Chapter IV: Introduction (2)

A. Can be scratched by a fingernail
Amphibole asbestos White Made up of 1-2½
slender, flexible
fibers that can
be pulled apart
Cerargyrite Shiny white or Light greenish 1-1½
gray color; appears
waxy; knife cuts
it smoothly;
turns violet
brown to black
when exposed to
light
Muscovite (white White Light colored; 2-2½
mica) splits into thin,
flat, transparent
sheets that will
bend without
breaking
Sulfur White or pale Greenish; will 1½-2½
yellow burn with a blue
flame; commonly
found as
crystals, crusts,
or grains
Talc White Light greenish 1
color; knife cuts
it smoothly;
feels soapy or
greasy; splits
into thin flakes
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Amphibole asbestos White Made up of 1-2½
slender, flexible
fibers that can
be pulled apart
Biotite (black mica) White Dark green; 2½-3
splits into thin,
flat, translucent
sheets that will
bend without
breaking
Calcite White Dilute 3
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Halite White Greenish tint; 2½
salty taste;
dissolves in
water;
cube-shaped
cleavage fragments
Muscovite (white White Light colored; 2-2½
mica) splits into thin,
flat, transparent
sheets that will
bend without
breaking
Serpentine White Two kinds: silky 2½-4
and fibrous, waxy
and platy
Sulfur White or pale Greenish; will 1½-2½
yellow burn with a blue
flame; commonly
found as
crystals, crusts,
or grains
C. 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
shaped like
octahedrons;
crystals commonly
cubes
Malachite Green Bright green 3½-4
color; dilute
hydrochloric acid
will fizz on
malachite
Serpentine White Two kinds: silky 2½-4
and fibrous, waxy
and platy
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near 90°
Garnet White Commonly occurs 6½-7
as crystals

12. NONMETALLIC luster, BLUE color

A. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Anhydrite White Commonly occurs 3-3½
as sugary-looking
masses
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
B. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Anhydrite White Commonly occurs 3-3½
as sugary-looking
masses
Azurite Blue Bright, intense 3½-4
blue color;
dilute
hydrochloric acid
will fizz on
azurite
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
Fluorite White Cleavage in 4 4
directions can
give fragments
that are shaped
like octahedrons;
crystals commonly
cube-shaped
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull; milky
white and
bluish-white
precious opal
shows plays of
colors
C. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near 90°
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull; milky
white and
bluish-white
precious opal
shows plays of
colors
Quartz White Curved conchoidal 7
fracture; occurs
as crystalline
quartz and as
bluish
_chalcedony_
D. Cannot be scratched by quartz
Topaz White Perfect basal 8
cleavage gives
flat, plate-like
fragments; notice
hardness

13. NONMETALLIC luster, RED or PINK color

A. Can be scratched by a fingernail
Gypsum White Reddish; soft; 2
occurs as
crystals or as
fibrous,
granular,
compact, or
earthy masses
Hematite Dark reddish brown Brownish-red 1+
color; soft and
earthy
Sulfur White or pale Reddish; will 1½-2½
yellow burn with blue
flame; commonly
found as
crystals, crusts,
or grains
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Anhydrite White Pinkish tint; 3-3½
commonly occurs
as sugary-looking
masses
Barite White Pinkish tint; 3-3½
rather heavy;
cleavage
fragments may be
flat and slab-like
Calcite White Pink color; 3
dilute
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Cinnabar Dark red Dark red or 2½
bright
yellowish-red
color; shiny,
brilliant luster
when pure; dull
and earthy when
impure; heavy
Halite White Reddish tint; 2½
salty taste;
dissolves in
water;
cube-shaped
cleavage fragments
Sulfur White or pale Reddish; will 1½-2½
yellow burn with blue
flame; commonly
found as
crystals, crusts,
or grains
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Anhydrite White Pinkish tint; 3-3½
commonly occurs
as sugary-looking
masses
Barite White Pinkish tint; 3-3½
rather heavy;
cleavage
fragments may
look flat and
slab-like
Dolomite White Pink color; 3½-4
commonly occurs
as granular
masses and as
rhomb-shaped
crystals; dilute
hydrochloric acid
may fizz slightly
on dolomite
Fluorite White Pink color; 4
cleavage in 4
directions can
give fragments
that are shaped
like octahedrons;
crystals commonly
cubes
Opal White Reddish color; 5-6
curved,
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near 90°
Garnet White Commonly occurs 6½-7
as crystals
Opal White Reddish color; 5-6
curved,
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull
Quartz White Curved, 7
conchoidal
fracture; occurs
as _rose quartz_,
as pink _chert_,
and as _agate_
and _jasper_

14. NONMETALLIC luster, PURPLE or VIOLET color

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.

