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.
Greenish, silky [amphibole asbestos] from northeastern Gillespie County, Texas.
The [asbestos] occurs in veins in [Precambrian] [metamorphic rocks] in southern Llano County, northwestern Blanco County, and northeastern Gillespie County. These deposits are small.
A variety of the mineral [serpentine] called [chrysotile] [asbestos] is the kind most used by industry. Its fibers are commonly flexible enough and strong enough to be woven into cloth. This cloth is made into articles, such as fireproof suits, gloves, and theater curtains. Some chrysotile has been found in [Precambrian] [metamorphic] rocks in northwestern Blanco County, but it does not break into fibers fine enough or flexible enough to be called asbestos.
Azurite. See [Copper Minerals].
Barite
[Barite], barium sulfate, is a fairly common mineral in Texas. It has a glassy or a pearly luster, and it is [transparent] to [translucent]. Barite is colorless, white, brownish, bluish, yellowish, or reddish. When rubbed across a [streak] plate, it gives a white streak. It is not extremely hard—you can scratch it with a pocket knife, although not with a fingernail.
[Barite] is distinctive because of its weight and [cleavage]. It cleaves in three directions, and some [cleavage fragments] are flat or platy. For a mineral with a nonmetallic luster, barite is heavy—it has a [specific gravity] of 4.5.
[Barite] [cleavage fragment] from west Texas.
[Barite] commonly occurs as prism-shaped and as flat crystals, as [granular] masses, as cleavable masses, and as rounded masses called [nodules]. In Texas, some of it was deposited in [sedimentary rocks] by underground waters. As the waters seeped through these rocks, mineral matter came out of solution to form the barite. Some of the barite in Texas also formed from solutions that came from hot magmas.
A number of [barite] deposits have been found in Texas, but many of them are small. Barite occurs in [Precambrian] [metamorphic rocks] in Gillespie and Llano counties, in [Pennsylvanian] [shale] in Brewster County, in [Permian] shales in Baylor and Taylor counties, and in Permian limestones in Culberson County. It is found in Triassic red shales in Howard County and in [Cretaceous] [sedimentary rocks] in Brewster, Brown, Hudspeth, Jeff Davis, Kinney, and Val Verde counties. In Live Oak County, barite occurs in [Tertiary] bentonitic clays. Barite is being mined from a deposit in the Seven Heart Gap area northeast of Van Horn in Culberson County.
[Barite] is used in a number of ways. It is a source of barium chemicals, and it also is powdered and used as an ingredient in paint. The oil industry uses large amounts of barite. In drilling for oil by the rotary method, water and muds are pumped down the hole to aid drilling. Barite is added to these drilling [fluids] to make them heavy, since high-pressure gases are not as likely to blow heavy fluids out of the hole.
Basalt
[Basalt] is a heavy [igneous] rock that is black, dark gray, or dark brown. This rock is made up chiefly of a [feldspar] mineral, such as labradorite, and a pyroxene mineral, such as augite. Other minerals may be present.
The mineral grains of some basalts are so small that you cannot distinguish them even with a magnifying glass. Other basalts, however, are porphyritic, which means that they contain larger, easily seen crystals and grains of [feldspar] and pyroxene scattered either through a mass of the small mineral grains or through glassy material.
Some basalts contain many small holes. These holes, called vesicles, were formed when bubbles of gas were trapped in the hardening [magma]. Later, solutions moving through the rocks may have deposited another mineral—such as [calcite] or [chalcedony]—in some of the vesicles.
[Basalt] forms from molten rock material that hardens either on or beneath the surface—it can be [extrusive] or [intrusive]. Much of the basalt now found in the [Trans-Pecos] country of west Texas formed from [lava] that flowed out onto the surface during the [Tertiary] [Period]. A few of the places where basalt occurs in west Texas are the Chinati Mountains of Presidio County, the Chisos Mountains of Brewster County, the Davis Mountains of Jeff Davis County, and the Van Horn Mountains of Culberson and Hudspeth counties.
[Basalt] from Brewster County, Texas.
Several varieties of [basalt] occur in the Balcones [fault] region of Bandera, Comal, Hays, Kinney, Medina, Travis, and Uvalde counties. These basalts formed from molten [magma] that forced its way into rocks just below the earth’s surface.
Some [basalt], which is known commercially as trap rock, is produced in Uvalde County. It is crushed and used for railroad ballast, road building material, and as concrete aggregate.
Bentonite. See [Clay].
Biotite. See [Mica].
Braunite. See [Manganese Minerals].
Calcite
[Calcite], calcium carbonate, is one of the most abundant minerals in Texas. It is the chief mineral in [limestone] and in some [marble]. It also serves as the cementing material in many sandstones. Crystals, grains, and cleavable masses of calcite, which have been deposited by underground water, occur in cracks and cavities in many of the [igneous], [metamorphic], and [sedimentary rocks] of Texas. Calcite also occurs as cave, spring, and stream deposits and as [caliche].
[Calcite] is [transparent] or [translucent], and—depending on the variety—its luster is glassy to earthy. Most calcite is white or colorless, but it can be a shade of pink, blue, green, brown, yellow, or gray. It gives a white [streak] when you rub it across a streak plate.
Two properties of [calcite] to notice are the hardness and the [cleavage]. This mineral cleaves perfectly in three directions that are not at right angles to each other, and some of the [cleavage fragments] are rhombohedrons. Calcite is rather soft—you can scratch it with a copper penny but not with a fingernail. A drop or two of dilute hydrochloric acid also will help you to identify this mineral. The acid will readily fizz and bubble when it is placed on calcite.
[Calcite] has perfect rhombohedral [cleavage]. The three directions of cleavage are not at right angles to each other.
[Calcite] occurs in more different kinds of crystal shapes than any other mineral. Some of these crystals are flat and tabular; some are rhombohedrons; some are prisms. Pointed crystals, called dog-tooth spar, and twinned crystals have been found in the Terlingua area of Brewster County in west Texas. A somewhat unusual occurrence of calcite crystals is in geodes. Some of these are found in Lower [Cretaceous] rocks west of Austin in Travis County.
[Transparent] crystals and [cleavage] fragments of [calcite] show a property called double refraction (other minerals show it, too). To test this property, you can mark a single dot on paper. When you look at the dot through a piece of clear calcite, you will see two dots instead of one. This happens because a ray of light is bent (refracted) and is split into two rays as it enters the mineral. These two rays travel through the calcite in slightly different directions, and each carries an image of the dot through the mineral. The two images that you see are at the points where the two rays leave the calcite.
[Calcite] that is deposited at springs, along river and creek banks, and in caves is known as [travertine]. Cave forms of travertine, including stalagmites and stalactites, occur in several caves in Texas. Another kind of travertine is called calcareous tufa or calcareous sinter. It is a porous and spongy-looking material deposited from water carrying dissolved [limestone] and is found around the openings of some springs and along some creek and river banks.
A dull, earthy [calcite] deposit, known as [caliche], occurs in areas of Texas that have scant rainfall, such as the [High Plains], west Texas, and the southwestern part of the [Gulf Coastal Plain]. Caliche commonly is found mixed with other materials, such as [clay], [sand], or [gravel]. This substance may be firm and compact or loose and powdery.
It is thought that [caliche] forms when ground moisture, containing dissolved calcium bicarbonate, moves upward. In dry areas of the country, this moisture evaporates. As it does, it leaves a crust of calcium carbonate in the form of caliche on or near the surface of the ground.
[Calcite] crystals (dog-tooth spar) from the Terlingua area of Brewster County, Texas.
[Caliche] is quarried in many counties in Texas and is used chiefly as road material and as an aggregate.
Caliche. See [Calcite].
Carnotite. See [Uranium Minerals].
Cassiterite
[Cassiterite], tin dioxide, is the mineral that serves as the chief source of tin. Tin does not corrode and tarnish, and one of its main uses is in the making of tin cans. (Actually, our tin cans are made from thin sheets of steel that have been coated with a protective layer of tin.)
[Cassiterite] has either a nonmetallic or a submetallic luster. Some specimens are brilliant and shiny; others are dull. Cassiterite may be [translucent] to [transparent]. It may be black, brown, gray, reddish brown, or yellowish brown. When rubbed across a [streak] plate, this mineral leaves a pale brown, a pale yellow, or a white streak. Cassiterite is quite heavy—it has a [specific gravity] of 6.8 to 7.1. It is too hard to be scratched by an average pocket knife.
Sometimes, prospectors use a chemical test to help them identify [cassiterite]. They put small pieces of metallic zinc into a jar or test tube containing dilute hydrochloric acid. Then they add a few fragments of the mineral that they suspect is cassiterite. If the fragments are cassiterite, they become covered with a pale gray coating of metallic tin.
[Cassiterite]’s most common crystal shape is a short, 8-sided prism with pyramids at each end, but perfect crystals are not often found. Most Texas cassiterite does not show a crystal shape. Instead, it occurs as [crystalline] masses in [igneous rocks] and as loose [pebbles] that have weathered out of these rocks.
[Cassiterite] occurs in a number of places in the United States but not in large quantities. A small amount of cassiterite has been found in [quartz] veins in [Precambrian] [granite] in both central Texas and west Texas. In El Paso County, the cassiterite is found on the east side of the Franklin Mountains a few miles north of El Paso, where some of it has been mined. In central Texas, cassiterite occurs in the Streeter area of Mason County.
When the [granite] rocks in these areas were formed, probably not all of the hot magmas cooled and hardened at the same time. The [fluids] given off by the remaining magmas contained tin and several other [elements]. It is believed that these fluids moved up into cracks in the granite rocks and formed the [cassiterite].
Celestite
[Celestite] is a strontium sulfate mineral. It is colorless, white, yellow, or gray. Light blue specimens of this mineral also are found, and it is because of this sky-like color that celestite gets its name. The word celestite comes from the Latin word caelestis, meaning of the sky.
[Celestite] has a glassy to a pearly luster, and it is either [transparent] or [translucent]. It gives a white [streak] when rubbed across a streak plate. Celestite has a [specific gravity] of 3.95 to 3.97. It is, however, lighter than [barite], a mineral that it resembles. Celestite is not very hard—a knife will scratch it, although your fingernail will not. It cleaves in three directions, and some of the fragments are flat and slabby.
[Celestite] [cleavage fragment] from Lampasas County, Texas.
[Celestite] occurs commonly either as prism-shaped or flat crystals and as cleavable, [granular], or fibrous [crystalline] masses. In Texas, it is found in geodes, as rounded [nodules], or as bedded or layer-like deposits in limestones and other [sedimentary rocks]. In Real County, celestite occurs on the walls of a cave in [Cretaceous] [limestone].
Some [celestite] may be deposited by sea water, but much of the Texas celestite is believed to have been deposited by underground water that seeped through cracks and pores in the limestones and other [sedimentary rocks]. This water picked up and dissolved strontium compounds that were scattered in small amounts through the rocks. Then, it re-deposited the strontium in the rocks as celestite.
In Texas, beds of [celestite] occur in [Permian] rocks in Coke, Fisher, and Nolan counties and in Lower [Cretaceous] rocks in Brown, Comanche, and Mills counties. Celestite geodes and [nodules] are found in Lower Cretaceous [limestone] rocks in Lampasas, Travis, and Williamson counties, and in Permian rocks in Coke, Fisher, Nolan, and Taylor counties.
[Celestite] is one of two minerals (the other mineral is strontianite, strontium carbonate) used as a source of strontium. Strontium compounds give a crimson-red color to a flame, so they are used in fireworks, tracer bullets, and flares. Perhaps you have seen a red flare set out on the highway at night to warn motorists that a truck has stalled. The chances are good that the flare’s red flame was due to a strontium compound. Some of the Texas celestite has been mined, but most of the strontium minerals now used in the United States are imported from England and Mexico.
Cerargyrite. See [Silver Minerals].
Chalcedony. See [Quartz].
Chalcocite. See [Copper Minerals].
Chalcopyrite. See [Copper Minerals].
Chalk. See [Limestone].
Chert ([Flint]). See [Quartz].
Chrysotile. See [Asbestos]; [Serpentine].
Cinnabar
[Cinnabar], which is mercuric sulfide, is the most common mercury mineral. It has a dark red or a bright yellowish-red color and is [transparent] to [translucent]. When rubbed across a [streak] plate, it leaves a dark red streak. If pure, cinnabar has a brilliant, shiny, nonmetallic luster. It is, however, commonly found mixed with impurities, such as [clay], [calcite], iron oxide, or bituminous material, and then it looks dull and earthy. Cinnabar is quite heavy—it has a [specific gravity] of 8.10. It is rather soft, and you can scratch it with a copper penny.
Some prospectors use a quick chemical test to identify [cinnabar]. They rub a clean, shiny copper coin with a mineral sample that has been moistened with a drop or two of dilute hydrochloric acid. If the sample is cinnabar, a light silvery-gray coating appears on the coin.
[Cinnabar] occurs as small crystals or as fine-grained or compact [crystalline] masses. It is found in veins that fill cracks in rocks and also occurs as crusts and coatings on rocks. It also may be widely scattered through rocks, such as limestones.
[Cinnabar] occurs in the Terlingua area of Brewster and Presidio counties in west Texas. It has been mined there, off and on, since about 1894, and during this time, mercury worth many millions of dollars has been produced.
Most of this west Texas [cinnabar] is found in cracks, pores, and breccia-filled cavities of [Cretaceous] limestones and clays. If you will look at the Texas [geologic map] (pp. [4]-5), you will see that [igneous rocks] occur in this district. Many millions of years ago during the [Tertiary] [Period], when these igneous rocks were still hot [magma], some of them pushed up under the Cretaceous rocks and emitted [fluids] containing mercury. The fluids moved upward through cracks and pores in the Cretaceous rocks where they deposited the mercury as cinnabar and as other mercury minerals.
[Cinnabar] crystals (dark) with [calcite] crystals (white) from the Terlingua area of Brewster County, Texas.
Mercury is an unusual [element]. Instead of occurring as a solid metal at ordinary room temperatures, as do [gold], silver, and lead, it remains a liquid until it is cooled to 38 degrees below zero Fahrenheit. Because the silvery little drops of liquid mercury roll about as if they were alive, this element long has been called quicksilver.
Mercury is used in a variety of ways. In some noiseless light-switches, a glass tube containing a small ball of mercury tilts when the switch is turned “on.” The mercury then rolls to the end of the tube that contains electrical contacts and quietly completes the electrical circuit. In other uses, mercury is added to silver, tin, and other metals to make fillings for teeth. Some medicines, such as calomel and mercurochrome, contain mercury. Fulminate of mercury helps to set off dynamite and other explosives. Mercury is used in many barometers and thermometers, and farmers use mercury poisons to control insects and fungi.
Mercury also commonly is used to obtain [gold] from its ores. One method of accomplishing this is to pass wet gold-bearing [gravel] or crushed rock over metal plates that are coated with mercury. The gold particles quickly mix with the mercury to form an amalgam, which later can be scraped off the plates. The gold is then recovered by heating the amalgam to drive off the mercury.
Clay
[Clay] is a smooth, soft, earthy rock made up of mineral particles no bigger than specks of dust. Some of the particles are clay minerals, which consist of aluminum, silicon, and other [elements]. In addition, tiny particles of [quartz], [calcite], and other minerals may also be present in the clay.
The [clay] particles are all that remain of rocks and of minerals, such as [feldspar], that have been broken into fragments or altered into clay minerals by weathering. Some clay remains at the place where it formed, but some is carried away and deposited elsewhere.
[Clay] is white, tan, brown, red, green, blue, gray—almost any color. When moist, it has an earthy odor. You can moisten a piece of clay enough to notice this just by breathing on it. Most clays, when wet, can be molded into many different shapes—that is, they are plastic, but when they are dry, they are firm and solid.
[Clay] is abundant in Texas and has a number of uses. Some goes to make portland cement, and some is baked or burned in a kiln to make brick, tile, sewer pipes, pottery, and other products. This kind of clay is obtained from [Tertiary] [formations] of the [Gulf Coastal Plain], from Upper [Cretaceous] formations in central Texas, and from [Pennsylvanian] formations in north-central Texas. (You can locate Tertiary, Cretaceous, and Pennsylvanian rocks on the Texas [geologic map], pp. [4]-5.)
