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

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.

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.

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 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.

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 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.

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.