Calcite is transparent or translucent, and—depending on the variety—its luster is glassy to earthy. Most calcite is white or colorless, but it can be a shade of pink, blue, green, brown, yellow, or gray. It gives a white streak when you rub it across a streak plate.

Two properties of calcite to notice are the hardness and the cleavage. This mineral cleaves perfectly in three directions that are not at right angles to each other, and some of the cleavage fragments are rhombohedrons. Calcite is rather soft—you can scratch it with a copper penny but not with a fingernail. A drop or two of dilute hydrochloric acid also will help you to identify this mineral. The acid will readily fizz and bubble when it is placed on calcite.

Calcite occurs in more different kinds of crystal shapes than any other mineral. Some of these crystals are flat and tabular; some are rhombohedrons; some are prisms. Pointed crystals, called _dog-tooth spar_, and twinned crystals have been found in the Terlingua area of Brewster County in west Texas. A somewhat unusual occurrence of calcite crystals is in geodes. Some of these are found in Lower Cretaceous rocks west of Austin in Travis County.

Transparent crystals and cleavage fragments of calcite show a property called _double refraction_ (other minerals show it, too). To test this property, you can mark a single dot on paper. When you look at the dot through a piece of clear calcite, you will see two dots instead of one. This happens because a ray of light is bent (refracted) and is split into two rays as it enters the mineral. These two rays travel through the calcite in slightly different directions, and each carries an image of the dot through the mineral. The two images that you see are at the points where the two rays leave the calcite.

Calcite that is deposited at springs, along river and creek banks, and in caves is known as _travertine_. Cave forms of travertine, including stalagmites and stalactites, occur in several caves in Texas. Another kind of travertine is called _calcareous tufa_ or _calcareous sinter_. It is a porous and spongy-looking material deposited from water carrying dissolved limestone and is found around the openings of some springs and along some creek and river banks.

A dull, earthy calcite deposit, known as _caliche_, occurs in areas of Texas that have scant rainfall, such as the High Plains, west Texas, and the southwestern part of the Gulf Coastal Plain. Caliche commonly is found mixed with other materials, such as clay, sand, or gravel. This substance may be firm and compact or loose and powdery.

It is thought that caliche forms when ground moisture, containing dissolved calcium bicarbonate, moves upward. In dry areas of the country, this moisture evaporates. As it does, it leaves a crust of calcium carbonate in the form of caliche on or near the surface of the ground.

Caliche is quarried in many counties in Texas and is used chiefly as road material and as an aggregate.

Caliche. _See_ Calcite.

Carnotite. _See_ Uranium Minerals.

Cassiterite

Cassiterite, tin dioxide, is the mineral that serves as the chief source of tin. Tin does not corrode and tarnish, and one of its main uses is in the making of tin cans. (Actually, our tin cans are made from thin sheets of steel that have been coated with a protective layer of tin.)

Cassiterite has either a nonmetallic or a submetallic luster. Some specimens are brilliant and shiny; others are dull. Cassiterite may be translucent to transparent. It may be black, brown, gray, reddish brown, or yellowish brown. When rubbed across a streak plate, this mineral leaves a pale brown, a pale yellow, or a white streak. Cassiterite is quite heavy—it has a specific gravity of 6.8 to 7.1. It is too hard to be scratched by an average pocket knife.

Sometimes, prospectors use a chemical test to help them identify cassiterite. They put small pieces of metallic zinc into a jar or test tube containing dilute hydrochloric acid. Then they add a few fragments of the mineral that they suspect is cassiterite. If the fragments are cassiterite, they become covered with a pale gray coating of metallic tin.

