Celestite

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 ascrystallinemasses inigneous rocksand as loosepebblesthat have weathered out of these rocks.

Cassiteriteoccurs in a number of places in the United States but not in large quantities. A small amount of cassiterite has been found inquartzveins inPrecambriangranitein 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 thegraniterocks in these areas were formed, probably not all of the hot magmas cooled and hardened at the same time. Thefluidsgiven off by the remaining magmas contained tin and several otherelements. It is believed that these fluids moved up into cracks in the granite rocks and formed thecassiterite.

Celestiteis 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 wordcaelestis, meaningof the sky.

Celestitehas a glassy to a pearly luster, and it is eithertransparentortranslucent. It gives a whitestreakwhen rubbed across a streak plate. Celestite has aspecific gravityof 3.95 to 3.97. It is, however, lighter thanbarite, 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.

Celestitecleavage fragmentfrom Lampasas County, Texas.

Celestiteoccurs commonly either asprism-shaped or flat crystals and as cleavable,granular, or fibrouscrystallinemasses. In Texas, it is found in geodes, as roundednodules, or as bedded or layer-like deposits in limestones and othersedimentary rocks. In Real County, celestite occurs on the walls of a cave inCretaceouslimestone.

Somecelestitemay 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 othersedimentary 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 ofcelestiteoccur inPermianrocks in Coke, Fisher, and Nolan counties and in LowerCretaceousrocks in Brown, Comanche, and Mills counties. Celestite geodes andnodulesare found in Lower Cretaceouslimestonerocks in Lampasas, Travis, and Williamson counties, and in Permian rocks in Coke, Fisher, Nolan, and Taylor counties.

Celestiteis one of two minerals (the other mineral isstrontianite, 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.SeeSilver Minerals.

Chalcedony.SeeQuartz.

Chalcocite.SeeCopper Minerals.

Chalcopyrite.SeeCopper Minerals.

Chalk.SeeLimestone.

Chert(Flint).SeeQuartz.

Chrysotile.SeeAsbestos;Serpentine.

Cinnabar, which is mercuric sulfide, is the most common mercury mineral. It has a dark red or a bright yellowish-red color and istransparenttotranslucent. When rubbed across astreakplate, it leaves a dark red streak. If pure, cinnabar has a brilliant, shiny, nonmetallic luster. It is, however, commonly found mixed with impurities, such asclay,calcite, iron oxide, or bituminous material, and then it looks dull and earthy. Cinnabar is quite heavy—it has aspecific gravityof 8.10. It is rather soft, and you can scratch it with a copper penny.

Some prospectors use a quick chemical test to identifycinnabar. 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.

Cinnabaroccurs as small crystals or as fine-grained or compactcrystallinemasses. 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.

Cinnabaroccurs 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 Texascinnabaris found in cracks, pores, and breccia-filled cavities ofCretaceouslimestones and clays. If you will look at the Texasgeologic map(pp.4-5), you will see thatigneous rocksoccur in this district. Many millions of years ago during theTertiaryPeriod, when these igneous rocks were still hotmagma, some of them pushed up under the Cretaceous rocks and emittedfluidscontaining mercury. The fluids moved upward through cracks and pores in the Cretaceousrocks where they deposited the mercury as cinnabar and as other mercury minerals.

Cinnabarcrystals (dark) withcalcitecrystals (white) from the Terlingua area of Brewster County, Texas.

Mercury is an unusualelement. Instead of occurring as a solid metal at ordinary room temperatures, as dogold, 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 calledquicksilver.

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 obtaingoldfrom its ores. One method of accomplishing this is to pass wet gold-bearinggravelor crushed rock over metal plates that are coated with mercury. The gold particles quickly mix with the mercury to form anamalgam, which later can be scraped off the plates. The gold is then recovered by heating the amalgam to drive off the mercury.

Clayis 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 otherelements. In addition, tiny particles ofquartz,calcite, and other minerals may also be present in the clay.

Theclayparticles are all that remain of rocks and of minerals, such asfeldspar, 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.

Clayis 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.

Clayis 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 fromTertiaryformationsof theGulf Coastal Plain, from UpperCretaceousformations in central Texas, and fromPennsylvanianformations in north-central Texas. (You can locate Tertiary, Cretaceous, and Pennsylvanian rocks on the Texasgeologic map, pp.4-5.)

A special kind of white burningclaythat can be used to make chinaware is calledkaolinorchina clay. It contains particles of the clay mineralkaoliniteas 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 ofclay,bentonite, forms from weatheredvolcanic ash. Bentonite contains the clay mineralmontmorilloniteand 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 ofbentoniteoccur chiefly inEoceneTertiaryformationsof theGulf Coastal Plain, inCretaceousformations of the Big Bend area of west Texas, and inQuaternaryformations of the High Plains.

Bentoniteis used as a drilling-fluid additive in the rotary method of drilling for petroleum and gas.

Somebentoniteis used to absorb unwanted coloring material in petroleum and in vegetable oils. It is then known as ableachingclay. Bentonite bleaching clay is obtained from some of theTertiaryformationsalong the TexasGulf Coastal Plain.It has been produced in Angelina, Fayette, Gonzales,Jasper, Walker, and other counties in this area.

