Halite

Selenitegypsumcrystal from Bastrop County, Texas.

Seleniteis found in cracks and cavities in rocks. Good crystals have been collected at Gyp Hill, asaltdome southeast of Falfurrias in Brooks County, and some selenite has been mined there. Selenite crystals also occur scattered through clays, particularly along creek banks, in Lee, Fayette, Bastrop, and several other counties.

Another variety ofgypsumis known asfibrous gypsum. It is made up of slender, brittle, needle-like fibers that fill the cracks in some rocks. If fibrous gypsum has a silky or pearly luster, it is calledsatin spar. One of the places where satin spar occurs is inPermianrocks in Hardeman County.

Selenitegypsumrosettes from Nolan County, Texas.

Most of thefibrous gypsumandseleniteis formed by solutions. Some of these solutions develop when underground waters, seeping through rocks, pick up and dissolve minerals that containsulfur(such aspyrite). This dissolved material changes the water into very weak sulfuric acid.When the sulfuric acid meets calcium carbonate (as inlimestoneorcalcite), it combines with the calcium to form thegypsum.

Fibrous gypsumfrom Terlingua area, Brewster County, Texas.

Amassive, fine-grained, andtranslucentvariety ofgypsum, known asalabaster, is used for articles such as lamp bases, statuettes, vases, and book-ends.

A loose, earthy, crumbly variety ofgypsum, calledgypsite, is ordinarily found mixed with other materials, such asclay,sand, and soil. It occurs either at or near the surface of the ground. Gypsite is found in Culberson, Reeves, and other counties in west Texas.

Amassive,granularvariety ofgypsum, calledrock gypsum, may occur in large deposits. This is the gypsum that is used for making products such as plaster, wallboard, and some cements.

Deposits ofrock gypsumare found both underground and at the surface in Texas. Surface deposits occur inPermianrocks in several counties to the east of the TexasHigh Plains. They also occur in the area between the Pecos River and the Delaware and Apache Mountains in Culberson and Reeves counties. Some of the other surface deposits are found near the Malone Mountains in Hudspeth County and in LowerCretaceousrocks in Gillespie and Menard counties. Rock gypsum has been mined from the deposits in Fisher, Gillespie, Hardeman, Hudspeth, and Nolan counties. It also has been produced from the cap-rock at Hockleysaltdome in Harris County.

Gypsumand another mineral,anhydrite, have very nearly the same composition. Both are calcium sulfates. Gypsum, however, contains water of crystallization, and anhydrite does not. It is likely that most of the rock-gypsum deposits of Texas originally were beds of anhydrite. By absorbing water that seeped through it, the anhydrite changed into gypsum.

Halite, sodium chloride, is the tablesaltyou sprinkle on food for seasoning. This mineral ordinarily is white or colorless, but other materials cause it to be tinted red, blue, gray, brown, or green. When you rub halite across astreakplate, it leaves a white streak.

Becausehalitecleaves in three directions, all at right angles to each other, thecleavage fragmentsare shaped likecubes. You can see some of them by looking at a few grains of tablesaltthrough a magnifying glass.

Halitehas a salty taste and dissolves easily in water. It also istransparenttotranslucentand has a glassy luster. This mineral is soft enough for a copper penny to scratch it. Halite commonly occurs as cubic crystals and asgranularor compact masses.

In addition to its use as tablesalt, muchhalitegoes to make soda ash, chlorine, and other chemicals. A few of its other uses are in leather making, meat packing, and food canning.

Texas has large underground deposits ofhalite. These deposits, known asrock salt, occur in thePermiansubsurface basin of west Texas and in thesaltdomes of theGulf Coastal Plain. The Permian basin, which extends under parts of west Texas, New Mexico, Oklahoma, Colorado, and Kansas, is now completely filled withsediments. It appears level and flat when you travel across it and does not look at all like a basin or a valley. During Permian time, however, this area was covered by a saltysea. As the sea gradually dried up, the dissolved material that it contained was deposited as thick beds of halite,anhydrite, and other minerals. Later, these minerals were covered bysedimentary rockswhich were deposited on top of them. Now, the minerals are found many hundreds of feet below the surface. In Hutchinson, Mitchell, Ward, and Yoakum counties, some of this Permian basin salt has been produced (as brine) from wells that have been drilled into it.