Cassiterite’s most common crystal shape is a short, 8-sided prism with pyramids at each end, but perfect crystals are not often found. Most Texas cassiterite does not show a crystal shape. Instead, it occurs as crystalline masses in igneous rocks and as loose pebbles that have weathered out of these rocks.

Cassiterite occurs in a number of places in the United States but not in large quantities. A small amount of cassiterite has been found in quartz veins in Precambrian granite in both central Texas and west Texas. In El Paso County, the cassiterite is found on the east side of the Franklin Mountains a few miles north of El Paso, where some of it has been mined. In central Texas, cassiterite occurs in the Streeter area of Mason County.

When the granite rocks in these areas were formed, probably not all of the hot magmas cooled and hardened at the same time. The fluids given off by the remaining magmas contained tin and several other elements. It is believed that these fluids moved up into cracks in the granite rocks and formed the cassiterite.

Celestite

Celestite is a strontium sulfate mineral. It is colorless, white, yellow, or gray. Light blue specimens of this mineral also are found, and it is because of this sky-like color that celestite gets its name. The word celestite comes from the Latin word _caelestis_, meaning _of the sky_.

Celestite has a glassy to a pearly luster, and it is either transparent or translucent. It gives a white streak when rubbed across a streak plate. Celestite has a specific gravity of 3.95 to 3.97. It is, however, lighter than barite, a mineral that it resembles. Celestite is not very hard—a knife will scratch it, although your fingernail will not. It cleaves in three directions, and some of the fragments are flat and slabby.

Celestite occurs commonly either as prism-shaped or flat crystals and as cleavable, granular, or fibrous crystalline masses. In Texas, it is found in geodes, as rounded nodules, or as bedded or layer-like deposits in limestones and other sedimentary rocks. In Real County, celestite occurs on the walls of a cave in Cretaceous limestone.

Some celestite may be deposited by sea water, but much of the Texas celestite is believed to have been deposited by underground water that seeped through cracks and pores in the limestones and other sedimentary rocks. This water picked up and dissolved strontium compounds that were scattered in small amounts through the rocks. Then, it re-deposited the strontium in the rocks as celestite.

In Texas, beds of celestite occur in Permian rocks in Coke, Fisher, and Nolan counties and in Lower Cretaceous rocks in Brown, Comanche, and Mills counties. Celestite geodes and nodules are found in Lower Cretaceous limestone rocks in Lampasas, Travis, and Williamson counties, and in Permian rocks in Coke, Fisher, Nolan, and Taylor counties.

Celestite is one of two minerals (the other mineral is _strontianite_, strontium carbonate) used as a source of strontium. Strontium compounds give a crimson-red color to a flame, so they are used in fireworks, tracer bullets, and flares. Perhaps you have seen a red flare set out on the highway at night to warn motorists that a truck has stalled. The chances are good that the flare’s red flame was due to a strontium compound. Some of the Texas celestite has been mined, but most of the strontium minerals now used in the United States are imported from England and Mexico.

Cerargyrite. _See_ Silver Minerals.

Chalcedony. _See_ Quartz.

Chalcocite. _See_ Copper Minerals.

Chalcopyrite. _See_ Copper Minerals.

Chalk. _See_ Limestone.

Chert (Flint). _See_ Quartz.

Chrysotile. _See_ Asbestos; Serpentine.

Cinnabar

Cinnabar, which is mercuric sulfide, is the most common mercury mineral. It has a dark red or a bright yellowish-red color and is transparent to translucent. When rubbed across a streak plate, it leaves a dark red streak. If pure, cinnabar has a brilliant, shiny, nonmetallic luster. It is, however, commonly found mixed with impurities, such as clay, calcite, iron oxide, or bituminous material, and then it looks dull and earthy. Cinnabar is quite heavy—it has a specific gravity of 8.10. It is rather soft, and you can scratch it with a copper penny.

Some prospectors use a quick chemical test to identify cinnabar. They rub a clean, shiny copper coin with a mineral sample that has been moistened with a drop or two of dilute hydrochloric acid. If the sample is cinnabar, a light silvery-gray coating appears on the coin.

Cinnabar occurs as small crystals or as fine-grained or compact crystalline masses. It is found in veins that fill cracks in rocks and also occurs as crusts and coatings on rocks. It also may be widely scattered through rocks, such as limestones.