A special kind of white burning [clay] that can be used to make chinaware is called [kaolin] or china clay. It contains particles of the clay mineral kaolinite as well as several other clay minerals. Deposits of china clay occur in southern Jeff Davis County and in Real County near Leakey, but none is being produced.
Another kind of [clay], [bentonite], forms from weathered [volcanic ash]. Bentonite contains the clay mineral montmorillonite and looks smooth and soap-like. Fresh samples of this clay are white, pale green, or pale blue, but dried-out or weathered samples are tan, brown, yellow, or reddish. When wet, bentonite absorbs water, swells, and then has a jelly-like appearance.
Surface deposits of [bentonite] occur chiefly in [Eocene] [Tertiary] [formations] of the [Gulf Coastal Plain], in [Cretaceous] formations of the Big Bend area of west Texas, and in [Quaternary] formations of the High Plains.
[Bentonite] is used as a drilling-fluid additive in the rotary method of drilling for petroleum and gas.
Some [bentonite] is used to absorb unwanted coloring material in petroleum and in vegetable oils. It is then known as a bleaching [clay]. Bentonite bleaching clay is obtained from some of the [Tertiary] [formations] along the Texas [Gulf Coastal Plain]. It has been produced in Angelina, Fayette, Gonzales, [Jasper], Walker, and other counties in this area.
Another important use of [bentonite], and of other [clay], too, is as drilling mud. In the rotary method of drilling for oil and gas, mud is pumped down into the drilled hole. This mud carries the rock cuttings up to the surface, it cools the drilling tools, and it coats and seals the walls of the hole. Along the [Gulf Coastal Plain], drilling clay is obtained from [Tertiary] [formations].
Common Opal. See [Opal].
Copper Minerals ([Chalcocite], [Chalcopyrite], [Malachite], [Azurite])
A number of minerals containing copper, such as [chalcocite], [chalcopyrite], [malachite], and [azurite], occur in small deposits in Texas. They are found chiefly in the [Llano uplift] area of central Texas, in the Van Horn area of Culberson and Hudspeth counties in west Texas, and in a group of counties in north-central Texas.
Copper is an important [element]. Because it is an unusually good conductor of electricity (only silver, which costs much more, is a better one), it is used for many kinds of wires for switchboards, generators, motors, telephone and telegraph equipment, and light and power lines.
Manufacturers commonly combine copper with other [elements]. For example, some copper is mixed with zinc to make brass and with tin and a little zinc to make bronze. These mixtures are called alloys. Many products are made from copper alloys, including tubing, pipes, jewelry, pots, and pans. Even our coins contain copper.
Sometimes, a prospector uses a chemical test to find out if copper is present in a mineral. First, he crushes a small sample of what he believes is a copper mineral (such as [chalcocite], [chalcopyrite], [azurite], or [malachite]). He then puts the sample in a glass jar or test tube and pours in a small amount of dilute nitric acid (this acid, like hydrochloric acid, is poisonous). After the sample has dissolved in the acid, he adds enough ammonium hydroxide to make the solution alkaline. If the sample is a copper mineral, the solution turns a deep-blue color.
One of the [copper minerals], [chalcocite], copper sulfide, also is known as copper glance. It is a metallic mineral that commonly tarnishes to a dull black. By chipping off a fragment to obtain a fresh surface, you will see that it has a shiny lead-gray color. Chalcocite is rather soft, and it is [sectile], that is, a knife will cut through it as well as scratch it. When you rub chalcocite across a [streak] plate, it gives a grayish-black streak. This mineral commonly occurs as compact masses or as [granular] masses.
[Chalcocite], with its dark color, does not look at all like copper, which is a bright reddish brown. Chalcocite, however, is the chief copper mineral at the most important copper mine in Texas, the Hazel mine, which is about 15 miles northwest of Van Horn in Culberson County in west Texas. This mine, although now idle and almost filled with water, has produced about one and a half million pounds of copper along with more valuable silver ores. Here, the chalcocite and other minerals occur in material that fills large cracks in red [sandstone] of the [Precambrian] Hazel [Formation]. It is thought that long ago, molten [igneous] rock material far below the surface sent out hot solutions containing copper and other [elements]. These solutions moved upward and deposited minerals in the [fracture] zone in the sandstone.
[Chalcocite] occurs also in north-central Texas. It is found in Archer, Baylor, [Clay], Foard, Hardeman, King, Knox, Stonewall, and several other counties of this area. Here, it occurs in [Permian] [sedimentary] rocks (called “red beds”) as rounded masses, as scattered grains, and as petrified wood. Because these deposits are far from any [igneous] rocks, they apparently did not form in the same way as those at the Hazel mine. These north-central Texas deposits have never really been commercially developed. During the Civil War, however, some copper from this area was made into percussion caps for the Confederacy.
The Hazel copper-silver mine, Culberson County, Texas, as it appeared in 1951. Photograph by P. T. Flawn.
Another copper mineral, [chalcopyrite], is a copper-iron sulfide. It also is known as copper pyrites and yellow copper ore. This mineral has a metallic luster and a brass-yellow or a golden-yellow color. When rubbed across a [streak] plate, it gives a greenish-black streak. Chalcopyrite will tarnish and then has bronze, blue, purple, and other rainbow-like colors. This mineral is fairly soft—you can scratch it with a pocket knife. Because of chalcopyrite’s yellow color, it has often been mistaken for [gold]. For this reason, it, like iron [pyrite], is often called fool’s gold. (See Gold, [p. 60], for ways to tell them apart.)
[Chalcopyrite] commonly is found in compact masses that show no crystal shapes. These masses either are scattered through rocks or occur in material that fills cracks in rocks.
Some [chalcopyrite] is found in [Precambrian] [sandstone] at the Hazel mine and in other deposits in the Van Horn area of Culberson and Hudspeth counties. It also occurs in Precambrian rocks at the Sheridan and Pavitte prospects in Burnet County. These chalcopyrite localities are in districts where [igneous rocks] occur.
It is likely that, long ago, hot solutions containing copper moved upward, out of deeply buried molten [magma]. While still far below the surface, the solutions deposited the [chalcopyrite] in cracks and other openings in the nearby rocks.
Two [copper minerals] of Texas, [azurite] and [malachite], are copper carbonates. Azurite is commonly called chessylite and blue copper; malachite is called green copper carbonate. Because these minerals are carbonates, a drop of dilute hydrochloric acid will fizz and bubble when placed on either of them.
[Azurite] has a bright, intense blue color and leaves a blue [streak] when rubbed across a streak plate. [Malachite] has a bright green color and leaves a green streak. These minerals have a nonmetallic luster and a glassy to dull appearance. Commonly, they are [translucent], although some specimens of azurite are [transparent]. Both azurite and malachite are fairly soft—a pocket knife will scratch them, but a copper penny will not.
[Azurite] and [malachite] occur as individual crystals, but you are more likely to find them as crusts on rocks and on other minerals. Malachite is also found in rounded fibrous masses that resemble bunches of grapes (described then as botryoidal).
Both [azurite] and [malachite] are formed in the same way. Underground waters seep through rocks that contain deposits of [copper minerals] (such as [chalcocite] and [chalcopyrite]) and cause chemical reactions which change these minerals into malachite and azurite.
[Malachite] is more plentiful than [azurite], but both minerals can be found together. You can expect to find at least one of them at the same localities where [chalcocite], [chalcopyrite], and other [copper minerals] occur.
Coquina. See [Limestone].
Diatomite. See [Opal].
Dolomite
[Dolomite] is the name given both to a rock and to a mineral. The mineral is a calcium-magnesium carbonate and has a glassy or a pearly luster. It is any of a number of colors, such as white, pink, brown, or gray, or it can be colorless. Dolomite leaves a white [streak] on a streak plate and is [transparent] to [translucent]. It is not particularly hard and can be scratched with a pocket knife, although not with a copper penny. Dolomite cleaves perfectly in three directions, and some of the [cleavage fragments] are rhombohedrons. However, the [cleavages] of the individual mineral grains in specimens of fine-grained [massive] dolomite are not readily distinguishable.
[Dolomite] rock from the vicinity of Fairland, Burnet County, Texas.
Most Texas [dolomite] occurs as coarse-, medium-, and fine-grained [crystalline] masses as the chief mineral in dolomite rock and in dolomitic [marble]. It is also found as 6-sided crystals that are rhomb-shaped; when the faces are curved, they have a saddle-like appearance.
Crystals of the mineral [dolomite] commonly occur in cavities in the dolomite rocks. It is believed that they were deposited there by seeping underground waters. The waters dissolved some of the dolomite in the rocks and then re-deposited it as crystals.
[Dolomite] rock is made up mostly of [crystalline] grains of the mineral dolomite. In addition, [quartz] grains, [calcite], and other minerals may be present. Dolomite rock is almost any color—white, buff, pink brown, gray. It resembles some [limestone], and these two rocks actually are closely related.
To help tell them apart, dilute hydrochloric acid often is used. A few drops of this acid will readily fizz and bubble if the rock you put them on is a [limestone]. If the rock is [dolomite], the acid will effervesce only very little or not at all. (If, however, the acid is put on powdered dolomite, it then will fizz readily.) Dolomite is slightly harder than limestone, and it also is slightly heavier.
Some [dolomite] rocks formed directly from materials that were dissolved in sea water, and others are altered [limestone] rocks. Some limestones altered into dolomite on the sea floor by the addition of magnesium from the sea water. Others changed into dolomite much later after the sea had withdrawn and the limestones had become a part of the land; underground waters containing magnesium seeped through these limestones and altered them into dolomite.
Many of the [dolomite] rocks are found with limestones. In Texas they occur mostly in [Cambrian], [Ordovician], [Mississippian], [Pennsylvanian], [Permian], and [Cretaceous] [formations]. The [geologic map] (pp. [4]-5) indicates where these strata appear at the surface in Texas.
[Dolomite] is abundant in the Llano uplift area of central Texas—particularly in the [Cambrian] and [Ordovician] rocks. A number of these central Texas dolomites have been quarried for use as building stones. Some of them also have been crushed and used as a road-building material and as a stone aggregate that is mixed with cement to make concrete. This dolomite is also used as terrazzo chips (terrazzo floors are described with [serpentine] on [p. 88]). In addition, Ellenburger (Ordovician) dolomite from Burnet County was used during World War II as a source of the lightweight metal magnesium.
Dravite. See [Tourmaline].
Feldspar
[Feldspar] is the name given to a group of nonmetallic minerals that are much alike. Several of them are so similar that a petrographic microscope must be used to tell them apart. Each of the feldspar minerals is an aluminum silicate. Each of them contains, in addition, at least one of the following [elements]: potassium, sodium, calcium, and barium. The feldspar minerals that are found in Texas include [albite], a sodium-aluminum silicate, and [orthoclase] and [microcline], which are both potassium-aluminum silicates.
The [feldspar] minerals are [transparent] to [translucent] and have either glassy or pearly lusters. They can be white, cream, or a shade of red, brown, yellow, blue, gray, or green. When you rub a feldspar across a [streak] plate, it leaves a white streak. The feldspars are rather hard—a pocket knife will not scratch them, although a piece of [quartz] or a steel file will. These minerals have good [cleavage] in two directions. The cleavages meet at an angle of about 90°, so that the [cleavage fragments] have square corners.
[Feldspar] [cleavage fragment] from Llano County, Texas. The two directions of good [cleavage] meet at an angle of about 90°.
The feldspars are important rock-forming minerals. You can find them in [igneous rocks], such as [granite] or [pegmatite], and in [metamorphic rocks], such as [gneiss]. They also occur as fragments in [sedimentary rocks], such as some [sandstone] and [conglomerate].
Although the feldspars can originate in other ways, they form mostly from hot magmas that cool and crystallize into [igneous] rocks. These minerals occur in the rocks as grains, as cleavable masses, and as individual crystals. The crystals may be shaped like prisms, or they may be flat and slabby.
Good places to look for feldspars are in areas where granites, pegmatites, and other [intrusive] [igneous rocks] appear at the surface. The [pegmatite] rocks of Burnet, Gillespie, Llano, and Mason counties in the [Llano uplift] area of central Texas, and those of the Van Horn Mountains in Hudspeth and Culberson counties in west Texas, are especially good sources of [feldspar]. Large cleavable masses and crystals that are more than a foot long are found in some of these rocks.
The feldspars have a number of uses. Some of the [pegmatite] feldspars from Llano County in central Texas have been crushed and used as [granules] for built-up and composition roofs. In addition, some have been shipped to Mexico for glass-making. Some of the other uses of [feldspar] are in making porcelain, ceramic glazes, and scouring compounds. A few of the feldspar minerals, such as the variety of [microcline] known as amazonstone, are used as gemstones.
[Microcline] [feldspar] crystals from near [Granite] Shoals Lake, Llano County, Texas.
Fibrous Gypsum. See [Gypsum].
Flint. See [Quartz].
Fluorite
[Fluorite] is calcium fluoride. The fluorite that is mined and sent to market, however, commonly is found mixed with [quartz], [calcite], [limestone], or other rocks and minerals. Industry calls this mixture fluorspar.
[Fluorite] is a [transparent] to [translucent] mineral that has a glassy luster. It may be colorless, or it may be white, pink, green, purple, brown, or blue. Some specimens show more than one color. When you rub fluorite across a [streak] plate, it leaves a white streak. This mineral is not particularly hard—a pocket knife will scratch it, although a copper penny will not. Fluorite has perfect [cleavage] in four directions. By carefully breaking a specimen, you can obtain [cleavage fragments] that are shaped like [octahedrons].
[Fluorite] occurs as cleavable masses, as fine or coarse grains, and as crystals. Most of the crystals are [cubes], but some may be [octahedrons], [dodecahedrons], or combinations of these.
[Fluorite] has been found both in west Texas and in central Texas. In the [Llano uplift] area of central Texas, it occurs in a number of [Precambrian] [granite], [pegmatite], [schist], and [gneiss] rocks. The most important, although small, deposit in this area is near Spring Creek a few miles west of Burnet in Burnet County. Here, prospectors have dug holes and pits in gneiss and schist rocks and found layers of fluorite in them.
The largest known [fluorite] deposits in Texas (they are not particularly large when you compare them with the deposits in Illinois and Kentucky) are those in the Eagle Mountains of Hudspeth County. This fluorite occurs in both [igneous] and [sedimentary] rocks. Many years ago, probably during the late part of the [Tertiary] [Period], hot [magma] far below the surface gave off liquids and gases containing fluorine. These [fluids] moved up through large cracks (called [faults]) in [Cretaceous] limestones and Tertiary [igneous rocks] and deposited fluorite in them. In places, beds of [limestone] have been replaced by fluorite. Some of this west Texas fluorite has been mined and shipped to market.
[Fluorite] has octahedral [cleavage]. The four directions of perfect cleavage can result in [cleavage fragments] that are [octahedrons].
[Fluorite] is extremely important as a flux in steel-making to help the ingredients of the molten steel blend together. In addition, it combines with [sulfur], phosphorus, and other unwanted substances so that they can be removed from the steel. Other important uses of fluorite are in glass-making and in the manufacture of hydrofluoric acid. This acid is used in the aluminum industry as well as in industries that make high-octane gasoline, insecticides, and refrigerants for refrigerators and freezers.
Galena
[Galena], lead sulfide, is a shiny, lead-gray, metallic mineral that has a [specific gravity] of 7.4 to 7.6. It is soft enough to mark paper, and it leaves a grayish-black [streak] on a streak plate. This mineral cleaves perfectly in three directions, and the [cleavage fragments] have square corners—some are [cubes].
[Galena] occurs as cleavable masses, as fine or coarse grains, and as crystals, most of which are [cubes]. Galena commonly is associated with other minerals; for example, some of the west Texas galena either contains some silver (then called argentiferous galena) or occurs with it. Sphalerite, a zinc mineral, is commonly found with galena.
[Galena] is an important mineral because it is the chief source of lead. Compounds of lead, called white lead, red lead, and litharge, are used as paint pigments. Automobile batteries contain lead plates, and tetraethyl lead is added to gasoline to keep the car’s motor from knocking. Some other uses of lead are in bullets, type metal, solder, and cable coverings.
