Marble

Hollanditefrom Jeff Davis County, Texas.

Hollanditeoccurs in western Jeff Davis County in west Texas at what is called the Mayfield prospect. Here, it is found as rounded masses that occur in a vein near a largefaultin LowerCretaceouslimestonerocks.

Other manganese compounds,pyrolusiteandwad, are found in several important deposits near the Pecos River in western Val Verde County.Pyrolusiteis a manganese dioxide mineral. It is black,opaque, and so soft that it rubs off on your fingers like soot. Pyrolusite may begranularandmassiveor may be powdery. It also occurs as a fern-like coating on rocks.Wadis not really a mineral but is an impure, dull-black or brownish-black mixture of manganese oxide, water, and other substances. It can be soft enough to soil your fingers, or it can be too hard to scratch with apocket knife. Wad occurs in earthy or compact masses or in crusts or stains on rocks.

In Val Verde County, thewadandpyrolusiteare found mixed with soil,clay,gravel,sand, and plant remains. This material fills cracks in LowerCretaceouslimestones, it is scattered through gravels, and it is deposited in low places at the surface. The manganese in these deposits came fromlimestonerocks that have since weathered away. Rainwater trickled into these rocks and dissolved themanganese mineralsthey contained. This manganese was washed down toward the Pecos River and was deposited as wad and pyrolusite.

Marbleis ametamorphic rockmade up chiefly of sparkling grains ofcalciteordolomite, but other minerals may be present. The marble may be fine grained, medium grained, or coarse grained; commonly, all the mineral grains are about the same size.

Marblemay be of uniform color, banded, spotted, or streaked. If it is made up only of purecalciteordolomite, the marble is white. If, however, it contains carbonaceous material, such asgraphite, it is grayish or black.Limoniteimpurities cause the marble to be yellowish brown, and manganese oxides andhematitegive it a brownish, pinkish, or reddish color.

Marbleis a rather soft rock, and you can scratch it easily with a pocket knife. A few drops of dilute hydrochloric acid will bubble and fizz readily oncalcitemarble; ondolomitemarble, it may fizz slightly.

Marbleforms fromlimestoneor fromdolomiterock. Heat and pressure below the earth’s surface cause thecalciteand dolomite mineral grains in these rocks to recrystallize. A fine-grained limestone can be changed into a coarse-grained calcite marble. The marble is not made up of new and different minerals, but it has a new texture unlike that of the limestone. (To a builder, the wordmarblehas another meaning. He considers rocks such as unaltered limestone, unaltered dolomite, or evenserpentineto be marble, if they will take a high polish.)

Polished section ofPrecambrianmetamorphicmarblefrom Llano County, Texas.

Metamorphicmarbles occur at the surface in central Texas and in west Texas. Some of the west Texas occurrences are in the Van Horn area of Culberson and Hudspeth counties and in the Big Bendarea of Brewster County. In central Texas,Precambrianmarbles are found in Burnet, Gillespie, Llano, and Mason counties of theLlano upliftarea. Many of them are suitable for use as monument and building stones. Some of the Llano Countymarbleis quarried and used asgranulesfor roofs and as terrazzo chips for making colorful floors (described withserpentineonp. 88).

Martite.SeeHematite.

Micais not just one mineral but is the name given to a group of similar minerals. The mica minerals are easy to recognize. Because they have perfectcleavagein one direction, they split into thin, flat sheets. You can see through some mica sheets, and they are elastic enough to be bent back and forth. (Another mineral,selenitegypsum, also will split into thin, flat,transparentsheets, but selenite sheets break when you bend them.)

Micaminerals have perfectcleavagein one direction, resulting in thin, sheet-likecleavage fragments.

Two of themicaminerals that you are most likely to find in Texas aremuscoviteandbiotite. Both these minerals are potassium-aluminum silicates, and biotite, in addition, contains magnesium and iron. In general, muscovite is light colored, that is, it has a light brown, yellow, or green tint, or is colorless, and biotite is dark colored, commonly dark green, brown, or black. These minerals have glassy or pearly lusters and are rather soft—a copper penny scratches them. Thespecific gravityof biotite is 2.8 to 3.2, and that of muscovite is 2.76 to 3.1.

