Quartzite

Quartzcrystal, with inclusions, from Burnet County, Texas.

Quartzis found as crystals and as masses. Some of the masses are coarselycrystalline, but some are made up of extremely small crystalline particles calledcryptocrystallinequartz. Some of the cryptocrystalline varieties of quartz found in Texas arechalcedony,chert, andjasper. Some of the coarsely crystalline varietiesfound here areamethyst,milky quartz,rose quartz,smoky quartz, androck crystal.

Amethystgeode from the Alpine area of Brewster County, Texas.

A colorless, glassy variety ofquartz, calledrock 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 withigneous rocks, such as those of theLlano upliftarea 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 astourmaline, actinolite, or rutile, enclosed in them.

A clear, glassy variety ofquartz,amethyst, has a purple or violet color. It, likerock crystal, is commonly found in 6-sided prisms with pyramid-shaped ends and is also prized as a gemstone. Amethyst has been found inPrecambrianrocks in theLlano upliftarea 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 inCenozoicigneousrocks in the Sierra Blanca and Quitman Mountains of Hudspeth County and in the Alpine area of Brewster County.

Milky quartzfrom Burnet County, Texas.

A variety ofquartzwith a milk-white color and a glassy to greasy luster is calledmilky quartz. It occurs either as crystals or ascrystallinemasses. Very little light will pass through it. In central Texas, milky quartz occurs abundantly in thePrecambrianrocks of theLlano upliftarea 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 andgravelsalong many streams in Texas.

Somequartzhas 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 inigneous rocks, such as those of theLlano upliftarea of central Texas.

A kind ofquartzwith a smoky brown, a smoky yellow, or a dark brownish-black color is calledsmoky quartz. Its luster is glassy, and it may be eithertranslucentortransparent. Smoky quartz is commonly found as crystals that are shaped like 6-sided prisms with pyramid-like ends. It is commonly associated withigneous rocks, and beautiful specimens have been found in the Lake Buchanan area of Llano and Burnet counties in central Texas.

Smoky-quartz crystals from Burnet County, Texas.

Acryptocrystallinevariety ofquartz,chalcedony, has a waxy to dull luster and a tan, white, gray, or light-blue color. It istranslucentbut nottransparent. 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.

Chalcedonycommonly occurs in some of theTertiaryrocks of the Gulf Coastal Plain. For example, chalcedony associated withopalis found near Freer in northern Duval County. In theHigh Plainsof 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 inextrusiveigneous rocks.

Polishedagatefrom Rio Grandegravelsof Zapata County, Texas.

A variety ofchalcedonythat generally is made up of more than one color is calledagate(although agates consisting of severalshades 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 calledonyx. Agate that has a moss-like or tree-like design in it is calledmoss agate. Some agates make attractive gemstones when cut and polished.

Jasperfrom Uvalde County, Texas. Dark areas are brownish red; light areas are a yellowish-tan.

Muchagatehas been found filling cavities inCenozoicigneous rocksin Brewster, Presidio, and other counties in theTrans-Pecoscountry 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 byagate. Many specimens ofagatized woodhave been collected fromTertiaryformationsin Fayette, Gonzales, Lee, Washington, and other counties of theGulf Coastal Plain. (The agatized wood, along with opalized wood, occurs within about 20 miles of the boundary between no. 2 and no. 3 on thegeologic map, pp.4-5.)

A hard, compact, slightlytranslucentvariety ofcryptocrystallinequartzis calledjasper. It commonly has a red, brown, or yellow color due to the presence of an iron oxide, such ashematite. 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 rivergravels. Starr and other nearby counties along the Rio Grande have furnished a number of good specimens.

A hard, smooth, compact,translucentrock that is made up mostly ofcryptocrystallinequartzis calledchertorflint. 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 rivergravelsin Texas. It also occurs withlimestone, such as in the LowerCretaceousEdwards Limestone of central Texas and in theOrdovicianEllenburger strata in theLlano upliftarea. Chert also is found with the Ordovician rocks of the Marathon area of Brewster County.