[Galena] has perfect cubic [cleavage]. The three directions of cleavage are at right angles to each other resulting in cubic cleavage fragments.
[Galena] has been found in several areas of Texas and has been mined in central and west Texas. None, however, has been produced in [recent] years. Most of the galena mined in west Texas was obtained from silver mines, where the galena was a by-product. Some of the west Texas galena deposits are at Altuda Mountain east of Alpine in Brewster County, in the Eagle Mountains and the Quitman Mountains in Hudspeth County, and in the Chinati Mountains and the Shafter area in Presidio County. Most of the mining has been from the Shafter area (this area is described with [silver minerals] on [p. 90]).
In central Texas, several small [galena] deposits have been found in Blanco, Burnet, and other counties of the [Llano uplift] area. Some galena has been mined at Silver Creek in northwestern Burnet County. Here, galena occurs in cracks and as scattered grains in [Cambrian] limestones and sandstones.
It is probable that much of the [galena] in west Texas and in central Texas was formed when hot [magma] forced out solutions containing lead. These solutions moved up through cracks and other openings in the subsurface rocks and deposited the galena in them.
Small amounts of [galena], which likely had a different origin, have been found in Fisher, Foard, Hardeman, and Young counties. A little occurs also in rocks associated with [salt] in a number of the [Gulf Coastal Plain] salt domes.
Garnet
[Garnet] is not one mineral but is the name given to a group of several minerals that are very much alike. In fact, it often is impossible to tell some of them apart without using special laboratory tests.
[Garnet] crystal forms include: A, trapezohedron; B, [dodecahedron]; C and D, combination trapezohedron and dodecahedron.
The [garnet] minerals have glassy to resinous lusters and are [transparent] or [translucent]. A pocket knife will not scratch them, and some specimens are too hard even for [quartz] to scratch. Two of the garnet minerals most commonly found in Texas are [almandite], an iron-aluminum silicate, and [grossularite], a calcium-aluminium silicate. Almandite has a deep-red or a brownish-red color. Grossularite is pale green, brownish yellow, cinnamon brown, or rose red.
[Garnet] minerals occur as crystals and as masses that are scattered through some of the [metamorphic] and [igneous] rocks. After they have weathered out of these rocks, the garnets make up a part of many sands and sandstones. Because these minerals so commonly occur as crystals, it is helpful to learn to recognize the crystal shapes.
[Garnet] minerals are found in the [igneous] and [metamorphic rocks] of both central Texas and west Texas. In central Texas, they occur in ancient [Precambrian] [schist] and [pegmatite] rocks of the Llano uplift area. Some of these central Texas garnet localities are in northeastern Mason County, central and northwestern Llano County, west-central Burnet County, and northeastern Gillespie County.
In west Texas, garnets occur in [metamorphic rocks] in the Quitman Mountains, which are southwest of Sierra Blanca in Hudspeth County, and in the [Mica] Mine area, which is south of Van Horn near the Hudspeth-Culberson County line. Garnets also have been found in [igneous rocks] in the Franklin Mountains a few miles north of El Paso in El Paso County.
Garnets that are found in [metamorphic rocks] such as schists were formed when great forces squeezed and heated rocks far below the earth’s surface. This heat and pressure caused [elements] in the rocks to join together into different combinations to form new minerals, such as garnets. Garnets that occur scattered through [igneous] rocks, such as some pegmatites and granites, cooled and crystallized from hot, igneous [fluids] when the rocks themselves formed.
Most Texas garnets are not [transparent]. A few, however, are clear enough to be used as gemstones. These can be cut, polished, and mounted in rings, brooches, bracelets, and earrings. Although some [garnet] is widely used as an abrasive, none from Texas has been produced for this purpose.
Gneiss
[Gneiss] is a [metamorphic rock] that has parallel layers or bands. Some gneiss is made up of the same minerals (chiefly [feldspar] and [quartz]) as [granite], and it is then called granite gneiss. Several of the other kinds of gneiss are known as [mica] gneiss, [conglomerate] gneiss, gabbro gneiss, and hornblende gneiss. In order to be a gneiss, a metamorphic rock has to have bands or layers. These bands may be either straight or wavy and either wide or narrow. In most gneisses, you will find a layer made up of long or flat mineral grains next to a layer made up of the grains of an entirely different mineral. The bands may show color differences, too. For example, a pink layer made up of feldspar grains may be found next to a black layer made up of hornblende grains. The mineral grains interlock as they do in [igneous rocks], and they are generally large enough to be seen without a magnifying glass.
[Gneiss] from Blanco County, Texas, showing light and dark bands.
[Gneiss] can form from an [igneous] rock, such as [granite], or from a [sedimentary] rock, such as [sandstone]. Heat, [fluids], and pressures below the earth’s surface change these rocks into gneiss.
[Gneiss] that formed during [Precambrian] time is now seen at the surface in both west Texas and central Texas. In west Texas, it occurs principally in the Van Horn area of Culberson and Hudspeth counties. In central Texas, it is found in Blanco, Burnet, Gillespie, Llano, and Mason counties of the [Llano uplift] area.
One of the [Llano uplift] rocks is called the Valley Spring [Gneiss]. It generally has a light color (much of it is pinkish), and it is believed to have once been a [sandstone]. Another gneiss of this area, the Big Branch Gneiss, which has a medium to dark gray color, occurs in northern Gillespie and Blanco counties and is an altered [igneous] rock. Some of the Texas gneiss rocks are suitable for use as building stones.
Gold
[Gold] commonly occurs in nature as a single [element]—gold—but much native gold has a small amount of some other element, such as silver, copper, or iron, mixed with it.
Native [gold] is a shiny, yellow, metallic mineral that does not tarnish, and it leaves a shiny, golden-yellow [streak] when you rub it across a streak plate. If silver is present, the color and streak have a lighter shade. Pure gold is extremely heavy—its [specific gravity] is 19.3. Because it is malleable, this mineral will flatten into a thin sheet when hammered. It is ductile enough to be drawn out into wires. Gold is also soft—a pocket knife will scratch it easily. When it is to be used for ornaments and jewelry, gold is usually mixed with other metal, such as silver, copper, nickel, or palladium, to make it harder. The amount of gold that is present is then indicated by carats (or karats). Pure gold is 24 carats. If you have a gold ring that has 14 K stamped inside it, you know that it is made of a mixture of 14 parts gold and 10 parts of other metal.
[Gold] commonly occurs in nature as plates, scales, or grains. Some of the grains are large enough to be called nuggets. It also is found in a wire-like shape described as filiform, it occurs in a network, called reticulate, and it can have a branching and fern-like shape, described as dendritic. Gold is not often found as individual crystals.
Several other minerals, such as [pyrite], [chalcopyrite], and [mica], are sometimes mistaken for [gold]. None of these, however, is malleable and ductile, and none is nearly as heavy as gold. Pyrite and chalcopyrite have dark-colored streaks unlike that of gold. Mica cleaves so perfectly that it can be split into thin, flat sheets, but gold has no [cleavage] at all.
The best places to look for [gold] are in areas near [igneous rocks] and along the creeks and rivers that drain these areas. It is thought that most gold originally was carried up from molten [igneous] rock by hot solutions. The solutions moved into cracks and other openings in nearby rocks and deposited the gold, commonly along with [quartz]. Later, some of these gold-bearing rocks weathered away. The gold that the rocks contained either remained at the spot or was washed into creeks and rivers. These transported accumulations of loose gold are called placer deposits.
Placer [gold] in very small quantities has been found in some of the stream [gravels] of Texas.
No really important [gold] deposit has ever been found in Texas, although traces and small amounts have been reported in several areas. A little gold has been found in the [Llano uplift] area of central Texas. It occurs in [quartz] veinlets that cut through some of the [Precambrian] [metamorphic rocks] of Llano, Mason, northeastern Gillespie, and west-central Burnet counties. Many years ago, a small amount of gold was mined northeast of Llano in Llano County from the Heath mine. Some gold also has been found in sands and [gravels] along streams, such as along Sandy Creek and its tributaries, in parts of this Llano uplift area.
In the Trans-Pecos country of west Texas, small amounts of [gold] have been found in the Van Horn area of Culberson and Hudspeth counties, in the Quitman Mountains district of Hudspeth County, and in the country around Shafter in Presidio County. Most of the small quantity of gold that was mined in west Texas was obtained as a by-product from the Presidio mine in the Shafter district (described with [silver minerals] on [p. 90]).
Small amounts of [gold] have been reported from other parts of Texas. Some of these localities are in [Eocene] [Tertiary] sandstones in the [Gulf Coastal Plain], in [Cretaceous] limestones in Irion, Uvalde, and Williamson counties, and in [sand] and [gravel] in Howard and Taylor counties. None of these deposits has been found to have any commercial value.
Granite
[Granite] is an [intrusive] [igneous] rock that is made up chiefly of [crystalline] grains or crystals of [quartz] and a [feldspar] mineral, such as [orthoclase] or [microcline]. Several other minerals, including [mica] and hornblende, may also be present.
All of the mineral grains in [granite] are about the same size, and you can distinguish them without using a magnifying glass. A granite may be coarse grained, medium grained, or fine grained. When you examine this rock, you will see that its grains are not cemented but interlocked like the pieces of a jigsaw puzzle. The color of granite, which is pink, red, gray, or brownish, depends chiefly on the color of its [feldspar] grains.
Most granites formed from hot, molten [magma] that slowly cooled and hardened far below the earth’s surface. Because of this slow cooling, fairly large mineral grains were formed.
Polished section of pink [granite] from Gillespie County, Texas.
Granites are now seen at the surface in several areas of Texas. They were gradually uncovered as the areas became higher and the overlying rocks slowly weathered away. One of these areas is the [Llano uplift] of central Texas where the granites occur in Blanco, Burnet, Gillespie, Llano, and Mason counties. These granites formed during [Precambrian] time and are believed to be about a billion years old. (Scientists are now able to determine the age of some rocks accurately by very precisely measuring the relative amounts of isotopes produced by decay of radioactive minerals.)
Texas State Capitol building at Austin is made of Burnet County [granite] obtained from Granite Mountain near [Marble] Falls, Texas.
Granites also appear at the surface in the Trans-Pecos country of west Texas. Some of these areas include the Franklin Mountains of El Paso County, the Quitman Mountains of Hudspeth County, the Chisos Mountains of Brewster County, and the Chinati Mountains of Presidio County.
Red, pink, and gray granites from quarries in the [Llano uplift] area are widely used as building stones and monument stones. A large quarry at [Granite] Mountain just west of [Marble] Falls in Burnet County has supplied pink granite for buildings in many parts of the United States. The Texas Capitol building and several other State buildings in Austin are made of this granite.
Graphite
[Graphite] is a mineral that is made up of a single [element]—carbon. (Diamond, although it does not look at all like graphite, is a [crystalline] form of carbon.) Graphite is a steel-gray or black mineral that commonly has a metallic luster. It is not heavy and is extremely soft. Graphite will soil your fingers and leave a black mark on paper. This mineral cleaves perfectly in one direction and splits into thin flakes that feel greasy.
To help distinguish [graphite] from molybdenite, a mineral it resembles, you can use a shiny, glazed surface, such as is found on a saucer or a plate, to test its [streak]. When rubbed across this kind of surface, graphite will leave a black streak, but molybdenite will leave a greenish one.
[Graphite] commonly occurs as scales, as sheet-like layers, or as compact masses. It may be found mixed with [clay] or other impurities, and it then looks dull and earthy. Crystals of graphite, which are seldom found, are 6-sided and flat.
[Graphite] occurs in Llano, Burnet, and other counties in the [Llano uplift] area of central Texas. One of the Nation’s most important graphite mines is located in the Clear Creek area several miles northwest of Burnet in Burnet County. Some graphite has also been mined near Lone Grove in Llano County. In addition, a graphite [schist], obtained south of Llano in Llano County, has been used as a filtering material.
[Graphite] is used in pencil lead, generator brushes, and lubricants.
All of this [graphite] occurs in extremely old [Precambrian] graphite [schist] rocks that we now see at the surface in this part of Texas. It is believed that the schists were once ancient [sedimentary rocks], such as shales, which contained organic matter. Long ago, great forces below the earth’s surface altered these rocks. When this happened, the organic material that they contained changed into the mineral we know as graphite.
[Graphite] has a number of uses. It is mixed with [clay] to make the pencil lead that we use for writing. It serves as a lubricant, either alone or mixed with oil, grease, or water. In addition, graphite is used to make generator brushes, stove and shoe polish, and special paints. Because it can stand great heat without melting, some graphite is mixed with clay to make the pots or crucibles that hold molten metals.
Grossularite. See [Garnet].
Gypsite. See [Gypsum].
Gypsum
[Gypsum] is a hydrous calcium sulfate. This mineral is normally colorless or white, but impurities cause it to appear gray, brownish, yellowish, or reddish. It is [transparent] or [translucent] and is not heavy. When you rub gypsum across a [streak] plate, it leaves a white streak. This mineral is so soft that a fingernail scratches it easily. Gypsum occurs in several varieties.
The colorless, glassy, and [transparent] variety of [gypsum] is called [selenite]. It is found as cleavable masses and as crystals that are prism-shaped or flat and diamond-shaped. It is not uncommon for two crystals to be joined together so that they have a swallow-tail shape—these crystals are twinned. Groups of flat selenite crystals arranged together so that they resemble flowers are called rosettes. Many of these have been found in Nolan County.
[Gypsum] has four directions of [cleavage]. One of these directions is so perfect that some [selenite] splits into thin, clear sheets that may be mistaken for [mica]; other selenite [cleavage fragments] may be mistaken for [calcite]. You can distinguish selenite sheets from calcite by testing their hardness (selenite is softer) and by putting a drop or two of dilute hydrochloric acid on them. The acid will fizz and bubble on calcite but not on the selenite gypsum. There is also a quick way to distinguish the thin selenite cleavage fragments from mica. After you carefully bend a thin sheet of mica, it will snap back to its original shape without breaking. Selenite gypsum, however, is not elastic. It will bend, but it will break if you try to straighten it again.
[Selenite] [gypsum] crystal from Bastrop County, Texas.
[Selenite] is found in cracks and cavities in rocks. Good crystals have been collected at Gyp Hill, a [salt] dome southeast of Falfurrias in Brooks County, and some selenite has been mined there. Selenite crystals also occur scattered through clays, particularly along creek banks, in Lee, Fayette, Bastrop, and several other counties.
Another variety of [gypsum] is known as [fibrous gypsum]. It is made up of slender, brittle, needle-like fibers that fill the cracks in some rocks. If fibrous gypsum has a silky or pearly luster, it is called [satin spar]. One of the places where satin spar occurs is in [Permian] rocks in Hardeman County.
[Selenite] [gypsum] rosettes from Nolan County, Texas.
Most of the [fibrous gypsum] and [selenite] is formed by solutions. Some of these solutions develop when underground waters, seeping through rocks, pick up and dissolve minerals that contain [sulfur] (such as [pyrite]). This dissolved material changes the water into very weak sulfuric acid. When the sulfuric acid meets calcium carbonate (as in [limestone] or [calcite]), it combines with the calcium to form the [gypsum].
[Fibrous gypsum] from Terlingua area, Brewster County, Texas.
A [massive], fine-grained, and [translucent] variety of [gypsum], known as [alabaster], is used for articles such as lamp bases, statuettes, vases, and book-ends.
A loose, earthy, crumbly variety of [gypsum], called [gypsite], is ordinarily found mixed with other materials, such as [clay], [sand], and soil. It occurs either at or near the surface of the ground. Gypsite is found in Culberson, Reeves, and other counties in west Texas.
A [massive], [granular] variety of [gypsum], called [rock gypsum], may occur in large deposits. This is the gypsum that is used for making products such as plaster, wallboard, and some cements.
Deposits of [rock gypsum] are found both underground and at the surface in Texas. Surface deposits occur in [Permian] rocks in several counties to the east of the Texas [High Plains]. They also occur in the area between the Pecos River and the Delaware and Apache Mountains in Culberson and Reeves counties. Some of the other surface deposits are found near the Malone Mountains in Hudspeth County and in Lower [Cretaceous] rocks in Gillespie and Menard counties. Rock gypsum has been mined from the deposits in Fisher, Gillespie, Hardeman, Hudspeth, and Nolan counties. It also has been produced from the cap-rock at Hockley [salt] dome in Harris County.