Saltdomes, which are huge, underground columns ofhalite, occur on theGulf Coastal Plain.

TheGulf Coastal Plainsaltdomes are huge and almost circular columns ofhalite, some of which are more than 2 miles wide. Some are less than 300 feet below the surface, but most of them are much deeper. These salt columns pushed upward many thousands of feet from great, deeply buried salt deposits. The halite is mined from shafts dug into the Hockley salt dome in Harris County and into the Grand Saline salt dome in Van Zandt County. Salt brines are produced from wells drilled into several salt domes of this area.

At the surface in Texas,haliteoccurs insaltlakes in Crane and Hudspeth counties and in alkali lakes on theHigh Plains. It is found also on the shores of bays and lagoons in Cameron, Kenedy, Kleberg, and Willacy counties, and it occurs at springs and seepages in various places in the State.

Hematite, iron oxide, the chief ore of iron, is found in many places in Texas but not in large deposits. This mineral mayhave a metallic luster and appear reddish brown, dark brown, steel gray, or black or it may occur as a soft, red, earth-like material calledred ocher.

Specular hematitefrom Carrizo Mountains, Hudspeth County, Texas.

Most metallichematiteis too hard for a pocket knife to scratch, butquartzor a steel file will scratch it. Hematite is fairly heavy, for it has aspecific gravityof 5.26. This mineral has nocleavage, but some specimens show three directions ofpartingthat are almost at right angles to each other. A great help in identifying hematite is the dark reddish-brownstreakit leaves when you rub it across a streak plate.

Somehematiteoccurs as rounded masses that resemble kidneys or bunches of grapes (then calledkidney ore); it also is found as flat crystals. Most of the Texas hematite occurs asgranularor compact masses. One of thesemassivevarieties is composed of shiny scales or plates and is calledmicaceousorspecular hematite. This variety has been found in Hudspeth County and in northeastern Mason County. Hematite also commonly occurs as cementing material in many Texas sandstones.

Somehematiteis formed by the alteration ofmagnetite, another iron mineral. This hematite is known asmartite, and some of it still has the crystal shape (anoctahedronor adodecahedron) that belonged to the magnetite. Most of the hematite found in theLlano upliftarea of central Texas is believed to be altered magnetite. In this central Texas area, somemassive,granularmartite has been mined at the Gamble prospect, a few miles southeast of Fredonia in northeastern Mason County, where it occurs as layers inPrecambriangneiss.

Small deposits ofhematiteoccur in other parts of Texas, too. Some of the west Texas localities include Sierra Blanca, the Quitman Mountains, and the Carrizo Mountains of Hudspeth County and the area around Shafter in Presidio County.

Hollandite.SeeManganese Minerals.

Hyalite.SeeOpal.

Jasper.SeeQuartz.

Kaolin.SeeClay.

Limestoneis asedimentaryrock made up chiefly ofcalcite, a calcium-carbonate mineral. This rock also commonly contains grains ofquartz,clayminerals, the mineraldolomite, or other materials. If a large amount of dolomite is present, the rock is calleddolomitic limestone. In some limestones, the mineral grains are too small to be distinguished from each other without a magnifying glass or a microscope, but in other limestones, the individual mineral grains are easily seen.

Purelimestoneis white, but if it containsclayor plant or animal matter it is light gray, dark gray, or black. Limestone also may be some shade of yellow, brown, or red. It is fairly soft and can be scratched with a knife. Because this rock containscalcite, an easy chemical test will help identify it: a drop or two of dilute hydrochloric acid will quickly fizz and bubble when placed on the limestone.