Geologists do not agree on whetherchertandflintare two names for one variety of rock, or whether each is a separate variety. Some, however, now givecherta geological meaning andflintan archaeological meaning. They use the wordchertto describe geologicalformationsor rock specimens. They give the nameflintto the same rock when it has been used by Indians in making arrowheads, scribers, scrapers, and spearheads.

Quartziteis either ametamorphic rockor asedimentaryrock. (The sedimentary kind of quartzite is described withsand and sandstoneonp. 86.)Metamorphicquartzite is made up mostly ofquartz. It forms when heat andfluidsbelow the earth’s surface cause the grains and cement of a quartzsandstoneto recrystallize. When this happens, the grains interlock and are no longer held together by cement. Metamorphic quartzite, likesedimentary quartzite, is a hard, firm rock thatbreaks through the quartz grains instead of between them.

AncientPrecambrianmetamorphicquartziteoccurs at the surface in theLlano upliftarea of central Texas, in the Van Horn area of west Texas, and in the Franklin Mountains north of El Paso in extreme west Texas.

Rhyoliteis a fine-grained or glassyigneousrock that commonly isextrusiveorvolcanic. It has a pink, red, tan, white, gray, purple, or black color. This rock, likegranite, is made up chiefly offeldsparand a silica mineral, such asquartz, 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 Texasrhyoliteformed from hot, moltenlava. This lava flowed out onto the surface either throughvolcaniccones 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,crystallinemineral 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 hasflow structure.

One variety ofrhyolitehas easily seen crystals and grains of minerals, such asfeldspar,quartz, andmica, scattered through a mass of the tinycrystallinegrains (in much the same way that raisins are scattered through a cake). The easily seen crystals and grains are calledphenocrysts, and the rock itself is called arhyolite porphyry.

Many rhyolites andrhyoliteporphyries occur in theTertiaryigneous rocksof 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.SeeQuartz.

Rock Gypsum.SeeGypsum.

Rock Salt.SeeHalite.

Rose Quartz.SeeQuartz.

Salt.SeeHalite.

Sandis a loose, uncementedsedimentarydeposit 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 asgranules; they are larger than those known assilt.

Many sands are made up chiefly of grains ofquartz. This mineral is plentiful and does not easily weather away. In addition, rock fragments and many other minerals, such asfeldspar,mica,gypsum,magnetite, andgarnet, are found assandgrains.

Rains wash many of thesandgrains and other weathered rock and mineral fragments into creeks and rivers. These streams may carry the sand and othersedimentslong 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 otherCenozoicformationsand in some of thePaleozoicandMesozoicformations of Texas.

Sandhas many uses. Muchbuilding sand, which is used in mortar and concrete, is produced from numerous sand andgravelpits in Texas. Purequartzsand thatcan be used to make glass is known asglass sand. Some of it is found in north-central Texas in LowerCretaceousformations. A large glass sand quarry is located at Santa Anna in Coleman County. Along theGulf Coastal Plain, sand that is used in glassmaking occurs inEoceneTertiarystrata.

A coarse-grainedsand,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 afilteringsand in purifying water. These types of sand have been produced from theGulf Coastal Plainas well as from other areas of Texas.

Sandgrains, when nature cements them together, make up thesedimentaryrocksandstone. 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 ascalcite(calcium carbonate),quartz,chalcedony, oropal, which are silica minerals, andlimoniteandhematite, which are iron oxides.Clayalso may serve as a cement. It is either deposited along with thesandor is formed from weatheredfeldsparsand grains.

The color of the cementing material helps determine the color of the rock. Iron oxide cement, for example, causes thesandstoneto 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 thesandgrains. Thus, the broken surface of the rock feels rough and gritty. Somequartzsand 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 asquartzite. Some of thissedimentary quartziteoccurs in the TexasGulf Coastal Plainin theTertiaryCatahoula strata. (Another kind of quartzite is described on pp.84-85.)

Sandstonefrom theEoceneWilcox Group of strata of northwestern Zavala County, Texas.