[Gypsum] and another mineral, [anhydrite], have very nearly the same composition. Both are calcium sulfates. Gypsum, however, contains water of crystallization, and anhydrite does not. It is likely that most of the rock-gypsum deposits of Texas originally were beds of anhydrite. By absorbing water that seeped through it, the anhydrite changed into gypsum.
Halite
[Halite], sodium chloride, is the table [salt] you sprinkle on food for seasoning. This mineral ordinarily is white or colorless, but other materials cause it to be tinted red, blue, gray, brown, or green. When you rub halite across a [streak] plate, it leaves a white streak.
Because [halite] cleaves in three directions, all at right angles to each other, the [cleavage fragments] are shaped like [cubes]. You can see some of them by looking at a few grains of table [salt] through a magnifying glass.
[Halite] has a salty taste and dissolves easily in water. It also is [transparent] to [translucent] and has a glassy luster. This mineral is soft enough for a copper penny to scratch it. Halite commonly occurs as cubic crystals and as [granular] or compact masses.
In addition to its use as table [salt], much [halite] goes to make soda ash, chlorine, and other chemicals. A few of its other uses are in leather making, meat packing, and food canning.
Texas has large underground deposits of [halite]. These deposits, known as [rock salt], occur in the [Permian] subsurface basin of west Texas and in the [salt] domes of the [Gulf Coastal Plain]. The Permian basin, which extends under parts of west Texas, New Mexico, Oklahoma, Colorado, and Kansas, is now completely filled with [sediments]. It appears level and flat when you travel across it and does not look at all like a basin or a valley. During Permian time, however, this area was covered by a salty sea. As the sea gradually dried up, the dissolved material that it contained was deposited as thick beds of halite, [anhydrite], and other minerals. Later, these minerals were covered by [sedimentary rocks] which were deposited on top of them. Now, the minerals are found many hundreds of feet below the surface. In Hutchinson, Mitchell, Ward, and Yoakum counties, some of this Permian basin salt has been produced (as brine) from wells that have been drilled into it.
[Salt] domes, which are huge, underground columns of [halite], occur on the [Gulf Coastal Plain].
The [Gulf Coastal Plain] [salt] domes are huge and almost circular columns of [halite], some of which are more than 2 miles wide. Some are less than 300 feet below the surface, but most of them are much deeper. These salt columns pushed upward many thousands of feet from great, deeply buried salt deposits. The halite is mined from shafts dug into the Hockley salt dome in Harris County and into the Grand Saline salt dome in Van Zandt County. Salt brines are produced from wells drilled into several salt domes of this area.
At the surface in Texas, [halite] occurs in [salt] lakes in Crane and Hudspeth counties and in alkali lakes on the [High Plains]. It is found also on the shores of bays and lagoons in Cameron, Kenedy, Kleberg, and Willacy counties, and it occurs at springs and seepages in various places in the State.
Hematite
[Hematite], iron oxide, the chief ore of iron, is found in many places in Texas but not in large deposits. This mineral may have a metallic luster and appear reddish brown, dark brown, steel gray, or black or it may occur as a soft, red, earth-like material called red ocher.
[Specular hematite] from Carrizo Mountains, Hudspeth County, Texas.
Most metallic [hematite] is too hard for a pocket knife to scratch, but [quartz] or a steel file will scratch it. Hematite is fairly heavy, for it has a [specific gravity] of 5.26. This mineral has no [cleavage], but some specimens show three directions of [parting] that are almost at right angles to each other. A great help in identifying hematite is the dark reddish-brown [streak] it leaves when you rub it across a streak plate.
Some [hematite] occurs as rounded masses that resemble kidneys or bunches of grapes (then called kidney ore); it also is found as flat crystals. Most of the Texas hematite occurs as [granular] or compact masses. One of these [massive] varieties is composed of shiny scales or plates and is called micaceous or [specular hematite]. This variety has been found in Hudspeth County and in northeastern Mason County. Hematite also commonly occurs as cementing material in many Texas sandstones.
Some [hematite] is formed by the alteration of [magnetite], another iron mineral. This hematite is known as [martite], and some of it still has the crystal shape (an [octahedron] or a [dodecahedron]) that belonged to the magnetite. Most of the hematite found in the [Llano uplift] area of central Texas is believed to be altered magnetite. In this central Texas area, some [massive], [granular] martite has been mined at the Gamble prospect, a few miles southeast of Fredonia in northeastern Mason County, where it occurs as layers in [Precambrian] [gneiss].
Small deposits of [hematite] occur in other parts of Texas, too. Some of the west Texas localities include Sierra Blanca, the Quitman Mountains, and the Carrizo Mountains of Hudspeth County and the area around Shafter in Presidio County.
Hollandite. See [Manganese Minerals].
Hyalite. See [Opal].
Jasper. See [Quartz].
Kaolin. See [Clay].
Limestone
[Limestone] is a [sedimentary] rock made up chiefly of [calcite], a calcium-carbonate mineral. This rock also commonly contains grains of [quartz], [clay] minerals, the mineral [dolomite], or other materials. If a large amount of dolomite is present, the rock is called dolomitic limestone. In some limestones, the mineral grains are too small to be distinguished from each other without a magnifying glass or a microscope, but in other limestones, the individual mineral grains are easily seen.
Pure [limestone] is white, but if it contains [clay] or plant or animal matter it is light gray, dark gray, or black. Limestone also may be some shade of yellow, brown, or red. It is fairly soft and can be scratched with a knife. Because this rock contains [calcite], an easy chemical test will help identify it: a drop or two of dilute hydrochloric acid will quickly fizz and bubble when placed on the limestone.
Limestones form in fresh water, such as in lakes, but most of them form in the seas. As some earlier-formed rocks are weathered, the calcium minerals that they contain are dissolved. Creeks and rivers carry this dissolved material to the sea. There, small animals, such as corals, crinoids, sponges, and foraminifers, take the dissolved material out of the water to build their calcium carbonate shells. Plants, such as algae, can take calcium carbonate out of solution too, and it collects on them. Shells, shell fragments, and plant remains accumulate on the sea floor, forming limy deposits that later become [limestone].
Limestones also originate in a slightly different way. When the temperature and chemical composition of the water permit, calcium carbonate precipitates as millions of tiny grains of [calcite] and forms a limy mud that is converted to [limestone]. Many limestones contain shell or plant fragments in addition to these tiny grains of calcite.
Polished section of Lower [Cretaceous] Edwards [Limestone] from Travis County, Texas, containing fossil gastropods.
There are several special kinds of [limestone]. If the rock is made up of many little rounded [calcite] grains that resemble fish eggs, it is called [öolitic limestone]. Another limestone, [chalk], is soft, white, and fine grained. It consists mostly of tiny shell fragments and fine-grained calcite. [Coquina] is a porous limestone made up of loosely cemented shells and shell fragments. Another special kind of limestone, known as [lithographic limestone], because it can be used in printing, is smooth, firm, and hard. Its mineral grains are too small to be recognized without a microscope. This kind of limestone breaks with a smooth, sometimes curved, [fracture]. Still another variety, [pulverulent limestone], is loose, soft, powdery, and white. It occurs in the Lower [Cretaceous] Edwards Limestone in Williamson and Bell counties of central Texas. Some of this limestone is used to polish rice grains, and it is added to livestock feeds to provide calcium for the animals.
[Limestone] quarry in Lower [Cretaceous] Edwards Limestone at Georgetown, Williamson County, Texas.
Much [limestone] is found at the surface in Texas in [Cambrian], [Ordovician], [Mississippian], [Pennsylvanian], [Permian], and [Cretaceous] [formations]. If you will look at numbers 5, 6, 9, 10, and 11 on the Texas [geologic map] (pp. [4]-5), you will see that these strata appear at the surface in central, north-central, and Trans-Pecos Texas.
[Limestone] has many important uses. Much Texas limestone is crushed and used as a road-building material and as an aggregate that is mixed with cement to make concrete. Farmers in some areas improve their crops by adding limestone to the soil. Limestone also is sent to the iron furnaces in east Texas to be used in the production of pig iron and steel.
Some of the Texas limestones are heated to a fairly high temperature in order to change them into lime (calcium oxide). Industry uses a large amount of lime in making chemicals, steel, glass, paper, and other products. Builders use it to make plasters, mortars, and stuccos. At plants in Comal, Johnson, Travis, and Williamson counties, lime is made from [Cretaceous] limestones.
Another important use of [limestone] is in making portland cement. The limestone is mixed with [clay] or [shale], and the mixture is burned in a kiln until it just begins to melt. Then it is allowed to cool. Next, it is finely ground and in order to keep the finished cement from hardening or setting too quickly when it is used, a retarder, such as [gypsum], is added. A number of cement-manufacturing plants in Texas use [Cretaceous] limestones, shales, and clays.
Many of the Texas limestones make excellent building stones. Some of them are quarried from [Pennsylvanian] and [Cretaceous] [formations] in north-central Texas and from Lower Cretaceous formations in counties near the [Llano uplift] of central Texas. A large quarry on the Williamson-Travis County line near Cedar Park in central Texas has supplied Cretaceous [limestone] for many buildings and monuments in the United States and Canada.
Limonite
[Limonite] is not really a definite mineral but is a mixture of iron oxides containing water. It is believed to be closely related to an iron mineral called goethite. Some limonite may be dull and earthy with the appearance of brownish-yellow or rusty brown [clay]. This variety is so soft that a fingernail will scratch it easily.
Other [limonite] has a dark brown or black color and a metallic or almost metallic luster. A copper penny will not scratch it, but a steel file will. This kind of limonite may have a shiny black surface that resembles glossy lacquer. The property that will help you most in identifying limonite is the rusty, yellowish-brown [streak] it leaves when rubbed across a streak plate.
[Limonite] has no [cleavage] and no crystal shape of its own. But crystals of other iron minerals, such as [pyrite] and [magnetite], alter to form limonite. It then occurs with a crystal shape that originally belonged to one of these other minerals. (Such false forms of minerals are called pseudomorphs.) Limonite also occurs as layers in rocks, as hollow or solid concretions, or as coatings on other minerals. It is found mixed with minerals such as clays and serves as the cementing material in some sandstones.
[Limonite] is found in many localities in Texas including Blanco, Brewster, Burnet, Llano, and San Saba counties. The most important limonite deposits in Texas, however, are in the eastern part of the State, particularly in Anderson, Cass, Cherokee, Henderson, Marion, Morris, Nacogdoches, Smith, and Upshur counties.
The east Texas [limonite] deposits occur mainly in Weches [sedimentary rocks]. These rocks, which were deposited in the sea during [Eocene] [Tertiary] time, contain [clay] along with greensands. (Greensands are small, soft grains that contain glauconite, a mineral composed of iron, silicon, and several other [elements].) Later, as the sea retreated, these [sediments] became a part of the land. Waters seeping through the sediments changed into weak solutions of carbonic and sulfuric acid that dissolved the iron out of some of the greensands. When conditions were favorable, this iron was re-deposited as an iron-carbonate mineral called siderite. Siderite was changed to limonite by weathering. Some siderite is still found in east Texas, and it is also mined along with the limonite as an iron ore.
East Texas iron ore has been mined from time to time ever since about 1855, and records show that a number of local iron furnaces once operated. The brown iron ore (as the [limonite] is also called) now is mined from open pits in Cass, Cherokee, and Morris counties.
This ore, after being washed, goes into blast furnaces at Lone Star (near Daingerfield) and at Houston. In the blast furnaces the ore is changed into metallic iron by mixing it with coke (made from coal) and [limestone] and blowing in blasts of hot air.
To make steel, the iron from the blast furnace (called pig iron) is put into open-hearth furnaces together with scrap iron, [limestone], and other materials. This mixture is heated and melted together to get rid of unwanted substances. Then other [elements], such as molybdenum, manganese, or nickel, are added to make steel with the right strength and toughness.
[Limonite] ore is changed to metallic iron in a blast furnace.
Skip car Hot Gases Blast Furnace Iron Ore [Limestone] Coke Fire Brick lining Steel Plate covering 3550°F Hot Air Blast Slag Slag Ladle Molten Iron Iron Ladle
Steel mills alongside the furnaces in Texas turn out many products, such as steel plates for oil tanks, ships, and tank cars and steel beams for framework in buildings and bridges. Some of their other products include pipes for the oil and chemical industries and wire for nails and fencing material.
Lithographic Limestone. See [Limestone].
Llanite
[Llanite] is a unique rock that is found only in Llano County in central Texas. This [intrusive] [igneous] rock is made up of easily seen crystals and grains of [quartz] and [feldspar] that are scattered through a brown-colored mass of extremely small mineral grains. The quartz is beautiful, sky-blue, and opal-like; the feldspar has a rusty pink color. (Because the quartz looks like [opal], this rock often is called [opaline granite].) The mineral grains that make up the brown-colored mass are so tiny that they can be identified only with a microscope. They are quartz, feldspar, [mica], [fluorite], and apatite.
[Llanite] formed during [Precambrian] time. Molten rock material forced its way upward into cracks that cut across [granite] and [schist] rocks while the rocks were still far underground. This hot [magma] remained in the cracks where it cooled and hardened to form long, narrow, wall-like masses (called dikes) of llanite. We can see some of the llanite dikes exposed at the earth’s surface to the north and northeast of Llano in Llano County because the overlying rocks have weathered away.
[Llanite] has been quarried from one of the dikes west of Babyhead in northern Llano County. Because llanite is both attractive and strong, it has been used as an ornamental stone and as a monument stone.
Magnetite
[Magnetite], iron oxide, is a black, metallic mineral with an outstanding physical property: it is magnetic—fragments of magnetite readily cling to a magnet. It also leaves a black [streak] when rubbed across a streak plate. Although this mineral is too hard to be scratched by the average pocket knife, a steel file will scratch it. Magnetite is fairly heavy—it has a [specific gravity] of 5.18.
[Magnetite] occurs as compact or [granular] masses, as scattered grains, and as crystals. Most of the crystals are [octahedrons], but some [dodecahedrons] are found. Magnetite helps make up a part of many [metamorphic] and [igneous] rocks, and it also occurs as tiny crystals and grains in some sands, sandstones, and other [sedimentary rocks].
Metallic iron, after leaving the blast furnace, is made into steel in an open-hearth furnace.
Open-Hearth Furnace Scrap metal Alloying [Elements] [Limestone] Furnace Interior Live Fuel Burner Air pre-heated
Most of the [magnetite] that has been found in Texas occurs in [Precambrian] [gneiss] and [schist] rocks of the [Llano uplift] area of central Texas, particularly in Llano County and in eastern Mason County. It occurs as thin layers, as thick lens-shaped deposits, and as scattered grains in the rocks. Probably at least a billion years ago these gneisses and schists were [sedimentary rocks], such as shales and sandstones. Some geologists believe that these rocks could have contained iron [sediments] (perhaps in the form of glauconite). Great forces below the earth’s surface crumpled and squeezed the sedimentary rocks and changed them into the [metamorphic] schist and gneiss rocks we see today. As this happened, the iron sediments in the rocks were changed into magnetite.
[Granular] [magnetite] fragments from northwest of Llano, Llano County, Texas, are attracted to a magnet.
At least some of the [magnetite] in this area (such as the deposit at Iron Mountain in Llano County) probably had a different sort of origin. Molten [igneous] rock material containing iron could have moved up into cracks in the ancient [sedimentary rocks]. Then the magnetite formed from this iron material when the igneous and sedimentary rocks were changed into the schists and gneisses of today.
None of the Llano and Mason County [magnetite] deposits is really very large. Nevertheless, prospecting and a little mining have been carried on from time to time at several deposits in this area. At Iron Mountain, which is about 12 miles northwest of Llano in Llano County, magnetite has been mined from open pits. Although magnetite is commonly used as a source of iron, the magnetite from this deposit was used as a heavy concrete aggregate.
Malachite. See [Copper Minerals].