Cinnabar occurs in the Terlingua area of Brewster and Presidio counties in west Texas. It has been mined there, off and on, since about 1894, and during this time, mercury worth many millions of dollars has been produced.

Most of this west Texas cinnabar is found in cracks, pores, and breccia-filled cavities of Cretaceous limestones and clays. If you will look at the Texas geologic map (pp. 4-5), you will see that igneous rocks occur in this district. Many millions of years ago during the Tertiary Period, when these igneous rocks were still hot magma, some of them pushed up under the Cretaceous rocks and emitted fluids containing mercury. The fluids moved upward through cracks and pores in the Cretaceous rocks where they deposited the mercury as cinnabar and as other mercury minerals.

Mercury is an unusual element. Instead of occurring as a solid metal at ordinary room temperatures, as do gold, silver, and lead, it remains a liquid until it is cooled to 38 degrees below zero Fahrenheit. Because the silvery little drops of liquid mercury roll about as if they were alive, this element long has been called _quicksilver_.

Mercury is used in a variety of ways. In some noiseless light-switches, a glass tube containing a small ball of mercury tilts when the switch is turned “on.” The mercury then rolls to the end of the tube that contains electrical contacts and quietly completes the electrical circuit. In other uses, mercury is added to silver, tin, and other metals to make fillings for teeth. Some medicines, such as calomel and mercurochrome, contain mercury. Fulminate of mercury helps to set off dynamite and other explosives. Mercury is used in many barometers and thermometers, and farmers use mercury poisons to control insects and fungi.

Mercury also commonly is used to obtain gold from its ores. One method of accomplishing this is to pass wet gold-bearing gravel or crushed rock over metal plates that are coated with mercury. The gold particles quickly mix with the mercury to form an _amalgam_, which later can be scraped off the plates. The gold is then recovered by heating the amalgam to drive off the mercury.

Clay

Clay is a smooth, soft, earthy rock made up of mineral particles no bigger than specks of dust. Some of the particles are clay minerals, which consist of aluminum, silicon, and other elements. In addition, tiny particles of quartz, calcite, and other minerals may also be present in the clay.

The clay particles are all that remain of rocks and of minerals, such as feldspar, that have been broken into fragments or altered into clay minerals by weathering. Some clay remains at the place where it formed, but some is carried away and deposited elsewhere.

Clay is white, tan, brown, red, green, blue, gray—almost any color. When moist, it has an earthy odor. You can moisten a piece of clay enough to notice this just by breathing on it. Most clays, when wet, can be molded into many different shapes—that is, they are plastic, but when they are dry, they are firm and solid.

Clay is abundant in Texas and has a number of uses. Some goes to make portland cement, and some is baked or burned in a kiln to make brick, tile, sewer pipes, pottery, and other products. This kind of clay is obtained from Tertiary formations of the Gulf Coastal Plain, from Upper Cretaceous formations in central Texas, and from Pennsylvanian formations in north-central Texas. (You can locate Tertiary, Cretaceous, and Pennsylvanian rocks on the Texas geologic map, pp. 4-5.)

A special kind of white burning clay that can be used to make chinaware is called _kaolin_ or _china clay_. It contains particles of the clay mineral _kaolinite_ as well as several other clay minerals. Deposits of china clay occur in southern Jeff Davis County and in Real County near Leakey, but none is being produced.

Another kind of clay, _bentonite_, forms from weathered volcanic ash. Bentonite contains the clay mineral _montmorillonite_ and looks smooth and soap-like. Fresh samples of this clay are white, pale green, or pale blue, but dried-out or weathered samples are tan, brown, yellow, or reddish. When wet, bentonite absorbs water, swells, and then has a jelly-like appearance.

Surface deposits of bentonite occur chiefly in Eocene Tertiary formations of the Gulf Coastal Plain, in Cretaceous formations of the Big Bend area of west Texas, and in Quaternary formations of the High Plains.

Some bentonite is used to absorb unwanted coloring material in petroleum and in vegetable oils. It is then known as a _bleaching clay_. Bentonite bleaching clay is obtained from some of the Tertiary formations along the Texas Gulf Coastal Plain. It has been produced in Angelina, Fayette, Gonzales, Jasper, Walker, and other counties in this area.

