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.

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.

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.

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.

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.

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.)

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.