Manganese Minerals ([Braunite], [Hollandite], [Pyrolusite])
Although manganese does not occur alone in nature as a native [element], it makes up a part of many minerals and compounds. This element has an important use in steel making, where it helps rid the steel of unwanted substances, such as oxygen and [sulfur], and, in addition, it is used to make tough, hard, manganese steel for armor plate, railroad tracks, safes, and steam shovels. Manganese has various uses outside the steel industry. It is added to copper and nickel to make alloys, it is used in the manufacture of dry-cell batteries, and (as manganese sulfate) it is used as a fertilizer.
[Manganese minerals] and compounds, such as [braunite], [hollandite], [pyrolusite], and [wad], occur in several counties in Texas. No large, commercial deposits have been found here.
Some manganese compounds and minerals are covered with a soft, sooty black material that will soil your fingers. This can help you recognize these minerals; however, a few non-manganese minerals, such as some [chalcocite], also have a black coating that soils your fingers in a similar way.
One of the [manganese minerals], [braunite], is a complex oxide of manganese that contains silica. It has a submetallic luster and is dark steel-gray or black. When rubbed across a [streak] plate, it leaves a steel-gray or a black streak. This mineral is too hard to be scratched by a pocket knife, but a piece of [quartz] or a steel file will scratch it. Braunite has a [specific gravity] of 4.75 to 4.82. It has four directions of [cleavage] that are parallel to the faces of a pyramid.
In the Spiller mine, about 15 miles northeast of Mason in Mason County, masses of [braunite] occur as lens-shaped layers in [Precambrian] [gneiss] and [quartzite] rocks. This braunite may have formed from another manganese mineral (possibly manganese [garnet]) that was exposed at the earth’s surface after the overlying rocks eroded away. As this other mineral weathered, it may have altered into braunite, or the braunite could have been deposited from solutions emanating from hot magmas before the great thickness of overlying rock was removed.
The mineral variety [hollandite] is a rare manganate of manganese and barium. It has a metallic luster, and its color is silvery gray or black. When you rub it across a [streak] plate, hollandite leaves a black streak. It has a [specific gravity] of 4.7 to 5. Hollandite is rather hard, but a steel file will scratch it.
[Hollandite] from Jeff Davis County, Texas.
[Hollandite] occurs in western Jeff Davis County in west Texas at what is called the Mayfield prospect. Here, it is found as rounded masses that occur in a vein near a large [fault] in Lower [Cretaceous] [limestone] rocks.
Other manganese compounds, [pyrolusite] and [wad], are found in several important deposits near the Pecos River in western Val Verde County. Pyrolusite is a manganese dioxide mineral. It is black, [opaque], and so soft that it rubs off on your fingers like soot. Pyrolusite may be [granular] and [massive] or may be powdery. It also occurs as a fern-like coating on rocks. Wad is not really a mineral but is an impure, dull-black or brownish-black mixture of manganese oxide, water, and other substances. It can be soft enough to soil your fingers, or it can be too hard to scratch with a pocket knife. Wad occurs in earthy or compact masses or in crusts or stains on rocks.
In Val Verde County, the [wad] and [pyrolusite] are found mixed with soil, [clay], [gravel], [sand], and plant remains. This material fills cracks in Lower [Cretaceous] limestones, it is scattered through gravels, and it is deposited in low places at the surface. The manganese in these deposits came from [limestone] rocks that have since weathered away. Rainwater trickled into these rocks and dissolved the [manganese minerals] they contained. This manganese was washed down toward the Pecos River and was deposited as wad and pyrolusite.
Marble
[Marble] is a [metamorphic rock] made up chiefly of sparkling grains of [calcite] or [dolomite], but other minerals may be present. The marble may be fine grained, medium grained, or coarse grained; commonly, all the mineral grains are about the same size.
[Marble] may be of uniform color, banded, spotted, or streaked. If it is made up only of pure [calcite] or [dolomite], the marble is white. If, however, it contains carbonaceous material, such as [graphite], it is grayish or black. [Limonite] impurities cause the marble to be yellowish brown, and manganese oxides and [hematite] give it a brownish, pinkish, or reddish color.
[Marble] is a rather soft rock, and you can scratch it easily with a pocket knife. A few drops of dilute hydrochloric acid will bubble and fizz readily on [calcite] marble; on [dolomite] marble, it may fizz slightly.
[Marble] forms from [limestone] or from [dolomite] rock. Heat and pressure below the earth’s surface cause the [calcite] and dolomite mineral grains in these rocks to recrystallize. A fine-grained limestone can be changed into a coarse-grained calcite marble. The marble is not made up of new and different minerals, but it has a new texture unlike that of the limestone. (To a builder, the word marble has another meaning. He considers rocks such as unaltered limestone, unaltered dolomite, or even [serpentine] to be marble, if they will take a high polish.)
Polished section of [Precambrian] [metamorphic] [marble] from Llano County, Texas.
[Metamorphic] marbles occur at the surface in central Texas and in west Texas. Some of the west Texas occurrences are in the Van Horn area of Culberson and Hudspeth counties and in the Big Bend area of Brewster County. In central Texas, [Precambrian] marbles are found in Burnet, Gillespie, Llano, and Mason counties of the [Llano uplift] area. Many of them are suitable for use as monument and building stones. Some of the Llano County [marble] is quarried and used as [granules] for roofs and as terrazzo chips for making colorful floors (described with [serpentine] on [p. 88]).
Martite. See [Hematite].
Mica
[Mica] is not just one mineral but is the name given to a group of similar minerals. The mica minerals are easy to recognize. Because they have perfect [cleavage] in one direction, they split into thin, flat sheets. You can see through some mica sheets, and they are elastic enough to be bent back and forth. (Another mineral, [selenite] [gypsum], also will split into thin, flat, [transparent] sheets, but selenite sheets break when you bend them.)
[Mica] minerals have perfect [cleavage] in one direction, resulting in thin, sheet-like [cleavage fragments].
Basal [Cleavage]
Two of the [mica] minerals that you are most likely to find in Texas are [muscovite] and [biotite]. Both these minerals are potassium-aluminum silicates, and biotite, in addition, contains magnesium and iron. In general, muscovite is light colored, that is, it has a light brown, yellow, or green tint, or is colorless, and biotite is dark colored, commonly dark green, brown, or black. These minerals have glassy or pearly lusters and are rather soft—a copper penny scratches them. The [specific gravity] of biotite is 2.8 to 3.2, and that of muscovite is 2.76 to 3.1.
[Mica] minerals occur in [igneous] rocks, such as [granite] and [pegmatite], and in [metamorphic rocks], such as [schist] and [gneiss]. They also are found as tiny flakes in some sandstones, limestones, and other [sedimentary rocks]. Most of the Texas mica is found in the [Llano uplift] area (particularly in Llano County) and in the Mica Mine area. (The Mica Mine area is in the Van Horn Mountains about 15 miles south of Van Horn in west Texas.) In both these areas, the mica minerals occur mostly in [Precambrian] pegmatites and mica schists.
The gleaming [mica] schists were once [igneous rocks] or [sedimentary rocks] such as sandstones and shales. Long ago, great forces beneath the earth’s surface changed the rocks into mica schists. The mica that is found in pegmatites formed from hot [fluids] of [igneous] origin when the [pegmatite] rock itself was formed.
Clusters of [mica] in the pegmatites are called books, because the thin sheets into which the mica splits look like pages. Some [muscovite] books up to 8 inches across are found in the Mica Mine area of the Van Horn Mountains.
The books or sheets of [muscovite] [mica] that occur in pegmatites are especially valuable to industry. Muscovite can stand great heat without melting, it is tough, it splits into thin sheets, and it lets very little heat and electricity pass through. Because of these properties, muscovite is used in fuses and as insulators in heating [elements] of electric irons and toasters. ([Biotite] is not used, because the iron it contains makes it a conductor of electricity.) Sheet muscovite also is widely used by the electronics industry as a non-conducting material in the manufacture of tubes and other products.
Both [muscovite] and [biotite] from [mica] [schist] rocks, as well as scrap pieces of sheet mica from pegmatites, are ground into flakes or powder. This ground-up mica has many uses, ranging from a powder coating for automobile inner-tubes to Christmas tree “snow.”
Only a small amount of [mica] has been mined in Texas. A fair grade of sheet mica occurs in the pegmatites at Mica Mine in west Texas, but the deposit is not large. In the pegmatites of the [Llano uplift] area of central Texas, no sheet mica has been found that is considered good enough for the requirements of industry. Mica suitable for grinding, however, is found in both these Texas areas.
Micaceous Hematite. See [Hematite].
Microcline. See [Feldspar].
Milky Quartz. See [Quartz].
Muscovite. See [Mica].
Native Silver. See [Silver Minerals].
Obsidian and [Vitrophyre]
[Obsidian] is a dark, glassy-looking [igneous] rock. Most obsidian contains the same chemical [elements] as [granite] and [rhyolite], since all three of these rocks can form from the same type of molten rock material. Obsidian, however, has no separate minerals, because its chemical elements are not combined in an orderly way. It is a natural glass.
Because it is a glass, we know that [obsidian] forms very quickly. One way for it to form is from the sudden cooling of hot, molten [lava] that flows out of volcanoes. If the lava cools and hardens before the separate minerals can crystallize, it becomes a natural glass, such as obsidian.
This rock is smooth and shiny. Most of it is black, but some can be dark green or dark brown. [Obsidian] allows light to pass through it, and it breaks with a curved, [conchoidal] [fracture]. The broken edges are very sharp.
Another glassy [igneous] rock that forms from fast-cooling [lava] is [vitrophyre]. It looks like [obsidian] except that it has crystals or [crystalline] mineral grains (which may be light colored) scattered through the dark glassy material.
[Obsidian] was used by the Indians to make arrowheads.
[Obsidian and vitrophyre] are found in the Big Bend area of Brewster and Presidio counties in west Texas. They occur with other [igneous rocks] that formed there during [Tertiary] time.
The Indians who long ago roamed this area used the smooth, shiny [vitrophyre] and [obsidian] to make some of their arrowheads and scrapers. Today, rock collectors pick up these attractive rocks for their collections, and some of them cut and polish obsidian and vitrophyre for use as gemstones.
Onyx. See [Quartz].
Öolitic Limestone. See [Limestone].
Opal
[Opal] is like hardened jelly or gelatin. It has no [crystalline] inner structure and no crystal shape of its own—it is [amorphous]. This mineral has almost the same chemical composition as [quartz]. Both are silicon dioxides (silica), but opal, in addition, contains water.
[Opal] can be almost any color—red, yellow, blue, brown, gray, white—or it can be colorless. It is [transparent] or [translucent] and appears glassy, resinous, greasy, or dull. Opal has a [specific gravity] of 1.9 to 2.2—this mineral is a little lighter than [quartz]. It also is softer than quartz. A copper penny will not scratch opal, but quartz will. Opal has a white [streak] and a curved, [conchoidal] [fracture] but no [cleavage].
[Opal] occurs in a number of places in Texas. In the Trans-Pecos country of west Texas, it fills cracks and cavities in some of the [extrusive] [igneous] rocks. It occurs on the [High Plains] of northwest Texas, and it is found in [Tertiary] [formations] of the [Gulf Coastal Plain] where it occurs as masses that fill cracks and cavities in [sedimentary rocks], as the cementing material in some sandstones (such as in the Catahoula [sandstone]), and as opalized wood.
Much [opal] forms from underground waters that contain silicon. These solutions move through the rocks and deposit the opal in them.
[Opal] is found in a number of varieties. Some show a beautiful, lustrous play of colors that comes from inside the specimens. These varieties are known as [precious opal] and are prized as gemstones. In Texas, some precious opal is found near Alpine in Brewster County. It has a milky white to bluish-white color, is [translucent], and shows a fiery orange, red, blue, and green play of colors.
The variety known as [common opal] shows no play of colors. It may be white, gray, bluish, reddish, greenish, or yellowish, and it is only slightly [translucent]. It is found in Brewster, Jeff Davis, Presidio, and other counties of the Trans-Pecos country of west Texas. It occurs also around some of the wet-weather ([playa]) lakes on the Texas [High Plains]. In the [Gulf Coastal Plain], common opal is found with [chalcedony] (a variety of [quartz]) in [Tertiary] [formations]. A south Texas locality sometimes visited by collectors is near Freer in Duval County.
A clear, commonly rounded, variety of [opal] that looks like ice is called [hyalite]. Two areas in which it has been found are in Presidio County in west Texas and in Llano County in central Texas.
Opalized wood from Washington County, Texas.
A variety of petrified wood, called opalized wood, is [opal] that replaced the fibers of a piece of wood. [Wood opal] is found at a number of places in the [Gulf Coastal Plain]. It occurs there in [Tertiary] [formations] within about 20 miles of the boundary line between areas 2 and 3 shown on the [geologic map] (pp. [4]-5).
A soft opaline material called [diatomite], or diatomaceous earth, is made up chiefly of the skeletons of diatoms—tiny, one-celled plants that live in fresh or [salt] water. These little plants are able to take silica from the water to make [opal] skeletons for themselves. When the diatom skeletons collect at the bottom of a lake or sea, they form the light, crumbly, white, gray, or cream-colored deposit of impure opal known as diatomite. Industry uses this material as a filter, as insulation, as an abrasive, and as a filler.
[Diatomite] formed in ancient lakes on the Texas [High Plains] during late [Tertiary] ([Pliocene]) and early [Quaternary] ([Pleistocene]) times. It is found in Armstrong, Crosby, Dickens, Ector, Hartley, and Lamb counties.
Opaline Granite. See [Llanite].
Orthoclase. See [Feldspar].
Pegmatite
Pegmatites occur in [igneous] rock areas, and most geologists consider them [intrusive] [igneous rocks]. They are made up of crystals and [crystalline] mineral grains that fit together—the grains are interlocked. The crystals and grains in pegmatites are larger than those of surrounding rocks, and some are huge, even larger than a man. However, there is a wide range of grain sizes in [pegmatite].
Some pegmatites cut through [igneous] or [metamorphic rocks] in such a way that they resemble walls (called dikes). Others are found as veins, as flat masses, or as odd-shaped bodies in rocks. Many pegmatites occur in granites and contain [feldspar], [quartz], [mica], and other minerals, as [granite] does. Some pegmatites occur with other kinds of [igneous rocks] and contain the same minerals as these rocks. A few pegmatites contain rare and unusual minerals.
Many geologists believe that pegmatites form from hot [fluids] of [igneous] origin that are left after other [igneous rocks], such as [granite], have already formed. These left-over fluids contain large amounts of aluminum, potassium, silicon, sodium, and several other [elements]. While the granite or other rocks are still far underground, this material pushes up into them, and may even partly dissolve them. Then it slowly cools and hardens into [pegmatite]. It is believed that, later, more fluids move into cracks in some pegmatites. This new material adds other minerals to the pegmatites and alters some of those minerals already there.
Quartz-feldspar [pegmatite] from Burnet County, Texas.
Some of the pegmatites we now see at the surface in Texas are probably about a billion years old. They formed during [Precambrian] time and occur with other extremely old rocks. One well-known Texas [pegmatite] area is the [Mica] Mine district of west Texas. It is about 15 miles south of Van Horn in the Van Horn Mountains of Culberson and Hudspeth counties. Another pegmatite area is in the [Llano uplift] of central Texas. These central Texas pegmatites occur in Burnet, Gillespie, Llano, and Mason counties.
Large crystals and grains of [feldspar], [mica], and [quartz] are found in the pegmatites of both these areas. A small amount of mica has been mined from the west Texas pegmatites, and feldspar has been produced from the central Texas pegmatites.
An extremely rare and unusual [pegmatite] occurs in the [Llano uplift] area at Baringer Hill, which is west of Burnet in Llano County. This pegmatite was once on the bank of the Colorado River, but when Buchanan Dam was built, the area was flooded. The Baringer Hill pegmatite now lies beneath the water of Lake Buchanan. Many rare minerals, which contain beryllium, cerium, thorium, uranium, yttrium, zirconium, and a number of other [elements], occur in this pegmatite. Some of these minerals, such as those containing yttrium and zirconium, glow or incandesce when they are heated. During the early part of this century, before the area was flooded, several of the yttrium minerals were mined and used in making lamp mantles.
Pitchblende. See [Uranium Minerals].
Precious Opal. See [Opal].
Pulverulent Limestone. See [Limestone].
Pumicite. See [Volcanic Ash].