Another important use of bentonite, and of other clay, too, is as _drilling mud_. In the rotary method of drilling for oil and gas, mud is pumped down into the drilled hole. This mud carries the rock cuttings up to the surface, it cools the drilling tools, and it coats and seals the walls of the hole. Along the Gulf Coastal Plain, drilling clay is obtained from Tertiary formations.

Common Opal. _See_ Opal.

Copper Minerals (Chalcocite, Chalcopyrite, Malachite, Azurite)

A number of minerals containing copper, such as _chalcocite_, _chalcopyrite_, _malachite_, and _azurite_, occur in small deposits in Texas. They are found chiefly in the Llano uplift area of central Texas, in the Van Horn area of Culberson and Hudspeth counties in west Texas, and in a group of counties in north-central Texas.

Copper is an important element. Because it is an unusually good conductor of electricity (only silver, which costs much more, is a better one), it is used for many kinds of wires for switchboards, generators, motors, telephone and telegraph equipment, and light and power lines.

Manufacturers commonly combine copper with other elements. For example, some copper is mixed with zinc to make _brass_ and with tin and a little zinc to make _bronze_. These mixtures are called _alloys_. Many products are made from copper alloys, including tubing, pipes, jewelry, pots, and pans. Even our coins contain copper.

Sometimes, a prospector uses a chemical test to find out if copper is present in a mineral. First, he crushes a small sample of what he believes is a copper mineral (such as chalcocite, chalcopyrite, azurite, or malachite). He then puts the sample in a glass jar or test tube and pours in a small amount of dilute nitric acid (this acid, like hydrochloric acid, is poisonous). After the sample has dissolved in the acid, he adds enough ammonium hydroxide to make the solution alkaline. If the sample is a copper mineral, the solution turns a deep-blue color.

One of the copper minerals, _chalcocite_, copper sulfide, also is known as _copper glance_. It is a metallic mineral that commonly tarnishes to a dull black. By chipping off a fragment to obtain a fresh surface, you will see that it has a shiny lead-gray color. Chalcocite is rather soft, and it is sectile, that is, a knife will cut through it as well as scratch it. When you rub chalcocite across a streak plate, it gives a grayish-black streak. This mineral commonly occurs as compact masses or as granular masses.

Chalcocite, with its dark color, does not look at all like copper, which is a bright reddish brown. Chalcocite, however, is the chief copper mineral at the most important copper mine in Texas, the Hazel mine, which is about 15 miles northwest of Van Horn in Culberson County in west Texas. This mine, although now idle and almost filled with water, has produced about one and a half million pounds of copper along with more valuable silver ores. Here, the chalcocite and other minerals occur in material that fills large cracks in red sandstone of the Precambrian Hazel Formation. It is thought that long ago, molten igneous rock material far below the surface sent out hot solutions containing copper and other elements. These solutions moved upward and deposited minerals in the fracture zone in the sandstone.

Chalcocite occurs also in north-central Texas. It is found in Archer, Baylor, Clay, Foard, Hardeman, King, Knox, Stonewall, and several other counties of this area. Here, it occurs in Permian sedimentary rocks (called “red beds”) as rounded masses, as scattered grains, and as petrified wood. Because these deposits are far from any igneous rocks, they apparently did not form in the same way as those at the Hazel mine. These north-central Texas deposits have never really been commercially developed. During the Civil War, however, some copper from this area was made into percussion caps for the Confederacy.

Another copper mineral, _chalcopyrite_, is a copper-iron sulfide. It also is known as _copper pyrites_ and _yellow copper ore_. This mineral has a metallic luster and a brass-yellow or a golden-yellow color. When rubbed across a streak plate, it gives a greenish-black streak. Chalcopyrite will tarnish and then has bronze, blue, purple, and other rainbow-like colors. This mineral is fairly soft—you can scratch it with a pocket knife. Because of chalcopyrite’s yellow color, it has often been mistaken for gold. For this reason, it, like iron pyrite, is often called _fool’s gold_. (See Gold, p. 60, for ways to tell them apart.)

Chalcopyrite commonly is found in compact masses that show no crystal shapes. These masses either are scattered through rocks or occur in material that fills cracks in rocks.

