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.
[Illustration: The Hazel copper-silver mine, Culberson County, Texas, as
it appeared in 1951. Photograph by P. T. Flawn.]
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.