Pyrite
[Pyrite] is a shiny, pale golden-yellow or brassy-yellow metallic mineral. This mineral, an iron disulfide, is so often mistaken for [gold] that it is widely known by the nickname fool’s gold.
Except for their similar color and luster, [pyrite] and [gold] are really very different. When you rub pyrite across a [streak] plate, it leaves a black, a greenish-black, or a brownish-black streak, but the streak of gold is gold-colored. Pyrite is too hard for the average pocket knife to scratch, but a knife will scratch gold easily. Pyrite is brittle and readily breaks, but gold is malleable and flattens out when hit with a hammer. Pyrite is only about 5 times as heavy as an equal volume of water, but pure gold is over 19 times as heavy. And pyrite may have a brown or a multicolored tarnish on it, but gold never tarnishes.
[Pyrite] veins in white [marble] from Llano County, Texas.
Cubic crystals of [pyrite].
[Pyrite] is a common mineral and is found in many of the [igneous], [metamorphic], and [sedimentary rocks] of Texas. It may be scattered through the rocks, or it may fill cracks and cavities in them. This mineral occurs as [granular] and compact masses, as rounded masses, or as crystals. The crystals are commonly [cubes], [pyritohedrons], or [octahedrons]. In some crystals, the shapes are combined (such as a cube with an octahedron or two pyritohedrons grown through each other). You may notice that the sides of some cubes and pyritohedrons have fine, parallel grooves (called striae or striations) on them.
[Pyrite] originates in a number of different ways. Some of it forms, along with other minerals in [igneous rocks], from hot magmas. It also forms in [metamorphic rocks] by the same processes that produce these rocks. Some of the pyrite in [limestone] and other [sedimentary rocks] is formed when the rocks themselves are deposited by seas or streams. Pyrite also is deposited by the hot [fluids] that are given off by magmas. These fluids travel up into cracks and other openings in rocks and then form pyrite as well as other minerals. Much pyrite forms in still another way. As water seeps through rocks, it dissolves some of the iron minerals that they contain. When, under certain conditions, these iron solutions mix with hydrogen sulfide (this is the gas that makes some water smell like rotten eggs), pyrite is formed.
[Pyrite] alters easily. Because of this, most builders carefully check the [limestone], [granite], [marble], or whatever other building stone they plan to use to be sure that it does not contain large amounts of pyrite. When exposed to the weather, pyrite changes to [limonite] and causes an unsightly rust stain.
[Pyrite] is used as a source of [sulfur], and it is produced for this purpose in several states. In Texas, however, no pyrite deposits have been found that are large enough to be mined.
Pyrolusite. See [Manganese Minerals].
Quartz
[Quartz], silicon dioxide, is one of the most common minerals. It is glassy, waxy, greasy, or dull and is [transparent] or [translucent]. Pure quartz is colorless, but impurities make some varieties white, black, or a shade of red, yellow, blue, violet, or brown. Quartz is a hard mineral. It scratches window glass and cannot be scratched by a pocket knife or even by a steel file. It has a [specific gravity] of 2.65. The curved, [conchoidal] [fracture] shown by many specimens helps identify it.
[Quartz] is plentiful in Texas. It occurs in [igneous rocks], such as [granite], [llanite], and [pegmatite]; in [metamorphic rocks], such as [quartzite], [schist], and [gneiss]; and in [sedimentary rocks], such as some [sandstone], [conglomerate], and [breccia].
[Quartz] crystal, with inclusions, from Burnet County, Texas.
[Quartz] is found as crystals and as masses. Some of the masses are coarsely [crystalline], but some are made up of extremely small crystalline particles called [cryptocrystalline] quartz. Some of the cryptocrystalline varieties of quartz found in Texas are [chalcedony], [chert], and [jasper]. Some of the coarsely crystalline varieties found here are [amethyst], [milky quartz], [rose quartz], [smoky quartz], and [rock crystal].
[Amethyst] geode from the Alpine area of Brewster County, Texas.
A colorless, glassy variety of [quartz], called [rock crystal], is clear enough to see through. It is found as crystals that are 6-sided prisms with pyramid-like faces on the ends. This variety is commonly associated with [igneous rocks], such as those of the [Llano uplift] area of central Texas and of the Trans-Pecos country of west Texas. It is commonly used as a gemstone and is made into necklaces, earrings, and other jewelry. Some specimens of rock crystal have slender, needle-like crystals of other minerals, such as [tourmaline], actinolite, or rutile, enclosed in them.
A clear, glassy variety of [quartz], [amethyst], has a purple or violet color. It, like [rock crystal], is commonly found in 6-sided prisms with pyramid-shaped ends and is also prized as a gemstone. Amethyst has been found in [Precambrian] rocks in the [Llano uplift] area of central Texas. (Amethyst Hill, a locality well known to collectors for many years, is in northeastern Gillespie County.) In west Texas, amethyst has been found in [Cenozoic] [igneous] rocks in the Sierra Blanca and Quitman Mountains of Hudspeth County and in the Alpine area of Brewster County.
[Milky quartz] from Burnet County, Texas.
A variety of [quartz] with a milk-white color and a glassy to greasy luster is called [milky quartz]. It occurs either as crystals or as [crystalline] masses. Very little light will pass through it. In central Texas, milky quartz occurs abundantly in the [Precambrian] rocks of the [Llano uplift] area in Blanco, Burnet, Gillespie, Llano, and Mason counties. It also is found in some of the rocks of the Trans-Pecos country of west Texas, such as in the Carrizo Mountains of Culberson and Hudspeth counties. Other good places to look for this variety of quartz are in the sands and [gravels] along many streams in Texas.
Some [quartz] has a glassy to a greasy luster and a rose or pink color. [Rose quartz], as this variety is called, commonly occurs as masses rather than as individual crystals. It can be found along some of the streams in Texas and also in [igneous rocks], such as those of the [Llano uplift] area of central Texas.
A kind of [quartz] with a smoky brown, a smoky yellow, or a dark brownish-black color is called [smoky quartz]. Its luster is glassy, and it may be either [translucent] or [transparent]. Smoky quartz is commonly found as crystals that are shaped like 6-sided prisms with pyramid-like ends. It is commonly associated with [igneous rocks], and beautiful specimens have been found in the Lake Buchanan area of Llano and Burnet counties in central Texas.
Smoky-quartz crystals from Burnet County, Texas.
A [cryptocrystalline] variety of [quartz], [chalcedony], has a waxy to dull luster and a tan, white, gray, or light-blue color. It is [translucent] but not [transparent]. Chalcedony does not have its own crystal shape but instead is found in masses that line or fill cracks, pores, and other cavities in rocks. It is formed when water containing silicon slowly seeps into these openings in the rocks and deposits the silicon dioxide there as chalcedony.
[Chalcedony] commonly occurs in some of the [Tertiary] rocks of the Gulf Coastal Plain. For example, chalcedony associated with [opal] is found near Freer in northern Duval County. In the [High Plains] of west Texas, it is found in alkali-lake deposits, such as at Shafter Lake in Andrews County and at Cedar Lake in Gaines County. In the Trans-Pecos country of west Texas, it can be found filling small cavities in [extrusive] [igneous rocks].
Polished [agate] from Rio Grande [gravels] of Zapata County, Texas.
A variety of [chalcedony] that generally is made up of more than one color is called [agate] (although agates consisting of several shades of a single color are also found). The colors may be spread out unevenly so that the agate has a cloudy appearance, or they can be arranged in wavy, in straight, or in concentric lines or bands. If the bands are straight and parallel, the specimen is called [onyx]. Agate that has a moss-like or tree-like design in it is called moss agate. Some agates make attractive gemstones when cut and polished.
[Jasper] from Uvalde County, Texas. Dark areas are brownish red; light areas are a yellowish-tan.
Much [agate] has been found filling cavities in [Cenozoic] [igneous rocks] in Brewster, Presidio, and other counties in the [Trans-Pecos] country of west Texas. It has been found also in an area about 10 to 15 miles wide along the Rio Grande, mostly in southern Webb County and in Zapata and Starr counties.
Trees and other plants have been replaced by [agate]. Many specimens of [agatized wood] have been collected from [Tertiary] [formations] in Fayette, Gonzales, Lee, Washington, and other counties of the [Gulf Coastal Plain]. (The agatized wood, along with opalized wood, occurs within about 20 miles of the boundary between no. 2 and no. 3 on the [geologic map], pp. [4]-5.)
A hard, compact, slightly [translucent] variety of [cryptocrystalline] [quartz] is called [jasper]. It commonly has a red, brown, or yellow color due to the presence of an iron oxide, such as [hematite]. Some jasper is made up of irregular bands of more than one of these colors. This variety of quartz often is polished to make attractive gem or ornamental stones. It has been collected at several localities in Texas, particularly from creek and river [gravels]. Starr and other nearby counties along the Rio Grande have furnished a number of good specimens.
A hard, smooth, compact, [translucent] rock that is made up mostly of [cryptocrystalline] [quartz] is called [chert] or [flint]. It is white, black, or some shade of gray, brown, or pink, and its luster is waxy, slightly glassy, or dull. Chert is found in many creek and river [gravels] in Texas. It also occurs with [limestone], such as in the Lower [Cretaceous] Edwards Limestone of central Texas and in the [Ordovician] Ellenburger strata in the [Llano uplift] area. Chert also is found with the Ordovician rocks of the Marathon area of Brewster County.
Geologists do not agree on whether [chert] and [flint] are two names for one variety of rock, or whether each is a separate variety. Some, however, now give chert a geological meaning and flint an archaeological meaning. They use the word chert to describe geological [formations] or rock specimens. They give the name flint to the same rock when it has been used by Indians in making arrowheads, scribers, scrapers, and spearheads.
Quartzite
[Quartzite] is either a [metamorphic rock] or a [sedimentary] rock. (The sedimentary kind of quartzite is described with [sand and sandstone] on [p. 86].) [Metamorphic] quartzite is made up mostly of [quartz]. It forms when heat and [fluids] below the earth’s surface cause the grains and cement of a quartz [sandstone] to recrystallize. When this happens, the grains interlock and are no longer held together by cement. Metamorphic quartzite, like [sedimentary quartzite], is a hard, firm rock that breaks through the quartz grains instead of between them.
Ancient [Precambrian] [metamorphic] [quartzite] occurs at the surface in the [Llano uplift] area of central Texas, in the Van Horn area of west Texas, and in the Franklin Mountains north of El Paso in extreme west Texas.
Rhyolite
[Rhyolite] is a fine-grained or glassy [igneous] rock that commonly is [extrusive] or [volcanic]. It has a pink, red, tan, white, gray, purple, or black color. This rock, like [granite], is made up chiefly of [feldspar] and a silica mineral, such as [quartz], but other minerals may be present. Both rhyolite and granite form from the same kind of molten rock material. Nevertheless, even though their compositions are the same, these two rocks do not look alike. Their textures differ because granite forms slowly and rhyolite forms quickly.
Much of the Texas [rhyolite] formed from hot, molten [lava]. This lava flowed out onto the surface either through [volcanic] cones or cracks in the ground. Some of the lava cooled and hardened too quickly for mineral grains to develop. This rapidly cooled lava formed a rhyolite rock that is made up, at least partly, of glass. In many of the rhyolites, [crystalline] mineral grains were able to form, but these grains are extremely small, and you may not be able to distinguish them even with a magnifying glass. Some rhyolite, because it hardened from moving, flowing lava, has streaks and bands of different colors and textures. This rhyolite has flow structure.
One variety of [rhyolite] has easily seen crystals and grains of minerals, such as [feldspar], [quartz], and [mica], scattered through a mass of the tiny [crystalline] grains (in much the same way that raisins are scattered through a cake). The easily seen crystals and grains are called phenocrysts, and the rock itself is called a rhyolite porphyry.
Many rhyolites and [rhyolite] porphyries occur in the [Tertiary] [igneous rocks] of the Trans-Pecos country of west Texas. Just a few of these localities include the Barrilla Mountains of Jeff Davis and Reeves counties, the Chisos Mountains of Brewster County, the Chinati Mountains of Presidio County, and the Davis Mountains of Jeff Davis County.
Rock Crystal. See [Quartz].
Rock Gypsum. See [Gypsum].
Rock Salt. See [Halite].
Rose Quartz. See [Quartz].
Salt. See [Halite].
Sand and Sandstone
[Sand] is a loose, uncemented [sedimentary] deposit made up of fragments of weathered rocks and minerals. These fragments must be of a certain size (between ¹/₁₆ millimeter and 2 millimeters in diameter) in order to be called sand grains. The largest sand grains are about the size of a pinhead. Sand grains are smaller than the fragments known as [granules]; they are larger than those known as silt.
Many sands are made up chiefly of grains of [quartz]. This mineral is plentiful and does not easily weather away. In addition, rock fragments and many other minerals, such as [feldspar], [mica], [gypsum], [magnetite], and [garnet], are found as [sand] grains.
Rains wash many of the [sand] grains and other weathered rock and mineral fragments into creeks and rivers. These streams may carry the sand and other [sediments] long distances before depositing them. Today, we find sands along the banks of many creeks and rivers in Texas and along the beaches of the Gulf of Mexico. The sand in the rivers is in transit to the Gulf. In addition, sand occurs at the surface in other [Cenozoic] [formations] and in some of the [Paleozoic] and [Mesozoic] formations of Texas.
[Sand] has many uses. Much building sand, which is used in mortar and concrete, is produced from numerous sand and [gravel] pits in Texas. Pure [quartz] sand that can be used to make glass is known as glass sand. Some of it is found in north-central Texas in Lower [Cretaceous] [formations]. A large glass sand quarry is located at Santa Anna in Coleman County. Along the [Gulf Coastal Plain], sand that is used in glassmaking occurs in [Eocene] [Tertiary] strata.
A coarse-grained [sand], blast sand, is used with compressed air to clean the walls of brick and stone buildings and to carve designs on monument stones, such as marbles. Some coarse sand is also used as a filtering sand in purifying water. These types of sand have been produced from the [Gulf Coastal Plain] as well as from other areas of Texas.
[Sand] grains, when nature cements them together, make up the [sedimentary] rock [sandstone]. Some sandstones form when underground water carrying dissolved mineral matter moves through loose sand. As the dissolved mineral matter comes out of solution, it forms a cement that binds the sand grains together.
The cement may be material such as [calcite] (calcium carbonate), [quartz], [chalcedony], or [opal], which are silica minerals, and [limonite] and [hematite], which are iron oxides. [Clay] also may serve as a cement. It is either deposited along with the [sand] or is formed from weathered [feldspar] sand grains.
The color of the cementing material helps determine the color of the rock. Iron oxide cement, for example, causes the [sandstone] to have a reddish, yellowish, or brownish color. Sandstones also are white, black, gray, green, or cream colored.
Ordinarily, sandstones break through the cementing material, not through the [sand] grains. Thus, the broken surface of the rock feels rough and gritty. Some [quartz] sand grains, however, are tightly cemented with silica to form an extremely hard and compact rock. If this rock breaks smoothly through the grains instead of between them, it is known as [quartzite]. Some of this [sedimentary quartzite] occurs in the Texas [Gulf Coastal Plain] in the [Tertiary] Catahoula strata. (Another kind of quartzite is described on pp. [84]-85.)
[Sandstone] from the [Eocene] Wilcox Group of strata of northwestern Zavala County, Texas.
Ordinary sandstones are seen at the surface in many localities in Texas, and a number of them have been used as building stones. Some of the places where sandstones occur are in the [Cambrian] and [Pennsylvanian] [formations] of the [Llano uplift] area of central Texas and in the Pennsylvanian, [Permian], and Lower [Cretaceous] formations of north-central Texas. [Tertiary] sandstones occur in the Texas [Gulf Coastal Plain], and Triassic sandstones are found along the edges of the Texas [High Plains]. [Sandstone] is also found in many formations of the Trans-Pecos country of west Texas.
Sandstone. See [Sand and Sandstone].
Satin Spar. See [Gypsum].
Schist
[Schist] is a [metamorphic rock] that splits easily along thin, generally parallel layers, called folia. These layers may be either straight or curved, and they are made up of [crystalline] grains of one or more than one mineral. This structure is called schistosity or foliation. When you examine schist, you will see that many of the mineral grains are flat or long, and that they are lined up in one direction to form the layers. Some schists have fairly large crystals (many with perfect shapes) scattered through them. For example, [mica] schists may contain beautiful crystals of [garnet].
