White Light colored; 2-2½
mica) splits into thin,
flat, transparent
sheets that will
bend without
breaking
Sulfur White or pale Will burn with a 1½-2½
yellow blue flame;
commonly found as
crystals, crusts,
or grains
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Biotite (black mica) White Dark brown; 2½-3
splits into thin,
flat sheets that
will bend without
breaking
Calcite White Dilute 3
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Halite White Salty taste; 2½
dissolves in
water;
cube-shaped
cleavage fragments
Muscovite (white White Light colored; 2-2½
mica) splits into thin,
flat, transparent
sheets that will
bend without
breaking
Sulfur White or pale Will burn with a 1½-2½
yellow blue flame;
commonly found as
crystals, crusts,
or grains
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Dolomite White Commonly occurs 3½-4
as granular
masses and as
rhomb-shaped
crystals; dilute
hydrochloric acid
may fizz slightly
on dolomite
Fluorite White Cleavage in 4 4
directions can
give fragments
that are shaped
like octahedrons;
crystals commonly
cube-shaped
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Cassiterite Pale brown, pale Brown, reddish 6-7
yellow, or white brown, or
yellowish brown;
heavy; dull to
brilliant luster
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near
90°; common in
granite and
pegmatite rocks
Garnet White Commonly occurs 6½-7
as crystals
Quartz White Curved conchoidal 7
fracture; brown
_smoky_ quartz
crystals commonly
6-sided prisms
with pyramid-like
ends;
_chalcedony_,
_chert_, and
_jasper_ may be a
shade brown, too
Tourmaline White Dark brown 7-7½
variety is
_dravite_; notice
hardness,
striations on
crystals, and
triangular cross
section of some
crystals
E. Cannot be scratched by quartz
Tourmaline White Dark brown 7-7½
variety is
_dravite_; notice
hardness,
striations on
crystals, and
triangular cross
section of some
crystals

10. NONMETALLIC luster, BLACK color

A. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Biotite (black mica) White Splits into thin, 2½-3
flat sheets that
will bend without
breaking
B. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Garnet White Commonly occurs 6½-7
as crystals
Quartz White Curved conchoidal 7
fracture;
brownish-black
smoky quartz
crystals commonly
6-sided prisms
with pyramid-like
ends;
_chalcedony_ and
_chert_ may be
black, too
Tourmaline White Black variety is 7-7½
_schorl_; notice
hardness,
striations on
crystals, and
triangular cross
section of some
crystals
C. Cannot be scratched by quartz
Tourmaline White Black variety is 7-7½
_schorl_; notice
hardness,
striations on
crystals, and
triangular cross
section of some
crystals

11. NONMETALLIC luster, GREEN color