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 theCambrianandPennsylvanianformationsof theLlano upliftarea of central Texas and in the Pennsylvanian,Permian, and LowerCretaceousformations of north-central Texas.Tertiarysandstonesoccur in the TexasGulf Coastal Plain, and Triassic sandstones are found along the edges of the TexasHigh Plains.Sandstoneis also found in many formations of the Trans-Pecos country of west Texas.

Sandstone.SeeSand and Sandstone.

Satin Spar.SeeGypsum.

Schistis ametamorphic rockthat splits easily along thin, generally parallel layers, calledfolia. These layers may be either straight or curved, and they are made up ofcrystallinegrains of one or more than one mineral. This structure is calledschistosityorfoliation. 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,micaschists may contain beautiful crystals ofgarnet.

Each kind ofschistis named for an outstanding mineral that it contains.Micaschist contains a large amount of mica. We also find hornblende schist, actinolite schist, chlorite schist,talcschist, andgraphiteschist. (Graphite schist is discussed with graphite onp. 63.)

Schists form from other rocks, such asgranite, gabbro, orshale. The rocks are changed into schists byfluidsand by heat and pressure below the earth’s surface.

Extremely ancient schists that formed duringPrecambriantime are exposed at the surface in the Allamoore—Van Horn area of west Texas and in theLlano upliftarea of central Texas. Geologists believe that the PacksaddleSchistof the Llano uplift area was onceshale. Good exposures of this schist are seen in the Honey Creek area near Packsaddle Mountain in Llano County.

Schorl.SeeTourmaline.

Sedimentary Quartzite.SeeSand and Sandstone.

Selenite.SeeGypsum.

Serpentineis 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 calledchrysotile; if it is layered and platy, it is known asantigorite. 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 calledchrysotileasbestos.

Light will pass through both these varieties ofserpentine, and both are soft enough to be scratched by a pocket knife. When rubbed across astreakplate, they leave white streaks.Antigoriteandchrysotilehave 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 aspseudomorphsin a crystal shape that originally belonged to another mineral.

Antigoriteandchrysotileare commonly found closely mixed withdolomite,talc,magnetite,calcite,pyrite, and several other minerals. These minerals make upserpentinerock (also calledserpentinite). 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 ismassive. Serpentine rock, like the serpentine minerals, is fairly soft—you can scratch it with a pocket knife.

In theLlano upliftarea of central Texas,serpentinerock is found amongPrecambrianmetamorphic rocks, such asgneissandschist. 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 twocounties. (A little fibrouschrysotileis found here, but it will not break into flexible enough threads to be called chrysotileasbestos.) Several other deposits of serpentine occur in northeastern Gillespie County and in southern Llano County.

It is believed that the Coal Creekserpentinewas formed from anigneousrock such asperidotite, which is made up chiefly of grains of the mineralolivine. 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 hotfluidsand great pressures far below the earth’s surface.

The Llano areaserpentinehas 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.

Serpentinerock 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 whitecalciteordolomitein it.

In theBalcones fault zonearea (shown on the Texasphysiographic outline map,p. 42) from Uvalde County to Williamson County,serpentineoccurs with UpperCretaceousrocks. 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.SeeSerpentine.

Shaleis asedimentaryrock made up of tightly packedclayand 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, othersedimentswere deposited on top of them. The weight of these new sediments squeezed the clays and muds together to form firm, compact shale.

Shalelooks very much like some clays. It, likeclay, 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.

Shaleis fairly abundant in Texas, especially inMississippian,Pennsylvanian, andCretaceousformations. For example, Pennsylvanian shales are found at the surface in north-central Texas, in the area around theLlano upliftof central Texas, and in the Marathon and Solitario uplifts of west Texas.

Many ofshale’s uses are the same as those ofclay. 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 whereCretaceouslimestones, which also are used in cement making, and Cretaceous shales are found near each other at the surface.

Oilshale, from which petroleum can be obtained by heating, has been found in central Texas. It occurs inMississippianformationsin Lampasas, McCulloch, and San Saba counties. Because oil is much less expensive to obtain from wells, it is not produced from these shales.

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