Some chalcopyrite is found in Precambrian sandstone at the Hazel mine and in other deposits in the Van Horn area of Culberson and Hudspeth counties. It also occurs in Precambrian rocks at the Sheridan and Pavitte prospects in Burnet County. These chalcopyrite localities are in districts where igneous rocks occur.

It is likely that, long ago, hot solutions containing copper moved upward, out of deeply buried molten magma. While still far below the surface, the solutions deposited the chalcopyrite in cracks and other openings in the nearby rocks.

Two copper minerals of Texas, _azurite_ and _malachite_, are copper carbonates. Azurite is commonly called _chessylite_ and _blue copper_; malachite is called _green copper carbonate_. Because these minerals are carbonates, a drop of dilute hydrochloric acid will fizz and bubble when placed on either of them.

Azurite has a bright, intense blue color and leaves a blue streak when rubbed across a streak plate. Malachite has a bright green color and leaves a green streak. These minerals have a nonmetallic luster and a glassy to dull appearance. Commonly, they are translucent, although some specimens of azurite are transparent. Both azurite and malachite are fairly soft—a pocket knife will scratch them, but a copper penny will not.

Azurite and malachite occur as individual crystals, but you are more likely to find them as crusts on rocks and on other minerals. Malachite is also found in rounded fibrous masses that resemble bunches of grapes (described then as _botryoidal_).

Both azurite and malachite are formed in the same way. Underground waters seep through rocks that contain deposits of copper minerals (such as chalcocite and chalcopyrite) and cause chemical reactions which change these minerals into malachite and azurite.

Malachite is more plentiful than azurite, but both minerals can be found together. You can expect to find at least one of them at the same localities where chalcocite, chalcopyrite, and other copper minerals occur.

Coquina. _See_ Limestone.

Diatomite. _See_ Opal.

Dolomite

Dolomite is the name given both to a rock and to a mineral. The mineral is a calcium-magnesium carbonate and has a glassy or a pearly luster. It is any of a number of colors, such as white, pink, brown, or gray, or it can be colorless. Dolomite leaves a white streak on a streak plate and is transparent to translucent. It is not particularly hard and can be scratched with a pocket knife, although not with a copper penny. Dolomite cleaves perfectly in three directions, and some of the cleavage fragments are rhombohedrons. However, the cleavages of the individual mineral grains in specimens of fine-grained massive dolomite are not readily distinguishable.

Most Texas dolomite occurs as coarse-, medium-, and fine-grained crystalline masses as the chief mineral in dolomite rock and in dolomitic marble. It is also found as 6-sided crystals that are rhomb-shaped; when the faces are curved, they have a saddle-like appearance.

Crystals of the mineral dolomite commonly occur in cavities in the dolomite rocks. It is believed that they were deposited there by seeping underground waters. The waters dissolved some of the dolomite in the rocks and then re-deposited it as crystals.

Dolomite rock is made up mostly of crystalline grains of the mineral dolomite. In addition, quartz grains, calcite, and other minerals may be present. Dolomite rock is almost any color—white, buff, pink brown, gray. It resembles some limestone, and these two rocks actually are closely related.

To help tell them apart, dilute hydrochloric acid often is used. A few drops of this acid will readily fizz and bubble if the rock you put them on is a limestone. If the rock is dolomite, the acid will effervesce only very little or not at all. (If, however, the acid is put on powdered dolomite, it then will fizz readily.) Dolomite is slightly harder than limestone, and it also is slightly heavier.

Some dolomite rocks formed directly from materials that were dissolved in sea water, and others are altered limestone rocks. Some limestones altered into dolomite on the sea floor by the addition of magnesium from the sea water. Others changed into dolomite much later after the sea had withdrawn and the limestones had become a part of the land; underground waters containing magnesium seeped through these limestones and altered them into dolomite.

Many of the dolomite rocks are found with limestones. In Texas they occur mostly in Cambrian, Ordovician, Mississippian, Pennsylvanian, Permian, and Cretaceous formations. The geologic map (pp. 4-5) indicates where these strata appear at the surface in Texas.

