Part 6

A. Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife Fluorite White Cleavage in 4 4 directions can give fragments that are shaped like octahedrons; crystals commonly cubes B. Cannot be scratched by an ordinary pocket knife but can be scratched by quartz Quartz, variety: White Curved, 7 amethyst conchoidal fracture; _amethyst_ crystals commonly 6-sided prisms with pyramid-like ends

15. NONMETALLIC luster, COLORLESS

A. Can be scratched by a fingernail Cerargyrite Shiny white or Appears waxy; 1-1½ gray knife cuts it smoothly; turns violet brown to black when exposed to light Gypsum White Transparent 2 selenite variety commonly occurs as flat, diamond-shaped crystals; splits into thin, flat sheets that will not bend without breaking Muscovite (white White Splits into thin, 2-2½ mica) flat, transparent sheets that will bend without breaking B. Cannot be scratched by a fingernail but can be scratched by a copper penny Barite White Rather heavy; 3-3½ cleavage fragments may be flat and slab-like Calcite White Dilute 3 hydrochloric acid fizzes on calcite; perfect cleavage in 3 directions gives rhomb-shaped fragments Celestite White Not quite as 3-3½ heavy as barite; crystals commonly prism-shaped or flat-looking; some cleavage fragments are flat and slab-like Halite White Salty taste; 2½ dissolves in water; cube-shaped cleavage fragments Muscovite (white White Splits into thin, 2-2½ mica) flat, transparent sheets that will bend without breaking C. Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife Barite White Rather heavy; 3-3½ cleavage fragments may be flat and slab-like Celestite White Not quite as 3-3½ heavy as barite; crystals commonly prism-shaped or flat-looking; some cleavage fragments are flat and slab-like Dolomite White Commonly occurs 3½-4 as granular masses and as rhomb-shaped crystals; dilute hydrochloric acid may fizz slightly on dolomite Fluorite White Cleavage in 4 4 directions can give fragments that are shaped like octahedrons; crystals commonly cubes Opal White Curved, 5-6 conchoidal fracture; transparent _hyalite_ variety resembles ice D. Cannot be scratched by an ordinary pocket knife but can be scratched by quartz Opal White Curved, 5-6 conchoidal fracture; transparent _hyalite_ variety resembles ice Quartz White Curved, 7 conchoidal fracture; _rock crystal_ quartz commonly 6-sided prism with pyramid-like ends E. Cannot be scratched by quartz Topaz White Perfect basal 8 cleavage gives flat, plate-like fragments; notice hardness

How To Use the Rock Identification Charts

In the rock identification charts on pages 40-41, the Texas rocks described in this book are arranged in four major groups according to their texture.

1. _Glassy_ (the rocks are smooth, dark, and shiny) 2. _Compact, dull, or stony_ (the rocks are smooth and dull, but the individual grains are too small to be recognized) 3. _Granular_ (at least some of the individual grains of the rocks are large enough to be seen without a magnifying glass) 4. _Fragmental_ (the rocks are made up of fragments that are either loose or cemented together)

Consult Rock Chart 1, if the rock is glassy; Chart 2, if it is compact, dull, or stony; Chart 3, if it is granular; and Chart 4, if it is fragmental.

Two of the rock charts are subdivided. In Rock Chart 2, the compact, dull, or stony rocks are arranged according to hardness as follows:

A. Rocks that can be scratched by a fingernail B. Rocks that cannot be scratched by a fingernail but can be scratched by an ordinary pocket knife C. Rocks that cannot be scratched by an ordinary pocket knife

In Rock Chart 3, the granular rocks also are arranged according to _hardness_ into:

A. Rocks that can be scratched by an ordinary pocket knife B. Rocks that cannot be scratched by an ordinary pocket knife These harder rocks are subdivided into three groups: 1. Those that have grains of about equal size 2. Those with large easily seen grains that are scattered through a mass of finer grains 3. Those rocks whose grains are arranged in layers

In the “remarks” column of the rock identification charts are included further tests that will aid you in identifying the rock.

For a more complete rock determination chart, you can consult a textbook, such as _Rocks and Rock Minerals_, by L. V. Pirsson and A. Knopf.

Rock Identification Charts

_Chart _Rock_ _Remarks_ No._ 1. GLASSY appearance (rock is dark, smooth, and shiny)

Obsidian Entire rock is glassy Vitrophyre Crystalline grains are scattered through the dark glassy mass

2. COMPACT, DULL, OR STONY appearance (individual grains too small to be recognized)

A. Can be scratched by a fingernail Chalk Dilute hydrochloric acid fizzes on it Clay Earthy odor when breathed on Diatomite Crumbly Rock gypsum Made up of the mineral gypsum Soapstone Soapy or greasy feel B. Cannot be scratched by a fingernail but can be scratched by ordinary pocket knife Dolomite Dilute hydrochloric acid may fizz slightly on it; will fizz if rock is powdered Limestone Dilute hydrochloric acid fizzes on it Serpentine rock Commonly some shade of green Shale Breaks in flat, thin flakes; earthy odor C. Cannot be scratched by an ordinary pocket knife Basalt Dark colored and heavy Chert Hard, smooth, and porcelain-like Rhyolite Light to dark colored; may show streaks or flow structure