Limestones form in fresh water, such as in lakes, but most of them form in the seas. As some earlier-formed rocks are weathered, the calcium minerals that they contain are dissolved. Creeks and rivers carry this dissolved material to the sea. There, small animals, such as corals, crinoids, sponges, and foraminifers, take the dissolved material out of the water to build their calcium carbonate shells. Plants, such as algae, can take calcium carbonate out of solution too, and it collects on them. Shells, shell fragments, and plant remains accumulate on the sea floor, forming limy deposits that later becomelimestone.

Limestones also originate in a slightly different way. When the temperature and chemical composition of the water permit, calcium carbonate precipitates as millions of tiny grains ofcalciteand forms a limy mud that is converted tolimestone. Many limestones contain shell or plant fragments in addition to these tiny grains of calcite.

Polished section of LowerCretaceousEdwardsLimestonefrom Travis County, Texas, containing fossil gastropods.

There are several special kinds oflimestone. If the rock is made up of many little roundedcalcitegrains that resemble fisheggs, it is calledÃ¶olitic limestone. Another limestone,chalk, is soft, white, and fine grained. It consists mostly of tiny shell fragments and fine-grained calcite.Coquinais a porous limestone made up of loosely cemented shells and shell fragments. Another special kind of limestone, known aslithographic limestone, because it can be used in printing, is smooth, firm, and hard. Its mineral grains are too small to be recognized without a microscope. This kind of limestone breaks with a smooth, sometimes curved,fracture. Still another variety,pulverulent limestone, is loose, soft, powdery, and white. It occurs in the LowerCretaceousEdwards Limestone in Williamson and Bell counties of central Texas. Some of this limestone is used to polish rice grains, and it is added to livestock feeds to provide calcium for the animals.

Limestonequarry in LowerCretaceousEdwards Limestone at Georgetown, Williamson County, Texas.

Muchlimestoneis found at the surface in Texas inCambrian,Ordovician,Mississippian,Pennsylvanian,Permian, andCretaceousformations. If you will look at numbers 5, 6, 9, 10, and 11 on the Texasgeologic map(pp.4-5), you will see that these strata appear at the surface in central, north-central, and Trans-Pecos Texas.

Limestonehas many important uses. Much Texas limestone is crushed and used as a road-building material and as an aggregate that is mixed with cement to make concrete. Farmers in some areas improve their crops by adding limestone to the soil. Limestone also is sent to the iron furnaces in east Texas to be used in the production of pig iron and steel.

Some of the Texas limestones are heated to a fairly high temperature in order to change them intolime(calcium oxide). Industry uses a large amount of lime in making chemicals, steel, glass, paper, and other products. Builders use it to make plasters, mortars, and stuccos. At plants in Comal, Johnson, Travis, and Williamson counties, lime is made fromCretaceouslimestones.

Another important use oflimestoneis in makingportland cement. The limestone ismixed withclayorshale, and the mixture is burned in a kiln until it just begins to melt. Then it is allowed to cool. Next, it is finely ground and in order to keep the finished cement from hardening or setting too quickly when it is used, aretarder, such asgypsum, is added. A number of cement-manufacturing plants in Texas useCretaceouslimestones, shales, and clays.

Many of the Texas limestones make excellent building stones. Some of them are quarried fromPennsylvanianandCretaceousformationsin north-central Texas and from Lower Cretaceous formations in counties near theLlano upliftof central Texas. A large quarry on the Williamson-Travis County line near Cedar Park in central Texas has supplied Cretaceouslimestonefor many buildings and monuments in the United States and Canada.

Limoniteis not really a definite mineral but is a mixture of iron oxides containing water. It is believed to be closely related to an iron mineral calledgoethite. Some limonite may be dull and earthy with the appearance of brownish-yellow or rusty brownclay. This variety is so soft that a fingernail will scratch it easily.

Otherlimonitehas a dark brown or black color and a metallic or almost metallic luster. A copper penny will not scratch it, but a steel file will. This kind of limonite may have a shiny black surface that resembles glossy lacquer. The property that will help you most in identifying limonite is the rusty, yellowish-brownstreakit leaves when rubbed across a streak plate.

