Part 23

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 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.

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.