3. GRANULAR appearance (at least some of the individual grains are large enough to be seen without a magnifying glass)

A. Can be scratched by an ordinary pocket knife Limestone Dilute hydrochloric acid will fizz on it Marble Dilute hydrochloric acid will fizz on calcite marble, and it may fizz slightly on dolomite marble Rock gypsum Made up of the mineral gypsum Rock salt Has a salty taste; made up of the mineral _halite_ B. Generally cannot be scratched by an ordinary pocket knife (some schist is softer) Grains are of about equal size (equigranular) 1. Granite Quartz and feldspar grains interlock Pegmatite Large interlocking grains; commonly feldspar, quartz, mica Quartzite Rock breaks across the quartz grains Sandstone Rock breaks through the cement but around the sand grains Easily seen grains are scattered through a mass of finer grains 2. Basalt Dark colored, heavy rock Llanite Rusty pink feldspar and blue quartz grains embedded in a brownish rock mass Rhyolite porphyry Light to dark colored rock; may show streaks or flow structure Grains are arranged in layers 3. Gneiss Interlocking grains are in straight or wavy bands Schist Splits in thin layers; some schist is soft enough to be scratched by a knife

4. FRAGMENTAL appearance (rocks are made up of fragments that are either loose or cemented together)

Breccia Angular, gravel-size fragments that are cemented together Conglomerate Rounded, gravel-size fragments that are cemented together Coquina Shells and shell fragments that are cemented together Gravel Loose fragments Pulverulent Loose, powdery fragments; dilute limestone hydrochloric acid fizzes on them Sand Loose fragments no larger than a pinhead Sandstone Sand fragments that are cemented together Volcanic ash Loose, fine, gritty particles

DESCRIPTIONS OF SOME TEXAS ROCKS AND MINERALS

The pages that follow contain descriptions of Texas rocks and minerals. The descriptions are given in alphabetical order, except that related varieties are described together. For example, agate, amethyst, chert, flint, jasper, onyx, and chalcedony are discussed under quartz, because they are varieties of quartz. The descriptions include the properties of the rock or mineral that will help you identify it and also include information on where the rock or mineral can be found in Texas, some of its uses, and how it may have formed. The chart on page 99 lists chemical composition, specific gravity, and hardness of various Texas minerals.

Agate. _See_ Quartz.

Agatized Wood. _See_ Quartz.

Alabaster. _See_ Gypsum.

Albite. _See_ Feldspar.

Almandite. _See_ Garnet.

Amethyst. _See_ Quartz.

Amphibole Asbestos. _See_ Asbestos.

Anhydrite

Anhydrite, calcium sulfate, is a rather soft mineral that you can scratch with a pocket knife, although not with a fingernail. It has a glassy or a pearly luster and is transparent or translucent. Most anhydrite is white, but impurities cause it to be grayish, bluish, or reddish. When rubbed across a streak plate, anhydrite gives a white streak. This mineral has an uneven fracture, and it cleaves in three directions that are at right angles to each other. It commonly occurs as rectangular cleavage fragments or as sugary crystalline masses.

Anhydrite resembles dolomite, limestone, or gypsum. You can use a hardness test to distinguish it from gypsum (anhydrite is harder) and an acid test to distinguish it from limestone and dolomite. A drop of dilute hydrochloric acid will fizz when you put it on limestone or powdered dolomite. On anhydrite, the acid does not fizz.

Anhydrite occurs at several places in Texas. It is, for example, seen in bluffs along the Double Mountain Fork and the Salt Fork of the Brazos River in north-central Texas. Most of the Texas anhydrite, however, occurs underground. In the Gulf Coastal Plain, the anhydrite is found below the surface in salt domes. (Salt domes are described with halite on p. 66 and with sulfur on p. 91.)

Another anhydrite locality is in the subsurface Permian basin of west Texas. Oil wells drilled in this basin penetrate great, thick deposits of massive anhydrite. The anhydrite was deposited during the Permian Period from a sea that covered this area. As the sea gradually evaporated, the mineral matter that was dissolved in it came out of solution to form anhydrite, halite, and several other minerals.

Antigorite. _See_ Serpentine.

Argentite. _See_ Silver Minerals.

Asbestos

Asbestos is not really any one particular mineral. It is the name given to several minerals that occur in masses of slender, delicate fibers. In the more typical kinds of asbestos, these fibers—when pulled apart—resemble soft, fluffy, silk strings.