Limonitehas nocleavageand no crystal shape of its own. But crystals of other iron minerals, such aspyriteandmagnetite, alter to form limonite. It then occurs with a crystal shape that originally belonged to one of these other minerals. (Such false forms of minerals are calledpseudomorphs.) Limonite also occurs as layers in rocks, as hollow or solid concretions, or as coatings on other minerals. It is found mixed with minerals such as clays and serves as the cementing material in some sandstones.

Limoniteis found in many localities in Texas including Blanco, Brewster, Burnet, Llano, and San Saba counties. The most important limonite deposits in Texas, however, are in the eastern part of the State, particularly in Anderson, Cass, Cherokee, Henderson, Marion, Morris, Nacogdoches, Smith, and Upshur counties.

The east Texaslimonitedeposits occur mainly in Wechessedimentary rocks. These rocks, which were deposited in the sea duringEoceneTertiarytime, containclayalong with greensands. (Greensands are small, soft grains that containglauconite, a mineral composed of iron, silicon, and several otherelements.) Later, as the sea retreated, thesesedimentsbecame a part of the land. Waters seeping through the sediments changed into weak solutions of carbonic and sulfuric acid that dissolved the iron out of some of the greensands. When conditions were favorable, this iron was re-deposited as an iron-carbonate mineral calledsiderite. Siderite was changed to limonite by weathering. Some siderite is still found in east Texas, and it is also mined along with the limonite as an iron ore.

East Texas iron ore has been mined from time to time ever since about 1855, and records show that a number of local iron furnaces once operated. The brown iron ore (as thelimoniteis also called) now is mined from open pits in Cass, Cherokee, and Morris counties.

This ore, after being washed, goes into blast furnaces at Lone Star (near Daingerfield) and at Houston. In the blast furnaces the ore is changed into metallic iron by mixing it with coke (made from coal) andlimestoneand blowing in blasts of hot air.

To make steel, the iron from the blast furnace (calledpig iron) is put into open-hearth furnaces together with scrap iron,limestone, and other materials. This mixture is heated and melted together to get rid of unwanted substances. Then otherelements, such as molybdenum, manganese,or nickel, are added to make steel with the right strength and toughness.

Limoniteore is changed to metallic iron in a blast furnace.

Steel mills alongside the furnaces in Texas turn out many products, such as steel plates for oil tanks, ships, and tank cars and steel beams for framework in buildings and bridges. Some of their other products include pipes for the oil and chemical industries and wire for nails and fencing material.

Lithographic Limestone.SeeLimestone.

Llaniteis a unique rock that is found only in Llano County in central Texas.Thisintrusiveigneousrock is made up of easily seen crystals and grains ofquartzandfeldsparthat are scattered through a brown-colored mass of extremely small mineral grains. The quartz is beautiful, sky-blue, and opal-like; the feldspar has a rusty pink color. (Because the quartz looks likeopal, this rock often is calledopaline granite.) The mineral grains that make up the brown-colored mass are so tiny that they can be identified only with a microscope. They are quartz, feldspar,mica,fluorite, and apatite.

Llaniteformed duringPrecambriantime. Molten rock material forced its way upward into cracks that cut acrossgraniteandschistrocks while the rocks were still far underground. This hotmagmaremained in the cracks where it cooled and hardened to form long, narrow, wall-like masses (calleddikes) of llanite. We can see some of the llanite dikes exposed at the earthâ€™s surface to the north and northeast of Llano in Llano County because the overlying rocks have weathered away.

Llanitehas been quarried from one of the dikes west of Babyhead in northern Llano County. Because llanite is both attractive and strong, it has been used as an ornamental stone and as a monument stone.

Magnetite, iron oxide, is a black, metallic mineral with an outstanding physical property: it is magneticâ€”fragments of magnetite readily cling to a magnet. It also leaves a blackstreakwhen rubbed across a streak plate. Although this mineral is too hard to be scratched by the average pocket knife, a steel file will scratch it. Magnetite is fairly heavyâ€”it has aspecific gravityof 5.18.