Micaminerals occur inigneousrocks, such asgraniteandpegmatite, and inmetamorphic rocks, such asschistandgneiss. They also are found as tiny flakes in some sandstones, limestones, and othersedimentary rocks. Most of the Texas mica is found in theLlano upliftarea (particularly in Llano County) and in the Mica Mine area. (The Mica Mine area is in the Van Horn Mountains about 15 miles south of Van Horn in west Texas.) In both these areas, the mica minerals occur mostly inPrecambrianpegmatites and mica schists.

The gleamingmicaschists were onceigneous rocksorsedimentary rockssuch as sandstones and shales. Long ago, great forces beneath the earth’s surface changed the rocks into mica schists. The mica that is found in pegmatites formed from hotfluidsofigneousorigin when thepegmatiterock itself was formed.

Clusters ofmicain the pegmatites are calledbooks, because the thin sheets into which the mica splits look like pages. Somemuscovitebooks up to 8 inches across are found in the Mica Mine area of the Van Horn Mountains.

The books or sheets ofmuscovitemicathat occur in pegmatites are especially valuable to industry. Muscovite can stand great heat without melting, it is tough, it splits into thin sheets, and it lets very little heat and electricity pass through. Because of these properties, muscovite is used in fuses and as insulators in heatingelementsof electric irons and toasters. (Biotiteis not used, because the iron it contains makes it a conductor of electricity.) Sheet muscovite also is widely used by the electronics industry as a non-conducting material in the manufacture of tubes and other products.

Bothmuscoviteandbiotitefrommicaschistrocks, as well as scrap pieces of sheet mica from pegmatites, are ground into flakes or powder. This ground-up mica has many uses, ranging from a powder coatingfor automobile inner-tubes to Christmas tree “snow.”

Only a small amount ofmicahas been mined in Texas. A fair grade of sheet mica occurs in the pegmatites at Mica Mine in west Texas, but the deposit is not large. In the pegmatites of theLlano upliftarea of central Texas, no sheet mica has been found that is considered good enough for the requirements of industry. Mica suitable for grinding, however, is found in both these Texas areas.

Micaceous Hematite.SeeHematite.

Microcline.SeeFeldspar.

Milky Quartz.SeeQuartz.

Muscovite.SeeMica.

Native Silver.SeeSilver Minerals.

Obsidianis a dark, glassy-lookingigneousrock. Most obsidian contains the same chemicalelementsasgraniteandrhyolite, since all three of these rocks can form from the same type of molten rock material. Obsidian, however, has no separate minerals, because its chemical elements are not combined in an orderly way. It is a natural glass.

Because it is a glass, we know thatobsidianforms very quickly. One way for it to form is from the sudden cooling of hot, moltenlavathat flows out of volcanoes. If the lava cools and hardens before the separate minerals can crystallize, it becomes a natural glass, such as obsidian.

This rock is smooth and shiny. Most of it is black, but some can be dark green or dark brown.Obsidianallows light to pass through it, and it breaks with a curved,conchoidalfracture. The broken edges are very sharp.

Another glassyigneousrock that forms from fast-coolinglavaisvitrophyre. It looks likeobsidianexcept that it has crystals orcrystallinemineral grains (which may be light colored) scattered through the dark glassy material.

Obsidianwas used by the Indians to make arrowheads.

Obsidian and vitrophyreare found in the Big Bend area of Brewster and Presidio counties in west Texas. They occur with otherigneous rocksthat formed there duringTertiarytime.

The Indians who long ago roamed this area used the smooth, shinyvitrophyreandobsidianto make some of their arrowheads and scrapers. Today, rock collectors pick up these attractive rocks for their collections, and some of them cut and polish obsidian and vitrophyre for use as gemstones.

Onyx.SeeQuartz.

Öolitic Limestone.SeeLimestone.

Opalis like hardened jelly or gelatin. It has nocrystallineinner structure and no crystal shape of its own—it isamorphous. This mineral has almost the same chemical composition asquartz. Both are silicon dioxides (silica), but opal, in addition, contains water.

Opalcan be almost any color—red, yellow, blue, brown, gray, white—or it can be colorless. It istransparentortranslucentand appears glassy, resinous, greasy, or dull. Opal has aspecific gravityof 1.9 to 2.2—this mineral is a little lighter thanquartz. It also is softer than quartz. A copper penny will not scratch opal, but quartz will. Opal has a whitestreakand a curved,conchoidalfracturebut nocleavage.

