Silver has many uses. Likegold, 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 Texassilver mineralsincludeargentite,cerargyrite, andnative silver. Although the argentite and native silver commonly found there are mixed withgalena, a lead mineral, or withchalcocite, a copper mineral, they also occur separately.
Theelementsilver is found alone asnative 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 aspecific gravityof 10.5) and soft (a pocket knife scratches it easily). When you rub it across astreakplate, 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 shapedcubesandoctahedrons, or as irregular masses. It may have a net-like appearance (calledreticulate), or it may be shaped like little needles (described then asacicular). It occurs in wires (then calledfiliform) or as scales or plates.
Prospector.
Prospector.
Two of the Texassilver minerals,argentiteandcerargyrite, do not resemble silver at all.Argentite, a silver sulfide, is also calledsilver glance. It is a dark, lead-gray mineral with a metallic luster that weathers to a dull black. When you rub it across astreakplate, argentite gives a shiny, blackish to lead-gray streak. This mineral is soft enough to leave a mark on paper. It has aspecific gravityof 7.3, and it issectileenough 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(orhorn silver) is a silver chloride. This mineral has a nonmetallic luster and istransparenttotranslucent. 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. Likeargentite, it issectile. This mineral has aspecific gravityof 5.5, and it commonly occurs as irregular masses and as crusts.
Thesesilver mineralshave been mined at a number of places inTrans-PecosTexas. The largest silver mine in Texas, the Presidio mine, is located near Shafter in south-central Presidio County. It containsargentite,cerargyrite, andnative silver, along withgalenaand 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 andgold. There are several other lead-silver mines in this Shafter area, but none has produced as much as the Presidio mine.
In this mine, thesilver mineralsoccur mostly in large, flat deposits inPermianlimestoneand othersedimentaryrocks. The minerals are believed to have been deposited there—probably duringTertiarytime—by solutions that came from hotmagmafar 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 otherelements. 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.Galenathat contains silver (calledargentiferous 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. Somecerargyritehas 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 withcopper mineralsonp. 52.)
Smoky Quartz.SeeQuartz.
Soapstone.SeeTalcandSoapstone.
Specular Hematite.SeeHematite.
Sulfuris one of Texas’ most valuable minerals. It consists of only a singleelement, sulfur. This mineral has a resinous luster and istransparenttotranslucent. Sulfur ordinarily is yellow, but impurities cause it to look greenish, brownish, reddish, or grayish. When you rub it across astreakplate, 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 aconchoidalto unevenfracture. When it gets hot enough (478° Fahrenheit), sulfur will burn. For this reason, it often is calledbrimstone.
Sulfurdoes 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 ofsulfurare 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.
Nativesulfurdeposits 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 thesaltdomes.
Thesaltdomes are huge (from about half a mile to more than 2 miles across), column-shaped masses made up ofhaliteand someanhydrite. These masses have pushed up toward the surface through thousands of feet ofsand,clay, and othersedimentary rocks. On top of many of the salt columns is a covering oflimestone(calcite), anhydrite, andgypsumknown as thecap-rock. It is in this cap-rock that thesulfuris found.
It is thought that when the masses ofhaliteandanhydritepushed 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 thesaltcolumn. Then, a part of this anhydrite was altered into thegypsum,limestone, andsulfurthat 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-rocksulfurdeposits 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 ofsulfurmining, 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.
Sulfurhas been obtained from a number of the Texas Gulf Coastsaltdomes 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,sulfuroccurs inPermianrocks 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 CastileGypsumand in the surfacegravel, gypsum,sand, andclaythat cover most of thisformation.
Sulfurhas 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.
Sulfuris obtained from the cap-rock ofGulf Coastal Plainsaltdomes by the Frasch process.
Sulfuris obtained from the cap-rock ofGulf Coastal Plainsaltdomes by the Frasch process.
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 astreakplate, it leaves a white streak.
Talccleaves perfectly in one direction, and thecleavage fragmentsare thin, flat, and sheet-like. Itsfractureis uneven. This mineral has a soaplike or greasy feel, and it issectile—a knife will cut through it. Talc is not particularly heavy—it has aspecific gravityof 2.7 to 2.8. This mineral seldom occurs with a crystal shape. More commonly it ismassiveand isgranularor layered.
Talcis 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, andmagnetite. This combination of talc with other minerals forms a soft, greasy or soapy-feelingmetamorphic rockcalledsoapstone. The talc in this rock may be difficult to identify without special laboratory tests.
In Texas,talc and soapstoneare found inPrecambrianmetamorphicrocks. 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.
Talcschistfrom the Allamoore area of Hudspeth County, Texas.
Talcschistfrom the Allamoore area of Hudspeth County, Texas.
Deposits ofsoapstone, containingtalc, occur in theLlano upliftarea of central Texas withschist,gneiss, andserpentinerocks in northeastern Gillespie, northwestern Blanco, and southern Llano counties. Smaller deposits occur in northeastern Mason County and in northwestern and southeastern Llano County.
TheLlano upliftarea soapstones are light green to light buff. It is thought that some of them were onceigneous rocksthat contained magnesium minerals.Fluids, along with great heat and pressures below the earth’s surface, changed theseigneousrocks intosoapstone.
Some of thisLlano upliftareasoapstoneis mined from open pits near Willow City in Gillespie County. It is used mostly in making insect powders and roofinggranules. In addition, some of the central Texas soapstones have been used for hearths and for fireplace linings.
Topaz, an aluminum fluorosilicate, is a mineral especially prized by collectors because many specimens are gemstones. Topaz istransparent, has a glassy luster, and is quite hard (neitherquartznor 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—itsspecific gravityis 3.4 to 3.6. It cleaves perfectly in one direction (called basalcleavage), and some of thecleavage fragmentshave a flat, slabby appearance.