Magnetiteoccurs as compact orgranularmasses, as scattered grains, and as crystals. Most of the crystals areoctahedrons, but somedodecahedronsare found. Magnetite helps make up a part of manymetamorphicandigneousrocks, and it also occurs as tiny crystals and grains in some sands, sandstones, and othersedimentary rocks.

Metallic iron, after leaving the blast furnace, is made into steel in an open-hearth furnace.

Most of themagnetitethat has been found in Texas occurs inPrecambriangneissandschistrocks of theLlano upliftarea of central Texas, particularly in Llano County and in eastern Mason County. It occurs as thin layers, as thick lens-shaped deposits, and as scattered grains in the rocks. Probably at least a billion years ago these gneisses and schists weresedimentary rocks, such as shales andsandstones. Some geologists believe that these rocks could have contained ironsediments(perhaps in the form ofglauconite). Great forces below the earthâ€™s surface crumpled and squeezed the sedimentary rocks and changed them into themetamorphicschist and gneiss rocks we see today. As this happened, the iron sediments in the rocks were changed into magnetite.

Granularmagnetitefragments from northwest of Llano, Llano County, Texas, are attracted to a magnet.

At least some of themagnetitein this area (such as the deposit at Iron Mountain in Llano County) probably had a different sort of origin. Moltenigneousrock material containing iron could have moved up into cracks in the ancientsedimentary rocks. Then the magnetite formed from this iron material when the igneous and sedimentary rocks were changed into the schists and gneisses of today.

None of the Llano and Mason Countymagnetitedeposits is really very large. Nevertheless, prospecting and a little mining have been carried on from time to time at several deposits in this area. At Iron Mountain, which is about 12 miles northwest of Llano in Llano County, magnetite has been mined from open pits. Although magnetite is commonly used as a source of iron, the magnetite from this deposit was used as a heavy concrete aggregate.

Malachite.SeeCopper Minerals.

Although manganese does not occur alone in nature as a nativeelement, it makes up a part of many minerals and compounds. This element has an important use in steel making, where it helps rid the steel of unwanted substances, such as oxygen andsulfur, and, in addition, it is used to make tough, hard, manganese steel for armor plate, railroad tracks, safes, and steam shovels. Manganese has various uses outside the steel industry. It is added to copper and nickel to make alloys, it is used in the manufacture of dry-cell batteries, and (as manganese sulfate) it is used as a fertilizer.

Manganese mineralsand compounds, such asbraunite,hollandite,pyrolusite, andwad, occur in several counties in Texas. No large, commercial deposits have been found here.

Some manganese compounds and minerals are covered with a soft, sooty black material that will soil your fingers. This can help you recognize these minerals; however, a few non-manganese minerals, such as somechalcocite, also have a black coating that soils your fingers in a similar way.

One of themanganese minerals,braunite, is a complex oxide of manganese that contains silica. It has a submetallic luster and is dark steel-gray or black. When rubbed across astreakplate, it leaves a steel-gray or a black streak. This mineral is too hard to be scratched by a pocket knife, but a piece ofquartzor a steel file will scratch it. Braunite has aspecific gravityof 4.75 to4.82. It has four directions ofcleavagethat are parallel to the faces of a pyramid.

In the Spiller mine, about 15 miles northeast of Mason in Mason County, masses ofbrauniteoccur as lens-shaped layers inPrecambriangneissandquartziterocks. This braunite may have formed from another manganese mineral (possibly manganesegarnet) that was exposed at the earthâ€™s surface after the overlying rocks eroded away. As this other mineral weathered, it may have altered into braunite, or the braunite could have been deposited from solutions emanating from hot magmas before the great thickness of overlying rock was removed.

The mineral varietyhollanditeis a rare manganate of manganese and barium. It has a metallic luster, and its color is silvery gray or black. When you rub it across astreakplate, hollandite leaves a black streak. It has aspecific gravityof 4.7 to 5. Hollandite is rather hard, but a steel file will scratch it.

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