Each kind of [schist] is named for an outstanding mineral that it contains. [Mica] schist contains a large amount of mica. We also find hornblende schist, actinolite schist, chlorite schist, [talc] schist, and [graphite] schist. (Graphite schist is discussed with graphite on [p. 63].)
Schists form from other rocks, such as [granite], gabbro, or [shale]. The rocks are changed into schists by [fluids] and by heat and pressure below the earth’s surface.
Extremely ancient schists that formed during [Precambrian] time are exposed at the surface in the Allamoore—Van Horn area of west Texas and in the [Llano uplift] area of central Texas. Geologists believe that the Packsaddle [Schist] of the Llano uplift area was once [shale]. Good exposures of this schist are seen in the Honey Creek area near Packsaddle Mountain in Llano County.
Schorl. See [Tourmaline].
Sedimentary Quartzite. See [Sand and Sandstone].
Selenite. See [Gypsum].
Serpentine
[Serpentine] is the name given both to a rock and to a mineral. The mineral serpentine (a hydrous magnesium silicate) is found in two different forms. If it is fibrous, it is called [chrysotile]; if it is layered and platy, it is known as [antigorite]. Antigorite is brownish green and smooth and waxy looking. Some of it can be split into thin sheets. Chrysotile is made up of greenish, silky fibers, which may be brittle and break apart in large pieces. If, however, the fibers can be pulled apart into soft flexible, little threads, the mineral is called chrysotile [asbestos].
Light will pass through both these varieties of [serpentine], and both are soft enough to be scratched by a pocket knife. When rubbed across a [streak] plate, they leave white streaks. [Antigorite] and [chrysotile] have no crystal shapes of their own, but several other minerals can alter to form these two varieties of serpentine. Thus antigorite and chrysotile may be found as pseudomorphs in a crystal shape that originally belonged to another mineral.
[Antigorite] and [chrysotile] are commonly found closely mixed with [dolomite], [talc], [magnetite], [calcite], [pyrite], and several other minerals. These minerals make up [serpentine] rock (also called [serpentinite]). This rock ordinarily is some shade of green (such as whitish, yellowish, brownish, bluish, or dark blackish green), and it may be mottled. It is brittle or tough and generally is [massive]. Serpentine rock, like the serpentine minerals, is fairly soft—you can scratch it with a pocket knife.
In the [Llano uplift] area of central Texas, [serpentine] rock is found among [Precambrian] [metamorphic rocks], such as [gneiss] and [schist]. An especially large deposit in this area is known as the Coal Creek serpentine mass. It is over 3½ miles long, and at one place, it is almost 1½ miles wide. This mass of serpentine extends across the Blanco-Gillespie County line in the extreme northern parts of these two counties. (A little fibrous [chrysotile] is found here, but it will not break into flexible enough threads to be called chrysotile [asbestos].) Several other deposits of serpentine occur in northeastern Gillespie County and in southern Llano County.
It is believed that the Coal Creek [serpentine] was formed from an [igneous] rock such as peridotite, which is made up chiefly of grains of the mineral olivine. The peridotite may have been altered into serpentine by underground waters that seeped through it. It is possible, however, that other serpentines in the area were formed when rocks were altered by hot [fluids] and great pressures far below the earth’s surface.
The Llano area [serpentine] has been widely used in terrazzo floors. To make these floors, small pieces of serpentine and other colored rocks are put into cement that is spread over a concrete slab. Then, after the cement has hardened, it is ground to a flat, smooth surface and polished. The resulting terrazzo floor is both colorful and durable.
[Serpentine] rock also is cut into slabs, polished, and used as indoor building stones. Verde antique, a variety often seen in the lobbies of office buildings, consists of green serpentine rock with streaks of white [calcite] or [dolomite] in it.
In the [Balcones fault zone] area (shown on the Texas [physiographic outline map], [p. 42]) from Uvalde County to Williamson County, [serpentine] occurs with Upper [Cretaceous] rocks. The serpentine rock is seen at the surface in a few places (such as in Travis and Uvalde counties), but much of it is underground. In several oil fields of this area (as at Thrall field in Williamson County and at Lytton Springs field in Caldwell County), the serpentine rocks contain oil.
Serpentinite. See [Serpentine].
Shale
[Shale] is a [sedimentary] rock made up of tightly packed [clay] and mud particles. It has a smooth appearance because it is so fine grained. In fact, most of the particles in it are too small to be distinguished with a magnifying glass. These particles are the weathered remains of earlier rocks. They were carried by creeks and rivers to other parts of the land or to the sea, where they formed layers of clay and mud. Later, other [sediments] were deposited on top of them. The weight of these new sediments squeezed the clays and muds together to form firm, compact shale.
[Shale] looks very much like some clays. It, like [clay], can be almost any color. If the shale contains animal or plant matter, it is black, gray, or blue. If it contains iron oxide (many minerals containing iron alter to this material), it is a shade of red, yellow, or brown. Shale is soft and can be easily scratched by a knife. It also is brittle and crumbles easily. This rock has a property that will help you to distinguish it from clay: the particles that make up the shale were deposited in layers, and the shale splits into flat, thin flakes along these layers, which clay will not do.
[Shale] is fairly abundant in Texas, especially in [Mississippian], [Pennsylvanian], and [Cretaceous] [formations]. For example, Pennsylvanian shales are found at the surface in north-central Texas, in the area around the [Llano uplift] of central Texas, and in the Marathon and Solitario uplifts of west Texas.
Many of [shale]’s uses are the same as those of [clay]. Some of it can be used to make brick, tile, and other products, and some is often used instead of clay in making portland cement. Cement plants at Dallas, El Paso, Fort Worth, and Waco are located at places where [Cretaceous] limestones, which also are used in cement making, and Cretaceous shales are found near each other at the surface.
Oil [shale], from which petroleum can be obtained by heating, has been found in central Texas. It occurs in [Mississippian] [formations] in Lampasas, McCulloch, and San Saba counties. Because oil is much less expensive to obtain from wells, it is not produced from these shales.
Silver Minerals ([Argentite], [Cerargyrite], [Native Silver])
Silver has many uses. Like [gold], it is a beautiful metal that long has been used for coins and ornaments. A large amount of silver goes to make articles such as spoons, forks, platters, and trays. The photographic industry uses silver—much of the film for cameras is coated with a silver halide. Doctors and dentists use silver, too. The mixture that a dentist uses to fill teeth contains silver along with several other metals. Doctors sometimes use silver wire to fasten broken bones, and silver compounds and solutions, such as silver nitrate, are used in some kinds of medical treatment.
Perhaps more people have heard of legendary, lost silver mines of Texas than of the actual and important silver deposits found in the Trans-Pecos country of west Texas. Some of the west Texas [silver minerals] include [argentite], [cerargyrite], and [native silver]. Although the argentite and native silver commonly found there are mixed with [galena], a lead mineral, or with [chalcocite], a copper mineral, they also occur separately.
The [element] silver is found alone as [native silver]. When pure, it is rather easy to recognize. It is metallic and has a silver-white color that may tarnish to gray, black, or yellowish brown. Native silver is heavy (it has a [specific gravity] of 10.5) and soft (a pocket knife scratches it easily). When you rub it across a [streak] plate, native silver, unless it is tarnished, leaves a shiny, silver-white streak. This metal is so ductile that it can be drawn into a wire. It is also malleable and flattens when hit with a hammer.
Silver occurs as crystals, which are poorly shaped [cubes] and [octahedrons], or as irregular masses. It may have a net-like appearance (called reticulate), or it may be shaped like little needles (described then as acicular). It occurs in wires (then called filiform) or as scales or plates.
Prospector.
Two of the Texas [silver minerals], [argentite] and [cerargyrite], do not resemble silver at all. Argentite, a silver sulfide, is also called silver glance. It is a dark, lead-gray mineral with a metallic luster that weathers to a dull black. When you rub it across a [streak] plate, argentite gives a shiny, blackish to lead-gray streak. This mineral is soft enough to leave a mark on paper. It has a [specific gravity] of 7.3, and it is [sectile] enough to be cut smoothly (like soap) with a knife. In some places argentite is found as irregular masses or as a coating on rocks and other minerals.
Another silver mineral, [cerargyrite] (or horn silver) is a silver chloride. This mineral has a nonmetallic luster and is [transparent] to [translucent]. It resembles pearl-gray, white, greenish, or colorless wax. When exposed to the light it turns violet brown or black. Cerargyrite is soft—you can scratch it with a fingernail. Like [argentite], it is [sectile]. This mineral has a [specific gravity] of 5.5, and it commonly occurs as irregular masses and as crusts.
These [silver minerals] have been mined at a number of places in [Trans-Pecos] Texas. The largest silver mine in Texas, the Presidio mine, is located near Shafter in south-central Presidio County. It contains [argentite], [cerargyrite], and [native silver], along with [galena] and several other minerals. This mine is not open now, but in the years between 1885 and 1942, it produced a large amount of silver along with some lead and [gold]. There are several other lead-silver mines in this Shafter area, but none has produced as much as the Presidio mine.
In this mine, the [silver minerals] occur mostly in large, flat deposits in [Permian] [limestone] and other [sedimentary] rocks. The minerals are believed to have been deposited there—probably during [Tertiary] time—by solutions that came from hot [magma] far below the rocks. As they moved in along the layers of limestone, the solutions replaced portions of this rock with minerals containing silver, lead, and other [elements]. Later, water seeped into these deposits and dissolved some of the minerals. This dissolved material was then re-deposited, and it formed most of the minerals we now find there.
No silver is being mined in Texas at present, but it has, in the past, been produced from other Trans-Pecos mines. [Galena] that contains silver (called argentiferous galena) has been mined at the Bird mine at Altuda Mountain (about 14 miles east of Alpine) in northern Brewster County. It also has been obtained from mines in the Quitman Mountains and in the Eagle Mountains of Hudspeth County. Some [cerargyrite] has been mined at the Plata Verde mine near the Culberson-Hudspeth County line.
Several mines in the Van Horn area of Culberson and Hudspeth counties have produced silver along with copper. An important silver mine in this area is the now idle and flooded Hazel mine. (This mine is described with [copper minerals] on [p. 52].)
Smoky Quartz. See [Quartz].
Soapstone. See [Talc] and [Soapstone].
Specular Hematite. See [Hematite].
Sulfur
[Sulfur] is one of Texas’ most valuable minerals. It consists of only a single [element], sulfur. This mineral has a resinous luster and is [transparent] to [translucent]. Sulfur ordinarily is yellow, but impurities cause it to look greenish, brownish, reddish, or grayish. When you rub it across a [streak] plate, it leaves a white or a pale-yellow streak. Sulfur has a specific gravity of 2.04 to 2.09 and is soft enough to be scratched by a copper penny. It breaks with a [conchoidal] to uneven [fracture]. When it gets hot enough (478° Fahrenheit), sulfur will burn. For this reason, it often is called brimstone.
[Sulfur] does not conduct electricity and is a poor conductor of heat. You can test how poorly heat passes through it by holding a fragment of sulfur up to your ear. You may be able to hear a crackling sound. The sound results when the outer part of the fragment expands (due to the heat from your hand) while the inner part (which has received no heat) remains unchanged.
Crystals of [sulfur] are sometimes found, and most of them have either a double-pyramid shape or a flat, tabular shape. Sulfur also occurs as compact masses, as crusts, and as scattered grains.
Native [sulfur] deposits are found in two widely separated areas of Texas—one in west Texas and the other along the Gulf Coast in southeast Texas, extending over into Louisiana. In the Gulf Coast area, native sulfur is found on some of the [salt] domes.
The [salt] domes are huge (from about half a mile to more than 2 miles across), column-shaped masses made up of [halite] and some [anhydrite]. These masses have pushed up toward the surface through thousands of feet of [sand], [clay], and other [sedimentary rocks]. On top of many of the salt columns is a covering of [limestone] ([calcite]), anhydrite, and [gypsum] known as the cap-rock. It is in this cap-rock that the [sulfur] is found.
It is thought that when the masses of [halite] and [anhydrite] pushed toward the earth’s surface, some of the upper part of the halite dissolved. The anhydrite, however, did not dissolve, and it remained on top of the [salt] column. Then, a part of this anhydrite was altered into the [gypsum], [limestone], and [sulfur] that now are found in some of the cap-rocks. Laboratory experiments have shown that the sulfur in the cap-rocks likely formed through the action of sulfate-reducing bacteria. These bacteria, in the presence of petroleum, converted the sulfate in some of the anhydrite into hydrogen sulfide. Later, hydrogen sulfide was oxidized—perhaps by reaction with more of the anhydrite—to form the sulfur.
Most of the large cap-rock [sulfur] deposits are about 1,500 to 2,400 feet underground. At first, an attempt was made to get this sulfur out of the ground by digging shafts down to it, but loose, wet, caving sands and poisonous gases, such as hydrogen sulfide, made this mining method almost impossible. Finally, a chemist, Herman Frasch, found a way to obtain the sulfur by making use of sulfur’s low melting point. When sulfur gets slightly hotter than boiling water (235° to 247° Fahrenheit), it melts and becomes a dark, yellowish-brown liquid.
In the Frasch method of [sulfur] mining, a well is drilled into the salt-dome cap-rock, and three pipes, one inside the other, are put into the well. Superheated water under pressure (hotter than 212° Fahrenheit, the temperature at which water ordinarily turns into steam) is sent down one of the pipes to melt the sulfur in the cap-rock around the bottom of the well. Then, compressed air is sent down another of the pipes. This air presses against the liquid sulfur and forces it up to the surface through the third pipe. At the surface, the sulfur is poured into bins, where it cools and becomes a solid again, or it is transported molten, in pipelines and tankers.
[Sulfur] has been obtained from a number of the Texas Gulf Coast [salt] domes including Bryan Mound, Clemens dome, Damon Mound, and Hoskins Mound in Brazoria County; Palangana dome in Duval County; Long Point dome, Nash dome, and Orchard dome in Fort Bend County; High Island dome in Galveston County; Fannett dome and Spindletop dome in Jefferson County; Moss Bluff dome in Liberty County; Gulf dome in Matagorda County; and Boling dome in Wharton County.
In west Texas, [sulfur] occurs in [Permian] rocks both at the surface and underground. A small amount of sulfur has been mined in the Rustler Springs area of northeastern Culberson County and northwestern Reeves County, about 50 miles northwest of Pecos. There, scattered grains, crystals, and irregular masses of sulfur occur in cracks and in dissolved-out openings in the Castile [Gypsum] and in the surface [gravel], gypsum, [sand], and [clay] that cover most of this [formation].
[Sulfur] has many uses. It is used as an insect-killer, thus helping our food crops to grow. It is used in pulp and paper manufacturing and in the vulcanizing of rubber. Some other uses are in the making of paints, dyes, and explosives. A large amount of sulfur goes to make sulfuric acid, which itself has numerous uses in the chemical, steel, oil refining, and other industries.
[Sulfur] is obtained from the cap-rock of [Gulf Coastal Plain] [salt] domes by the Frasch process.
[Sulfur] Uncemented [Sediments] [Limestone] Sulfur-Bearing Limestone Hot Water Melted Sulfur [Anhydrite]
Talc and [Soapstone]
[Talc], a hydrous magnesium silicate, is an extremely soft mineral—your fingernail scratches it easily. It has a greasy or a pearly luster, and its color is white, light green, or gray. When rubbed across a [streak] plate, it leaves a white streak.
[Talc] cleaves perfectly in one direction, and the [cleavage fragments] are thin, flat, and sheet-like. Its [fracture] is uneven. This mineral has a soaplike or greasy feel, and it is [sectile]—a knife will cut through it. Talc is not particularly heavy—it has a [specific gravity] of 2.7 to 2.8. This mineral seldom occurs with a crystal shape. More commonly it is [massive] and is [granular] or layered.
[Talc] is not always found as a single, pure mineral. In nature, it commonly occurs mixed with one or more other minerals, such as tremolite, anthophyllite, chlorite, and [magnetite]. This combination of talc with other minerals forms a soft, greasy or soapy-feeling [metamorphic rock] called [soapstone]. The talc in this rock may be difficult to identify without special laboratory tests.