Several small deposits of _amphibole asbestos_ have been found in the Llano uplift area of central Texas. This asbestos is a variety of the mineral _tremolite_, a calcium-magnesium silicate. It has fibers that break rather easily, and it has a silky luster. It is a shade of green or gray and gives a white streak when rubbed across a streak plate. When you pull its fibers apart, you actually are breaking the mineral along its two directions of perfect cleavage. This amphibole asbestos is softer than other varieties of the mineral tremolite—a copper penny scratches it easily.

The asbestos occurs in veins in Precambrian metamorphic rocks in southern Llano County, northwestern Blanco County, and northeastern Gillespie County. These deposits are small.

A variety of the mineral _serpentine_ called _chrysotile asbestos_ is the kind most used by industry. Its fibers are commonly flexible enough and strong enough to be woven into cloth. This cloth is made into articles, such as fireproof suits, gloves, and theater curtains. Some chrysotile has been found in Precambrian metamorphic rocks in northwestern Blanco County, but it does not break into fibers fine enough or flexible enough to be called asbestos.

Azurite. _See_ Copper Minerals.

Barite

Barite, barium sulfate, is a fairly common mineral in Texas. It has a glassy or a pearly luster, and it is transparent to translucent. Barite is colorless, white, brownish, bluish, yellowish, or reddish. When rubbed across a streak plate, it gives a white streak. It is not extremely hard—you can scratch it with a pocket knife, although not with a fingernail.

Barite is distinctive because of its weight and cleavage. It cleaves in three directions, and some cleavage fragments are flat or platy. For a mineral with a nonmetallic luster, barite is heavy—it has a specific gravity of 4.5.

Barite commonly occurs as prism-shaped and as flat crystals, as granular masses, as cleavable masses, and as rounded masses called _nodules_. In Texas, some of it was deposited in sedimentary rocks by underground waters. As the waters seeped through these rocks, mineral matter came out of solution to form the barite. Some of the barite in Texas also formed from solutions that came from hot magmas.

A number of barite deposits have been found in Texas, but many of them are small. Barite occurs in Precambrian metamorphic rocks in Gillespie and Llano counties, in Pennsylvanian shale in Brewster County, in Permian shales in Baylor and Taylor counties, and in Permian limestones in Culberson County. It is found in Triassic red shales in Howard County and in Cretaceous sedimentary rocks in Brewster, Brown, Hudspeth, Jeff Davis, Kinney, and Val Verde counties. In Live Oak County, barite occurs in Tertiary bentonitic clays. Barite is being mined from a deposit in the Seven Heart Gap area northeast of Van Horn in Culberson County.

Barite is used in a number of ways. It is a source of barium chemicals, and it also is powdered and used as an ingredient in paint. The oil industry uses large amounts of barite. In drilling for oil by the rotary method, water and muds are pumped down the hole to aid drilling. Barite is added to these drilling fluids to make them heavy, since high-pressure gases are not as likely to blow heavy fluids out of the hole.

Basalt

Basalt is a heavy igneous rock that is black, dark gray, or dark brown. This rock is made up chiefly of a feldspar mineral, such as _labradorite_, and a pyroxene mineral, such as _augite_. Other minerals may be present.

The mineral grains of some basalts are so small that you cannot distinguish them even with a magnifying glass. Other basalts, however, are _porphyritic_, which means that they contain larger, easily seen crystals and grains of feldspar and pyroxene scattered either through a mass of the small mineral grains or through glassy material.

Some basalts contain many small holes. These holes, called _vesicles_, were formed when bubbles of gas were trapped in the hardening magma. Later, solutions moving through the rocks may have deposited another mineral—such as calcite or chalcedony—in some of the vesicles.

Basalt forms from molten rock material that hardens either on or beneath the surface—it can be extrusive or intrusive. Much of the basalt now found in the Trans-Pecos country of west Texas formed from lava that flowed out onto the surface during the Tertiary Period. A few of the places where basalt occurs in west Texas are the Chinati Mountains of Presidio County, the Chisos Mountains of Brewster County, the Davis Mountains of Jeff Davis County, and the Van Horn Mountains of Culberson and Hudspeth counties.

Several varieties of basalt occur in the Balcones fault region of Bandera, Comal, Hays, Kinney, Medina, Travis, and Uvalde counties. These basalts formed from molten magma that forced its way into rocks just below the earth’s surface.

Some basalt, which is known commercially as _trap rock_, is produced in Uvalde County. It is crushed and used for railroad ballast, road building material, and as concrete aggregate.

Bentonite. _See_ Clay.

Biotite. _See_ Mica.

Braunite. _See_ Manganese Minerals.

Calcite

Calcite, calcium carbonate, is one of the most abundant minerals in Texas. It is the chief mineral in limestone and in some marble. It also serves as the cementing material in many sandstones. Crystals, grains, and cleavable masses of calcite, which have been deposited by underground water, occur in cracks and cavities in many of the igneous, metamorphic, and sedimentary rocks of Texas. Calcite also occurs as cave, spring, and stream deposits and as caliche.