Another important use ofbentonite, and of otherclay, too, is asdrilling 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 theGulf Coastal Plain, drilling clay is obtained fromTertiaryformations.

Common Opal.SeeOpal.

A number of minerals containing copper, such aschalcocite,chalcopyrite,malachite, andazurite, occur in small deposits in Texas. They are found chiefly in theLlano upliftarea 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 importantelement. 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 otherelements. For example, some copper is mixed with zinc to makebrassand with tin and a little zinc to makebronze. These mixtures are calledalloys. 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 aschalcocite,chalcopyrite,azurite, ormalachite). 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 thecopper minerals,chalcocite, copper sulfide, also is known ascopper 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 issectile, that is, a knife will cut through it as well as scratch it. When you rub chalcocite across astreakplate, it gives a grayish-black streak. This mineral commonly occurs as compact masses or asgranularmasses.

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 redsandstoneof thePrecambrianHazelFormation. It is thought that long ago, moltenigneousrock material far below the surface sent out hot solutions containing copper and otherelements. These solutions moved upward and deposited minerals in thefracturezone in the sandstone.

Chalcociteoccurs 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 inPermiansedimentaryrocks (called “red beds”) as rounded masses, as scattered grains, and as petrified wood. Because these deposits are far from anyigneousrocks, 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 ascopper pyritesandyellow copper ore. This mineral has a metallic luster and a brass-yellow or a golden-yellow color. When rubbed across astreakplate, 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 forgold. For this reason, it, like ironpyrite, is often calledfool’s gold. (See Gold,p. 60, for ways to tell them apart.)

Chalcopyritecommonly 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.

Somechalcopyriteis found inPrecambriansandstoneat 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 whereigneous rocksoccur.

It is likely that, long ago, hot solutions containing copper moved upward, out of deeply buried moltenmagma. While still far below the surface, the solutions deposited thechalcopyritein cracks and other openings in the nearby rocks.

Twocopper mineralsof Texas,azuriteandmalachite, are copper carbonates. Azurite is commonly calledchessyliteandblue copper; malachite is calledgreen copper carbonate. Because these minerals are carbonates, a drop of dilute hydrochloric acid will fizz and bubble when placed on either of them.

Azuritehas a bright, intense blue color and leaves a bluestreakwhen rubbed across a streak plate.Malachitehas a bright green color and leaves a green streak. These minerals have a nonmetallic luster and a glassy to dull appearance. Commonly, they aretranslucent, although some specimens of azurite aretransparent. Both azurite and malachite are fairly soft—a pocket knife will scratch them, but a copper penny will not.

Azuriteandmalachiteoccur as individualcrystals, 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 asbotryoidal).

Bothazuriteandmalachiteare formed in the same way. Underground waters seep through rocks that contain deposits ofcopper minerals(such aschalcociteandchalcopyrite) and cause chemical reactions which change these minerals into malachite and azurite.

Malachiteis more plentiful thanazurite, but both minerals can be found together. You can expect to find at least one of them at the same localities wherechalcocite,chalcopyrite, and othercopper mineralsoccur.

Coquina.SeeLimestone.

Diatomite.SeeOpal.

Dolomiteis 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 whitestreakon a streak plate and istransparenttotranslucent. 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 thecleavage fragmentsare rhombohedrons. However, thecleavagesof the individual mineral grains in specimens of fine-grainedmassivedolomite are not readily distinguishable.

Dolomiterock from the vicinity of Fairland, Burnet County, Texas.

Most Texasdolomiteoccurs as coarse-, medium-, and fine-grainedcrystallinemasses as the chief mineral in dolomite rock and in dolomiticmarble. 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 mineraldolomitecommonly 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.

Dolomiterock is made up mostly ofcrystallinegrains of the mineral dolomite. In addition,quartzgrains,calcite, and other minerals may be present. Dolomite rock is almost any color—white, buff, pink brown, gray. It resembles somelimestone, 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 alimestone. If the rock isdolomite, 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.

Somedolomiterocks formed directly from materials that were dissolved in sea water, and others are alteredlimestonerocks. 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 seepedthrough these limestones and altered them into dolomite.

Many of thedolomiterocks are found with limestones. In Texas they occur mostly inCambrian,Ordovician,Mississippian,Pennsylvanian,Permian, andCretaceousformations. Thegeologic map(pp.4-5) indicates where these strata appear at the surface in Texas.

Dolomiteis abundant in the Llano uplift area of central Texas—particularly in theCambrianandOrdovicianrocks. 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 withserpentineonp. 88). In addition, Ellenburger (Ordovician) dolomite from Burnet County was used during World War II as a source of the lightweight metal magnesium.

Dravite.SeeTourmaline.

Feldsparis 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 followingelements: potassium, sodium, calcium, and barium. The feldspar minerals that are found in Texas includealbite, a sodium-aluminum silicate, andorthoclaseandmicrocline, which are both potassium-aluminum silicates.

Thefeldsparminerals aretransparenttotranslucentand 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 astreakplate, it leaves a white streak. The feldspars are rather hard—a pocket knife will not scratch them, although a piece ofquartzor a steel file will. These minerals have goodcleavagein two directions. The cleavages meet at an angle of about 90°, so that thecleavage fragmentshave square corners.

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