Dolomite is abundant in the Llano uplift area of central Texas—particularly in the Cambrian and Ordovician rocks. A number of these central Texas dolomites have been quarried for use as building stones. Some of them also have been crushed and used as a road-building material and as a stone aggregate that is mixed with cement to make concrete. This dolomite is also used as terrazzo chips (terrazzo floors are described with serpentine on p. 88). In addition, Ellenburger (Ordovician) dolomite from Burnet County was used during World War II as a source of the lightweight metal magnesium.

Dravite. _See_ Tourmaline.

Feldspar

Feldspar is the name given to a group of nonmetallic minerals that are much alike. Several of them are so similar that a petrographic microscope must be used to tell them apart. Each of the feldspar minerals is an aluminum silicate. Each of them contains, in addition, at least one of the following elements: potassium, sodium, calcium, and barium. The feldspar minerals that are found in Texas include _albite_, a sodium-aluminum silicate, and _orthoclase_ and _microcline_, which are both potassium-aluminum silicates.

The feldspar minerals are transparent to translucent and have either glassy or pearly lusters. They can be white, cream, or a shade of red, brown, yellow, blue, gray, or green. When you rub a feldspar across a streak plate, it leaves a white streak. The feldspars are rather hard—a pocket knife will not scratch them, although a piece of quartz or a steel file will. These minerals have good cleavage in two directions. The cleavages meet at an angle of about 90°, so that the cleavage fragments have square corners.

The feldspars are important rock-forming minerals. You can find them in igneous rocks, such as granite or pegmatite, and in metamorphic rocks, such as gneiss. They also occur as fragments in sedimentary rocks, such as some sandstone and conglomerate.

Although the feldspars can originate in other ways, they form mostly from hot magmas that cool and crystallize into igneous rocks. These minerals occur in the rocks as grains, as cleavable masses, and as individual crystals. The crystals may be shaped like prisms, or they may be flat and slabby.

Good places to look for feldspars are in areas where granites, pegmatites, and other intrusive igneous rocks appear at the surface. The pegmatite rocks of Burnet, Gillespie, Llano, and Mason counties in the Llano uplift area of central Texas, and those of the Van Horn Mountains in Hudspeth and Culberson counties in west Texas, are especially good sources of feldspar. Large cleavable masses and crystals that are more than a foot long are found in some of these rocks.

The feldspars have a number of uses. Some of the pegmatite feldspars from Llano County in central Texas have been crushed and used as granules for built-up and composition roofs. In addition, some have been shipped to Mexico for glass-making. Some of the other uses of feldspar are in making porcelain, ceramic glazes, and scouring compounds. A few of the feldspar minerals, such as the variety of microcline known as _amazonstone_, are used as gemstones.

Fibrous Gypsum. _See_ Gypsum.

Flint. _See_ Quartz.

Fluorite

Fluorite is calcium fluoride. The fluorite that is mined and sent to market, however, commonly is found mixed with quartz, calcite, limestone, or other rocks and minerals. Industry calls this mixture _fluorspar_.

Fluorite is a transparent to translucent mineral that has a glassy luster. It may be colorless, or it may be white, pink, green, purple, brown, or blue. Some specimens show more than one color. When you rub fluorite across a streak plate, it leaves a white streak. This mineral is not particularly hard—a pocket knife will scratch it, although a copper penny will not. Fluorite has perfect cleavage in four directions. By carefully breaking a specimen, you can obtain cleavage fragments that are shaped like octahedrons.

Fluorite occurs as cleavable masses, as fine or coarse grains, and as crystals. Most of the crystals are cubes, but some may be octahedrons, dodecahedrons, or combinations of these.

Fluorite has been found both in west Texas and in central Texas. In the Llano uplift area of central Texas, it occurs in a number of Precambrian granite, pegmatite, schist, and gneiss rocks. The most important, although small, deposit in this area is near Spring Creek a few miles west of Burnet in Burnet County. Here, prospectors have dug holes and pits in gneiss and schist rocks and found layers of fluorite in them.

The largest known fluorite deposits in Texas (they are not particularly large when you compare them with the deposits in Illinois and Kentucky) are those in the Eagle Mountains of Hudspeth County. This fluorite occurs in both igneous and sedimentary rocks. Many years ago, probably during the late part of the Tertiary Period, hot magma far below the surface gave off liquids and gases containing fluorine. These fluids moved up through large cracks (called faults) in Cretaceous limestones and Tertiary igneous rocks and deposited fluorite in them. In places, beds of limestone have been replaced by fluorite. Some of this west Texas fluorite has been mined and shipped to market.