In Texas, [talc and soapstone] are found in [Precambrian] [metamorphic] rocks. In west Texas, talc occurs in an area about 20 miles long (just north of U. S. Highway 80 in the vicinity of Allamoore, Eagle Flat siding, and Talc Rock siding) in Hudspeth County. Some of this talc is mined from open pits and used by the ceramic industry to make wall tile. Some of it is finely ground, mixed with insect poison, and used as insect powders and dusts.
[Talc] [schist] from the Allamoore area of Hudspeth County, Texas.
Deposits of [soapstone], containing [talc], occur in the [Llano uplift] area of central Texas with [schist], [gneiss], and [serpentine] rocks in northeastern Gillespie, northwestern Blanco, and southern Llano counties. Smaller deposits occur in northeastern Mason County and in northwestern and southeastern Llano County.
The [Llano uplift] area soapstones are light green to light buff. It is thought that some of them were once [igneous rocks] that contained magnesium minerals. [Fluids], along with great heat and pressures below the earth’s surface, changed these [igneous] rocks into [soapstone].
Some of this [Llano uplift] area [soapstone] is mined from open pits near Willow City in Gillespie County. It is used mostly in making insect powders and roofing [granules]. In addition, some of the central Texas soapstones have been used for hearths and for fireplace linings.
Topaz
[Topaz], an aluminum fluorosilicate, is a mineral especially prized by collectors because many specimens are gemstones. Topaz is [transparent], has a glassy luster, and is quite hard (neither [quartz] nor a steel file will scratch it). The topaz that has been found in Texas is either colorless, pale blue, or sky blue. This mineral is fairly heavy—its [specific gravity] is 3.4 to 3.6. It cleaves perfectly in one direction (called basal [cleavage]), and some of the [cleavage fragments] have a flat, slabby appearance.
[Topaz] is commonly found as prism-shaped crystals, as [cleavage fragments], and as irregular grains. Some fragments of topaz look like [quartz]. Topaz, however, is harder and heavier than quartz, and it has perfect basal [cleavage], which quartz does not have.
In Texas, crystals, grains, and [cleavage fragments] of [topaz] occur in the Llano uplift area of central Texas. They are found near Streeter and Grit in west-central Mason County and near Katemcy in northern Mason County. Here, some of the topaz occurs in [Precambrian] [pegmatite] veins that cut through [granite] rocks. Most of the topaz, however, is found as [pebbles] in the [gravels] of nearby creeks, where it has washed after weathering out of the rocks.
[Topaz] crystal from near Streeter, Mason County, Texas.
[Topaz] probably originates when hot [fluids] move up out of molten [magma] into cracks and cavities in the surrounding rocks. There, the fluids react with [elements] in the rocks to form the topaz.
[Topaz] is a good gemstone because, in addition to its beauty, it is hard and is not easily marred by scratches. The Mason County topaz makes excellent gemstones. Most of it is beautiful and clear and is either colorless or of a pleasing blue color. These stones are cut, polished, and mounted in rings and other jewelry. A number of specimens of this Mason County topaz are displayed in museums.
Tourmaline
[Tourmaline] is a complex silicate of boron and aluminum. Other [elements], such as magnesium, sodium, lithium, calcium, iron, or fluorine, also may be present. This mineral has a glassy to resinous luster. Only the dark-colored varieties of tourmaline have been found in Texas. One is a black variety called [schorl], and another is a brown variety called [dravite]. Other kinds of tourmaline, although not found in Texas, are colorless or some shade of blue, yellow, red, pink, or green. Some crystals even show more than one color.
[Tourmaline] is too hard to scratch with a steel file, it has a [specific gravity] of 3 to 3.25, and it has a [conchoidal] to uneven [fracture]. Very little light passes through the dark varieties, and some fragments of [schorl] look like shiny, black coal.
[Tourmaline] occurs as masses without crystal shapes, but crystals are commonly found. The crystals are prism-shaped and have small vertical grooves, called striations, on the prism faces. When you look at some crystals from an end, you will see that the cross section is a triangle with the sides bowed outward.
Black [tourmaline] crystals with [milky quartz] from north of Llano, Llano County, Texas.
Both the black and the brown varieties of [tourmaline] have been found at several places in the [Llano uplift] of central Texas. One well-known locality is at Town Mountain north of Llano in Llano County. Here, the tourmaline occurs in [milky quartz] that is associated with [Precambrian] [granite] rocks. In west Texas, in Culberson and Hudspeth counties, black tourmaline occurs in [pegmatite] rocks in the Van Horn Mountains, the Carrizo Mountains, and the Wylie Mountains. In the Eagle Mountains of Hudspeth County, it is found in [metamorphic rocks] as well as in pegmatites.
Some [tourmaline] formed from hot [fluids] containing boron that were given off by magmas far below the earth’s surface. These fluids traveled up through cracks and other openings in overlying rocks. As the fluids reacted with other [elements] and compounds, the tourmaline formed.
The clear, light-colored varieties of [tourmaline] are much admired, and they are more widely used as gemstones than are the dark-colored varieties. Some collectors, however, find that the dark-colored Texas tourmalines, when cut and polished, make shiny, attractive gemstones.
Some [tourmaline] is used as grinding material, but no Texas tourmaline is produced for this purpose.
Travertine. See [Calcite].
Uranium Minerals ([Carnotite], [Uranophane], [Pitchblende])
In 1945, the world suddenly became aware of the awesome power of atomic energy when the [element] uranium was used to produce some of the first atomic bombs. Uranium does not occur alone in nature but is found combined with other elements in a number of minerals.
All of the [uranium minerals] are radioactive. The uranium they contain is gradually breaking down and changing into a [series] of 13 other [elements], called daughter elements. Each daughter element breaks down and changes into the next daughter element of the series. While breaking down, these elements give off particles and rays of energy.
This energy or radioactivity is made up of what are called alpha particles, beta particles, and gamma rays. You cannot see, hear, taste, smell, or feel them. The alpha and beta particles are weak and do not travel far. The gamma rays, however, can travel farther and can pass through seemingly solid material. Scientists have found that these rays can move through about 1 foot of rock, 2½ feet of water, and several hundred feet of air.
Prospectors searching for [uranium minerals] carry instruments that are able to detect this radioactivity. The uranium itself gives off only alpha particles, but some of its daughter [elements] give off gamma rays. These daughter elements are normally found with the uranium, and it is their strong gamma rays that the instruments are most apt to detect.
A Geiger counter is used to detect radioactivity.
One of the instruments used is the Geiger counter. It indicates radioactivity by means of a meter, a flashing light, or a clicking sound, which can be heard through earphones. Another instrument for detecting radioactivity is the scintillation counter. It is more sensitive than the Geiger counter and it can detect radioactivity from a greater distance. The scintillation counter can be used from an automobile or an airplane, but the Geiger counter must be quite close to the source of radioactivity to be of use.
Various [uranium minerals] have been found, mostly in small amounts, in a number of places in Texas. Some of these minerals, such as uraninite or [pitchblende], are heavy and dark colored. Others, including [carnotite], tyuyamunite, autunite, and [uranophane], are a shade of yellow or green. They are quite soft. Deposits of the light-colored uranium minerals have been mined from two areas of Texas. One of these areas is in Garza County on the Texas [High Plains], and the other is in Karnes and Live Oak counties in the [Gulf Coastal Plain].
One of the light-colored [uranium minerals], [carnotite], is a potassium-uranium vanadate, which has a bright canary-yellow or lemon-yellow color. This mineral is [transparent] to [translucent] and has an earthy or a pearly luster. Carnotite usually is found as crusts and as powdery masses. It is quite soft and can be scratched with a fingernail.
[Carnotite], along with tyuyamunite, autunite, and several other soft, yellowish or greenish [uranium minerals], is found in the Texas [Gulf Coastal Plain]. These minerals occur in the Jackson, Catahoula, and Oakville strata (which are [Tertiary] in age) in an area extending from Gonzales County to the Rio Grande (in parts of the area indicated by no. 2 and no. 3 on the [geologic map], pp. [4]-5). The largest deposits in this district have been found in the Karnes County area.
The [Gulf Coastal Plain] uranium minerals occur mostly with sandstones and clays in a sequence of strata that contains [volcanic ash]. It is believed that small scattered amounts of uranium compounds that were present in the volcanic ash [sediments] were dissolved by seeping underground water. These waters then moved into the sandstones and clays where they deposited the uranium as [carnotite] and as other [uranium minerals].
Another uranium mineral, [uranophane] (calcium-uranium silicate), also occurs in Texas. Uranophane has a yellow to yellow-orange color and a pearly to greasy luster. When rubbed across a [streak] plate, it leaves a light yellow to a light yellow-orange streak. It is soft enough to be scratched by a copper penny. Uranophane has been found in [extrusive] [igneous rocks] in northwestern Presidio County in west Texas.
A dark-colored uranium mineral, [pitchblende], is a variety of the mineral uraninite, uranium dioxide. Pitchblende does not occur with a crystal shape but rather as rounded and irregular-shaped masses. It is brownish black, greenish black, or black. If you rub it across a [streak] plate, pitchblende leaves a brownish-black streak. This mineral is heavy (it has a [specific gravity] of 6.5 to 8.5) and hard (a pocket knife will not scratch it, although a steel file will). Pitchblende has a submetallic luster and looks dull, greasy, or like pitch or tar.
Small amounts of [pitchblende] have been found at several places in Texas. One of these localities is a few miles west of Burnet in Burnet County in central Texas. Here, the pitchblende occurs in [Precambrian] [igneous rocks] that are associated with [gneiss]. In south Texas, some fine, scattered particles of pitchblende have been found about 325 feet below the surface in [Tertiary] ([Pliocene]) [sediments] that cover the Palangana [salt] dome in Duval County. No pitchblende is mined in Texas.
Uranophane. See [Uranium Minerals].
Vitrophyre. See [Obsidian] and [Vitrophyre].
Volcanic Ash ([Pumicite])
[Volcanic ash] deposits, which also are known as [pumicite], are loose and powdery. They are made up mostly of material that is thrown into the air when volcanoes erupt. If a volcano erupts with a violent explosion, the nearby rocks are blown into powder. Molten [lava] also is hurled into the air, where some of it immediately cools to become tiny bubbles and particles of glass. The winds may carry some of this fine material far away before depositing it.
Deposits of [volcanic ash] are white, bluish, greenish, yellowish, or grayish, and some of them glisten like snow in the sunlight. They feel rough and gritty. When examined under a microscope, this material shows the tiny curved and sharp-cornered particles of the broken [volcanic] glass. Deposits of volcanic ash may also contain [clay], silt, [sand], or other impurities.
Volcanoes, which may have been located in the Davis Mountains and in other areas of west Texas and in northern Mexico, erupted during [Tertiary] time. The [volcanic] ash that we find at the surface today in some of the Tertiary [formations] in Texas could have come from these volcanoes. Tertiary [volcanic ash] deposits occur in the Texas [Gulf Coastal Plain] (such as in Brazos, Fayette, Karnes, Polk, Starr, Trinity, and other counties) and in the [Trans-Pecos] country of west Texas.
[Volcanic ash] deposits of [Quaternary] ([Pleistocene]) age, which are less than a million years old, are found in a number of counties on the Texas [High Plains]. Farther to the east, ash deposits occur in Baylor, Dickens, Kent, and Wilbarger counties. This volcanic ash may have come from a volcano that erupted in northern New Mexico during Quaternary time.
[Volcanic ash] or [pumicite] has several commercial uses. Some is used to make pozzolan cement, and some is used in sweeping compounds, cleansing and scouring powders, and abrasive soaps. Pumicite has been mined in Dickens, Scurry, Starr, and several other counties of Texas.
Wad. See [Manganese Minerals].
Wood Opal. See [Opal].
COMPOSITION, HARDNESS, AND [SPECIFIC GRAVITY] OF SOME TEXAS MINERALS
For convenient reference, the Texas minerals described in this book are listed below, together with their chemical compositions, specific gravities, and hardness. You will be able to find similar information about additional minerals in mineralogy textbooks such as those noted on [page 24].
| Mineral | Composition | [Specific Gravity] | Hardness |
| [Albite] | NaAlSi₃O₈ | 2.62 | 6 |
| [Almandite] | Fe₃Al₂ (SiO₄)₂ | 4.2 | 7 |
| [Amphibole asbestos] | Ca₂Mg₅Si₈O₂₂(OH)₂ | 3.0-3.3 | 1-2½ |
| [Anhydrite] | CaSO₄ | 2.9 | 3-3½ |
| [Argentite] | Ag₂S | 7.3 | 2-2½ |
| [Azurite] | Cu₃(CO₃)₂(OH)₂ | 3.77 | 3½-4 |
| [Barite] | BaSO₄ | 4.5 | 3-3½ |
| [Biotite] | K(Mg, Fe)₃AlSi₃O₁₀(OH)₂ | 2.8-3.2 | 2½-3 |
| [Braunite] | 3MnMnO₃MnSiO₃ | 4.75-4.82 | 6-6½ |
| [Calcite] | CaCO₃ | 2.72 | 3 |
| [Carnotite] | K₂O·2UO₃·V₂O₅·nH₂O | 5.03 | 2 |
| [Cassiterite] | SnO₂ | 6.8-7.1 | 6-7 |
| [Celestite] | SrSO₄ | 3.95-3.97 | 3-3½ |
| [Cerargyrite] | AgCl | 5.5 | 1-1½ |
| [Chalcocite] | Cu₂S | 5.5-5.8 | 2½-3 |
| [Chalcopyrite] | CuFeS₂ | 4.1-4.3 | 3½-4 |
| [Cinnabar] | HgS | 8.10 | 2½ |
| [Dolomite] | CaMg(CO₃)₂ | 2.85 | 3½-4 |
| [Feldspar] (see Albite, [Microcline], [Orthoclase]) | |||
| [Fluorite] | CaF₂ | 3.18 | 4 |
| [Galena] | PbS | 7.4-7.6 | 2½ |
| [Garnet] (see Almandite, [Grossularite]) | |||
| [Gold] | Au | 15.0-19.3 | 2½-3 |
| [Graphite] | C | 2.2 | 1-2 |
| Grossularite | Ca₃Al₂(SiO₄)₃ | 3.53 | 6½ |
| [Gypsum] | CaSO₄·2H₂O | 2.32 | 2 |
| [Halite] | NaCl | 2.16 | 2½ |
| [Hematite] | Fe₂O₃ | 5.26 | 1-6½ |
| [Hollandite] | MnBaMn₁₆O₁₄ | 4.7-5 | 4-6 |
| [Limonite] | FeO(OH)·nH₂O | 3.6-4.0 | 1-5½ |
| [Magnetite] | Fe₃O₄ | 5.18 | 6 |
| [Malachite] | Cu₂CO₃(OH)₂ | 3.9-4.03 | 3½-4 |
| [Mica] (see [Muscovite], Biotite) | |||
| Microcline | KAlSi₃O₈ | 2.54-2.57 | 6 |
| Muscovite | KAl₃Si₃O₁₀(OH)₂ | 2.76-3.1 | 2-2½ |
| [Opal] | SiO₂·nH₂O | 1.9-2.2 | 5-6 |
| Orthoclase | KAlSi₃O₈ | 2.57 | 6 |
| [Pitchblende] | UO₂ | 6.5-8.5 | 5½ |
| [Pyrite] | FeS₂ | 5.02 | 6-6½ |
| [Pyrolusite] | MnO₂ | 4.75 | 1-2 |
| [Quartz] | SiO₂ | 2.65 | 7 |
| [Serpentine] | Mg₃Si₂O₅(OH)₄ | 2.48 | 3-4 |
| Silver | Ag | 10.5 | 2½-3 |
| [Sulfur] | S | 2.05-2.09 | 1½-2½ |
| [Talc] | Mg₃Si₄O₁₀(OH)₂ | 2.7-2.8 | 1 |
| [Topaz] | Al₂SiO₄(F,OH)₂ | 3.4-3.6 | 8 |
| [Tourmaline] | Complex silicate of boron and aluminum | 3.0-3.25 | 7-7½ |
| [Uranophane] | CaO·2UO₃·2SiO₂·7H₂O | 3.8-3.9 | 2-3 |