Opaloccurs in a number of places in Texas. In the Trans-Pecos country of west Texas, it fills cracks and cavities in some of theextrusiveigneousrocks. It occurs on theHigh Plainsof northwest Texas, and it is found inTertiaryformationsof theGulf Coastal Plainwhere it occurs as masses that fill cracks and cavities insedimentary rocks, as the cementing material in some sandstones (such as in the Catahoulasandstone), and as opalized wood.

Muchopalforms from underground waters that contain silicon. These solutions move through the rocks and deposit the opal in them.

Opalis found in a number of varieties. Some show a beautiful, lustrous play of colors that comes from inside the specimens. These varieties are known asprecious opaland are prized as gemstones. In Texas, some precious opal is found near Alpine in Brewster County. It has a milky white to bluish-white color, istranslucent, and shows a fiery orange, red, blue, and green play of colors.

The variety known ascommon opalshows no play of colors. It may be white, gray, bluish, reddish, greenish, or yellowish, and it is only slightlytranslucent. It is found in Brewster, Jeff Davis, Presidio, and other counties of the Trans-Pecos country of west Texas. It occurs also around some of the wet-weather (playa) lakes on the TexasHigh Plains. In theGulf Coastal Plain, common opal is found withchalcedony(a variety ofquartz) inTertiaryformations. A south Texas locality sometimes visited by collectors is near Freer in Duval County.

A clear, commonly rounded, variety ofopalthat looks like ice is calledhyalite. Two areas in which it has been found are in Presidio County in west Texas and in Llano County in central Texas.

Opalized wood from Washington County, Texas.

A variety of petrified wood, calledopalized wood, isopalthat replaced the fibers of a piece of wood.Wood opalis found at a number of places in theGulf Coastal Plain. It occurs there inTertiaryformationswithin about 20 miles of the boundary line between areas 2 and 3 shown on thegeologic map(pp.4-5).

A soft opaline material calleddiatomite, ordiatomaceous earth, is made up chiefly of the skeletons of diatoms—tiny, one-celled plants that live in fresh orsaltwater. These little plants are able to take silica from the water to makeopalskeletons for themselves. When the diatom skeletons collect at the bottom of a lake or sea, they form the light, crumbly, white, gray, or cream-colored deposit of impure opal known as diatomite. Industry uses this material as a filter, as insulation, as an abrasive, and as a filler.

Diatomiteformed in ancient lakes onthe TexasHigh Plainsduring lateTertiary(Pliocene) and earlyQuaternary(Pleistocene) times. It is found in Armstrong, Crosby, Dickens, Ector, Hartley, and Lamb counties.

Opaline Granite.SeeLlanite.

Orthoclase.SeeFeldspar.

Pegmatites occur inigneousrock areas, and most geologists consider themintrusiveigneous rocks. They are made up of crystals andcrystallinemineral grains that fit together—the grains are interlocked. The crystals and grains in pegmatites are larger than those of surrounding rocks, and some are huge, even larger than a man. However, there is a wide range of grain sizes inpegmatite.

Some pegmatites cut throughigneousormetamorphic rocksin such a way that they resemble walls (calleddikes). Others are found as veins, as flat masses, or as odd-shaped bodies in rocks. Many pegmatites occur in granites and containfeldspar,quartz,mica, and other minerals, asgranitedoes. Some pegmatites occur with other kinds ofigneous rocksand contain the same minerals as these rocks. A few pegmatites contain rare and unusual minerals.

Many geologists believe that pegmatites form from hotfluidsofigneousorigin that are left after otherigneous rocks, such asgranite, have already formed. These left-over fluids contain large amounts of aluminum, potassium, silicon, sodium, and several otherelements. While the granite or other rocks are still far underground, this material pushes up into them, and may even partly dissolve them. Then it slowly cools and hardens intopegmatite. It is believed that, later, more fluids move into cracks in some pegmatites. This new material adds other minerals to the pegmatites and alters some of those minerals already there.

Quartz-feldsparpegmatitefrom Burnet County, Texas.

Some of the pegmatites we now see at the surface in Texas are probably about a billion years old. They formed duringPrecambriantime and occur with other extremely old rocks. One well-known Texaspegmatitearea is theMicaMine district of west Texas. It is about 15 miles south of Van Horn in the Van Horn Mountains of Culberson and Hudspeth counties. Another pegmatite area is in theLlano upliftof central Texas. These central Texas pegmatitesoccur in Burnet, Gillespie, Llano, and Mason counties.

