Part 11

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.)

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.

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.