Topazis commonly found as prism-shaped crystals, ascleavage fragments, and as irregular grains. Some fragments of topaz look likequartz. Topaz, however, is harder and heavier than quartz, and it has perfect basalcleavage, which quartz does not have.
In Texas, crystals, grains, andcleavage fragmentsoftopazoccur 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 inPrecambrianpegmatiteveins that cut throughgraniterocks. Most of the topaz, however, is found aspebblesin thegravelsof nearby creeks, where it has washed after weathering out of the rocks.
Topazcrystal from near Streeter, Mason County, Texas.
Topazcrystal from near Streeter, Mason County, Texas.
Topazprobably originates when hotfluidsmove up out of moltenmagmainto cracks and cavities in the surrounding rocks. There, the fluids react withelementsin the rocks to form the topaz.
Topazis 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.
Tourmalineis a complex silicate of boron and aluminum. Otherelements, suchas 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 calledschorl, and another is a brown variety calleddravite. 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.
Tourmalineis too hard to scratch with a steel file, it has aspecific gravityof 3 to 3.25, and it has aconchoidalto unevenfracture. Very little light passes through the dark varieties, and some fragments ofschorllook like shiny, black coal.
Tourmalineoccurs as masses without crystal shapes, but crystals are commonly found. The crystals are prism-shaped and have small vertical grooves, calledstriations, 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.
Blacktourmalinecrystals withmilky quartzfrom north of Llano, Llano County, Texas.
Blacktourmalinecrystals withmilky quartzfrom north of Llano, Llano County, Texas.
Both the black and the brown varieties oftourmalinehave been found at several places in theLlano upliftof central Texas. One well-known locality is at Town Mountain north of Llano in Llano County. Here, the tourmaline occurs inmilky quartzthat is associated withPrecambriangraniterocks. In west Texas, in Culberson and Hudspeth counties, black tourmaline occurs inpegmatiterocks in the Van Horn Mountains, the Carrizo Mountains, and the Wylie Mountains. In the Eagle Mountains of Hudspeth County, it is found inmetamorphic rocksas well as in pegmatites.
Sometourmalineformed from hotfluidscontaining 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 otherelementsand compounds, the tourmaline formed.
The clear, light-colored varieties oftourmalineare 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.
Sometourmalineis used as grinding material, but no Texas tourmaline is produced for this purpose.
Travertine.SeeCalcite.
In 1945, the world suddenly became aware of the awesome power of atomic energy when theelementuraniumwas 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 theuranium mineralsare radioactive. The uranium they contain is gradually breaking down and changing into aseriesof 13 otherelements, calleddaughterelements. 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 orradioactivityis made up of what are called alpha particles, beta particles, and gamma rays. You cannot see, hear, taste, smell, or feel them. Thealpha 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 foruranium mineralscarry instruments that are able to detect this radioactivity. The uranium itself gives off only alpha particles, but some of its daughterelementsgive 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.
A Geiger counter is used to detect radioactivity.
One of the instruments used is theGeiger 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 thescintillation 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 quiteclose to the source of radioactivity to be of use.
Variousuranium mineralshave been found, mostly in small amounts, in a number of places in Texas. Some of these minerals, such as uraninite orpitchblende, are heavy and dark colored. Others, includingcarnotite, tyuyamunite, autunite, anduranophane, 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 TexasHigh Plains, and the other is in Karnes and Live Oak counties in theGulf Coastal Plain.
One of the light-coloreduranium minerals,carnotite, is a potassium-uranium vanadate, which has a bright canary-yellow or lemon-yellow color. This mineral istransparenttotranslucentand 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 greenishuranium minerals, is found in the TexasGulf Coastal Plain. These minerals occur in the Jackson, Catahoula, and Oakville strata (which areTertiaryin 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 thegeologic map, pp.4-5). The largest deposits in this district have been found in the Karnes County area.
TheGulf Coastal Plainuranium minerals occur mostly with sandstones and clays in a sequence of strata that containsvolcanic ash. It is believed that small scattered amounts of uranium compounds that were present in the volcanic ashsedimentswere dissolved by seeping underground water. These waters then moved into the sandstones and clays where they deposited the uranium ascarnotiteand as otheruranium 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 astreakplate, 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 inextrusiveigneous rocksin northwestern Presidio County in west Texas.
A dark-colored uranium mineral,pitchblende, is a variety of the mineraluraninite, 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 astreakplate, pitchblende leaves a brownish-black streak. This mineral is heavy (it has aspecific gravityof 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 ofpitchblendehave 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 inPrecambrianigneous rocksthat are associated withgneiss. In south Texas, some fine, scattered particles of pitchblende have been found about 325 feet below the surface inTertiary(Pliocene)sedimentsthat cover the Palanganasaltdome in Duval County. No pitchblende is mined in Texas.
Uranophane.SeeUranium Minerals.
Vitrophyre.SeeObsidianandVitrophyre.
Volcanic ashdeposits, which also are known aspumicite, 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. Moltenlavaalso 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 ofvolcanic ashare 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 brokenvolcanicglass. Deposits of volcanic ash may also containclay, 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 duringTertiarytime. Thevolcanicash that we find at the surface today in some of the Tertiaryformationsin Texas could have come from these volcanoes. Tertiaryvolcanic ashdeposits occur in the TexasGulf Coastal Plain(such as in Brazos, Fayette, Karnes, Polk, Starr, Trinity, and other counties) and in theTrans-Pecoscountry of west Texas.
Volcanic ashdeposits ofQuaternary(Pleistocene) age, which are less than a million years old, are found in a number of counties on the TexasHigh 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 ashorpumicitehas 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.SeeManganese Minerals.
Wood Opal.SeeOpal.