Large crystals and grains offeldspar,mica, andquartzare found in the pegmatites of both these areas. A small amount of mica has been mined from the west Texas pegmatites, and feldspar has been produced from the central Texas pegmatites.

An extremely rare and unusualpegmatiteoccurs in theLlano upliftarea at Baringer Hill, which is west of Burnet in Llano County. This pegmatite was once on the bank of the Colorado River, but when Buchanan Dam was built, the area was flooded. The Baringer Hill pegmatite now lies beneath the water of Lake Buchanan. Many rare minerals, which contain beryllium, cerium, thorium, uranium, yttrium, zirconium, and a number of otherelements, occur in this pegmatite. Some of these minerals, such as those containing yttrium and zirconium, glow or incandesce when they are heated. During the early part of this century, before the area was flooded, several of the yttrium minerals were mined and used in making lamp mantles.

Pitchblende.SeeUranium Minerals.

Precious Opal.SeeOpal.

Pulverulent Limestone.SeeLimestone.

Pumicite.SeeVolcanic Ash.

Pyriteis a shiny, pale golden-yellow or brassy-yellow metallic mineral. This mineral, an iron disulfide, is so often mistaken forgoldthat it is widely known by the nicknamefool’s gold.

Except for their similar color and luster,pyriteandgoldare really very different. When you rub pyrite across astreakplate, it leaves a black, a greenish-black, or a brownish-black streak, but the streak of gold is gold-colored. Pyrite is too hard for the average pocket knife to scratch, but a knife will scratch gold easily. Pyrite is brittle and readily breaks, but gold is malleable and flattens out when hit with a hammer. Pyrite is only about 5 times as heavy as an equal volume of water, but pure gold is over 19 times as heavy. And pyrite may have a brown or a multicolored tarnish on it, but gold never tarnishes.

Pyriteveins in whitemarblefrom Llano County, Texas.

Cubic crystals ofpyrite.

Pyriteis a common mineral and is found in many of theigneous,metamorphic, andsedimentary rocksof Texas. It may be scattered through the rocks, or it may fill cracks and cavities in them. This mineral occurs asgranularand compact masses, as rounded masses, or as crystals. The crystals are commonlycubes,pyritohedrons, oroctahedrons. In some crystals, the shapes are combined (such as a cube with an octahedron or two pyritohedrons grown through each other). You may notice that the sides of some cubes and pyritohedrons have fine, parallel grooves (calledstriaeorstriations) on them.

Pyriteoriginates in a number of different ways. Some of it forms, along with other minerals inigneous rocks, from hot magmas. It also forms inmetamorphic rocksby the same processes that produce these rocks. Some of the pyrite inlimestoneand othersedimentary rocksis formed when the rocks themselves are deposited by seas or streams. Pyrite also is deposited by the hotfluidsthat are given off by magmas. These fluids travel up into cracks and other openings in rocks and then form pyrite as well as other minerals. Much pyrite forms in still another way. As water seeps through rocks, it dissolves some of the iron minerals that they contain. When, under certain conditions, these iron solutions mix with hydrogen sulfide (this is the gas that makes some water smell like rotten eggs), pyrite is formed.

Pyritealters easily. Because of this, most builders carefully check thelimestone,granite,marble, or whatever other building stone they plan to use to be sure that it does not contain large amounts of pyrite. When exposed to the weather, pyrite changes tolimoniteand causes an unsightly rust stain.

Pyriteis used as a source ofsulfur, and it is produced for this purpose in several states. In Texas, however, no pyrite deposits have been found that are large enough to be mined.

Pyrolusite.SeeManganese Minerals.

Quartz, silicon dioxide, is one of the most common minerals. It is glassy, waxy, greasy, or dull and istransparentortranslucent. Pure quartz is colorless, but impurities make some varieties white, black, or a shade of red, yellow, blue, violet, or brown. Quartz is a hard mineral. It scratches window glass and cannot be scratched by a pocket knife or even by a steel file. It has aspecific gravityof 2.65. The curved,conchoidalfractureshown by many specimens helps identify it.

Quartzis plentiful in Texas. It occurs inigneous rocks, such asgranite,llanite, andpegmatite; inmetamorphic rocks, such asquartzite,schist, andgneiss; and insedimentary rocks, such as somesandstone,conglomerate, andbreccia.

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