Fluorite is extremely important as a flux in steel-making to help the ingredients of the molten steel blend together. In addition, it combines with sulfur, phosphorus, and other unwanted substances so that they can be removed from the steel. Other important uses of fluorite are in glass-making and in the manufacture of hydrofluoric acid. This acid is used in the aluminum industry as well as in industries that make high-octane gasoline, insecticides, and refrigerants for refrigerators and freezers.

Galena

Galena, lead sulfide, is a shiny, lead-gray, metallic mineral that has a specific gravity of 7.4 to 7.6. It is soft enough to mark paper, and it leaves a grayish-black streak on a streak plate. This mineral cleaves perfectly in three directions, and the cleavage fragments have square corners—some are cubes.

Galena occurs as cleavable masses, as fine or coarse grains, and as crystals, most of which are cubes. Galena commonly is associated with other minerals; for example, some of the west Texas galena either contains some silver (then called _argentiferous galena_) or occurs with it. Sphalerite, a zinc mineral, is commonly found with galena.

Galena is an important mineral because it is the chief source of lead. Compounds of lead, called white lead, red lead, and litharge, are used as paint pigments. Automobile batteries contain lead plates, and tetraethyl lead is added to gasoline to keep the car’s motor from knocking. Some other uses of lead are in bullets, type metal, solder, and cable coverings.

Galena has been found in several areas of Texas and has been mined in central and west Texas. None, however, has been produced in recent years. Most of the galena mined in west Texas was obtained from silver mines, where the galena was a by-product. Some of the west Texas galena deposits are at Altuda Mountain east of Alpine in Brewster County, in the Eagle Mountains and the Quitman Mountains in Hudspeth County, and in the Chinati Mountains and the Shafter area in Presidio County. Most of the mining has been from the Shafter area (this area is described with silver minerals on p. 90).

In central Texas, several small galena deposits have been found in Blanco, Burnet, and other counties of the Llano uplift area. Some galena has been mined at Silver Creek in northwestern Burnet County. Here, galena occurs in cracks and as scattered grains in Cambrian limestones and sandstones.

It is probable that much of the galena in west Texas and in central Texas was formed when hot magma forced out solutions containing lead. These solutions moved up through cracks and other openings in the subsurface rocks and deposited the galena in them.

Small amounts of galena, which likely had a different origin, have been found in Fisher, Foard, Hardeman, and Young counties. A little occurs also in rocks associated with salt in a number of the Gulf Coastal Plain salt domes.

Garnet

Garnet is not one mineral but is the name given to a group of several minerals that are very much alike. In fact, it often is impossible to tell some of them apart without using special laboratory tests.

The garnet minerals have glassy to resinous lusters and are transparent or translucent. A pocket knife will not scratch them, and some specimens are too hard even for quartz to scratch. Two of the garnet minerals most commonly found in Texas are _almandite_, an iron-aluminum silicate, and _grossularite_, a calcium-aluminium silicate. Almandite has a deep-red or a brownish-red color. Grossularite is pale green, brownish yellow, cinnamon brown, or rose red.

Garnet minerals occur as crystals and as masses that are scattered through some of the metamorphic and igneous rocks. After they have weathered out of these rocks, the garnets make up a part of many sands and sandstones. Because these minerals so commonly occur as crystals, it is helpful to learn to recognize the crystal shapes.

Garnet minerals are found in the igneous and metamorphic rocks of both central Texas and west Texas. In central Texas, they occur in ancient Precambrian schist and pegmatite rocks of the Llano uplift area. Some of these central Texas garnet localities are in northeastern Mason County, central and northwestern Llano County, west-central Burnet County, and northeastern Gillespie County.

In west Texas, garnets occur in metamorphic rocks in the Quitman Mountains, which are southwest of Sierra Blanca in Hudspeth County, and in the Mica Mine area, which is south of Van Horn near the Hudspeth-Culberson County line. Garnets also have been found in igneous rocks in the Franklin Mountains a few miles north of El Paso in El Paso County.