Texas Rocks and Minerals: An Amateur's Guide

Selenite is found in cracks and cavities in rocks. Good crystals have been collected at Gyp Hill, a salt dome 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 of gypsum is known as fibrous 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 called satin spar. One of the places where satin spar occurs is in Permian rocks in Hardeman County.

Most of the fibrous gypsum and selenite is formed by solutions. Some of these solutions develop when underground waters, seeping through rocks, pick up and dissolve minerals that contain sulfur (such as pyrite). This dissolved material changes the water into very weak sulfuric acid. When the sulfuric acid meets calcium carbonate (as in limestone or calcite), it combines with the calcium to form the gypsum.

A massive, fine-grained, and translucent variety of gypsum, known as alabaster, is used for articles such as lamp bases, statuettes, vases, and book-ends.

A loose, earthy, crumbly variety of gypsum, called gypsite, is ordinarily found mixed with other materials, such as clay, 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.

A massive, granular variety of gypsum, called rock gypsum, may occur in large deposits. This is the gypsum that is used for making products such as plaster, wallboard, and some cements.

Deposits of rock gypsum are found both underground and at the surface in Texas. Surface deposits occur in Permian rocks in several counties to the east of the Texas High 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 Lower Cretaceous rocks 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 Hockley salt dome in Harris County.

Gypsum and 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

Halite, sodium chloride, is the table salt you 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 a streak plate, it leaves a white streak.

Because halite cleaves in three directions, all at right angles to each other, the cleavage fragments are shaped like cubes. You can see some of them by looking at a few grains of table salt through a magnifying glass.

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

In addition to its use as table salt, much halite goes 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 of halite. These deposits, known as rock salt, occur in the Permian subsurface basin of west Texas and in the salt domes of the Gulf Coastal Plain. The Permian basin, which extends under parts of west Texas, New Mexico, Oklahoma, Colorado, and Kansas, is now completely filled with sediments. 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 salty sea. 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 by sedimentary rocks which 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.

The Gulf Coastal Plain salt domes are huge and almost circular columns of halite, 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, halite occurs in salt lakes in Crane and Hudspeth counties and in alkali lakes on the High 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

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

Most metallic hematite is too hard for a pocket knife to scratch, but quartz or a steel file will scratch it. Hematite is fairly heavy, for it has a specific gravity of 5.26. This mineral has no cleavage, but some specimens show three directions of parting that are almost at right angles to each other. A great help in identifying hematite is the dark reddish-brown streak it leaves when you rub it across a streak plate.

Some hematite occurs as rounded masses that resemble kidneys or bunches of grapes (then called kidney ore); it also is found as flat crystals. Most of the Texas hematite occurs as granular or compact masses. One of these massive varieties is composed of shiny scales or plates and is called micaceous or specular 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.

Some hematite is formed by the alteration of magnetite, another iron mineral. This hematite is known as martite, and some of it still has the crystal shape (an octahedron or a dodecahedron) that belonged to the magnetite. Most of the hematite found in the Llano uplift area of central Texas is believed to be altered magnetite. In this central Texas area, some massive, granular martite has been mined at the Gamble prospect, a few miles southeast of Fredonia in northeastern Mason County, where it occurs as layers in Precambrian gneiss.

Small deposits of hematite occur 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. See Manganese Minerals.

Hyalite. See Opal.

Jasper. See Quartz.

Kaolin. See Clay.

Limestone

Limestone is a sedimentary rock made up chiefly of calcite, a calcium-carbonate mineral. This rock also commonly contains grains of quartz, clay minerals, the mineral dolomite, or other materials. If a large amount of dolomite is present, the rock is called dolomitic 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.

Pure limestone is white, but if it contains clay or 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 contains calcite, 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 become limestone.

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 of calcite and forms a limy mud that is converted to limestone. Many limestones contain shell or plant fragments in addition to these tiny grains of calcite.

There are several special kinds of limestone. If the rock is made up of many little rounded calcite grains that resemble fish eggs, it is called öolitic limestone. Another limestone, chalk, is soft, white, and fine grained. It consists mostly of tiny shell fragments and fine-grained calcite. Coquina is a porous limestone made up of loosely cemented shells and shell fragments. Another special kind of limestone, known as lithographic 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 Lower Cretaceous Edwards 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.

Much limestone is found at the surface in Texas in Cambrian, Ordovician, Mississippian, Pennsylvanian, Permian, and Cretaceous formations. If you will look at numbers 5, 6, 9, 10, and 11 on the Texas geologic map (pp. 4-5), you will see that these strata appear at the surface in central, north-central, and Trans-Pecos Texas.

Limestone has 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 into lime (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 from Cretaceous limestones.

Another important use of limestone is in making portland cement. The limestone is mixed with clay or shale, 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, a retarder, such as gypsum, is added. A number of cement-manufacturing plants in Texas use Cretaceous limestones, shales, and clays.

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

Limonite

Limonite is 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 called goethite. Some limonite may be dull and earthy with the appearance of brownish-yellow or rusty brown clay. This variety is so soft that a fingernail will scratch it easily.

Other limonite has 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-brown streak it leaves when rubbed across a streak plate.

Limonite has no cleavage and no crystal shape of its own. But crystals of other iron minerals, such as pyrite and magnetite, 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 called pseudomorphs.) 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.

Limonite is 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 Texas limonite deposits occur mainly in Weches sedimentary rocks. These rocks, which were deposited in the sea during Eocene Tertiary time, contain clay along with greensands. (Greensands are small, soft grains that contain glauconite, a mineral composed of iron, silicon, and several other elements.) Later, as the sea retreated, these sediments became 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 called siderite. 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 the limonite is 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) and limestone and blowing in blasts of hot air.

To make steel, the iron from the blast furnace (called pig 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 other elements, such as molybdenum, manganese, or nickel, are added to make steel with the right strength and toughness.

Skip car

Hot Gases

Blast Furnace

Iron Ore

Limestone

Coke

Fire Brick lining

Steel Plate covering

3550°F

Hot Air Blast

Slag

Slag Ladle

Molten Iron

Iron Ladle

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. See Limestone.

Llanite

Llanite is a unique rock that is found only in Llano County in central Texas. This intrusive igneous rock is made up of easily seen crystals and grains of quartz and feldspar that 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 like opal, this rock often is called opaline 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.

Llanite formed during Precambrian time. Molten rock material forced its way upward into cracks that cut across granite and schist rocks while the rocks were still far underground. This hot magma remained in the cracks where it cooled and hardened to form long, narrow, wall-like masses (called dikes) of llanite. We can see some of the llanite dikes exposed at the earth’s surface to the north and northeast of Llano in Llano County because the overlying rocks have weathered away.

Llanite has 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

Magnetite, iron oxide, is a black, metallic mineral with an outstanding physical property: it is magnetic—fragments of magnetite readily cling to a magnet. It also leaves a black streak when 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 heavy—it has a specific gravity of 5.18.

Magnetite occurs as compact or granular masses, as scattered grains, and as crystals. Most of the crystals are octahedrons, but some dodecahedrons are found. Magnetite helps make up a part of many metamorphic and igneous rocks, and it also occurs as tiny crystals and grains in some sands, sandstones, and other sedimentary rocks.

Open-Hearth Furnace

Scrap metal

Alloying Elements

Limestone

Furnace Interior

Live Fuel Burner

Air pre-heated

Most of the magnetite that has been found in Texas occurs in Precambrian gneiss and schist rocks of the Llano uplift area 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 were sedimentary rocks, such as shales and sandstones. Some geologists believe that these rocks could have contained iron sediments (perhaps in the form of glauconite). Great forces below the earth’s surface crumpled and squeezed the sedimentary rocks and changed them into the metamorphic schist and gneiss rocks we see today. As this happened, the iron sediments in the rocks were changed into magnetite.

At least some of the magnetite in this area (such as the deposit at Iron Mountain in Llano County) probably had a different sort of origin. Molten igneous rock material containing iron could have moved up into cracks in the ancient sedimentary 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 County magnetite deposits 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. See Copper Minerals.

Manganese Minerals (Braunite, Hollandite, Pyrolusite)

Although manganese does not occur alone in nature as a native element, 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 and sulfur, 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 minerals and compounds, such as braunite, hollandite, pyrolusite, and wad, 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 some chalcocite, also have a black coating that soils your fingers in a similar way.

One of the manganese 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 a streak plate, it leaves a steel-gray or a black streak. This mineral is too hard to be scratched by a pocket knife, but a piece of quartz or a steel file will scratch it. Braunite has a specific gravity of 4.75 to 4.82. It has four directions of cleavage that are parallel to the faces of a pyramid.

In the Spiller mine, about 15 miles northeast of Mason in Mason County, masses of braunite occur as lens-shaped layers in Precambrian gneiss and quartzite rocks. This braunite may have formed from another manganese mineral (possibly manganese garnet) that was exposed at the earth’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 variety hollandite is 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 a streak plate, hollandite leaves a black streak. It has a specific gravity of 4.7 to 5. Hollandite is rather hard, but a steel file will scratch it.