Graphite is a mineral that is made up of a single element—carbon.
(_Diamond_, although it does not look at all like graphite, is a
crystalline form of carbon.) Graphite is a steel-gray or black mineral
that commonly has a metallic luster. It is not heavy and is extremely
soft. Graphite will soil your fingers and leave a black mark on paper.
This mineral cleaves perfectly in one direction and splits into thin
flakes that feel greasy.
To help distinguish graphite from _molybdenite_, a mineral it resembles,
you can use a shiny, glazed surface, such as is found on a saucer or a
plate, to test its streak. When rubbed across this kind of surface,
graphite will leave a black streak, but molybdenite will leave a
greenish one.
Graphite commonly occurs as scales, as sheet-like layers, or as compact
masses. It may be found mixed with clay or other impurities, and it then
looks dull and earthy. Crystals of graphite, which are seldom found, are
6-sided and flat.
Graphite occurs in Llano, Burnet, and other counties in the Llano uplift
area of central Texas. One of the Nation’s most important graphite mines
is located in the Clear Creek area several miles northwest of Burnet in
Burnet County. Some graphite has also been mined near Lone Grove in
Llano County. In addition, a graphite schist, obtained south of Llano in
Llano County, has been used as a filtering material.
[Illustration: Graphite is used in pencil lead, generator brushes, and
lubricants.]
All of this graphite occurs in extremely old Precambrian graphite schist
rocks that we now see at the surface in this part of Texas. It is
believed that the schists were once ancient sedimentary rocks, such as
shales, which contained organic matter. Long ago, great forces below the
earth’s surface altered these rocks. When this happened, the organic
material that they contained changed into the mineral we know as
graphite.
Graphite has a number of uses. It is mixed with clay to make the pencil
lead that we use for writing. It serves as a lubricant, either alone or
mixed with oil, grease, or water. In addition, graphite is used to make
generator brushes, stove and shoe polish, and special paints. Because it
can stand great heat without melting, some graphite is mixed with clay
to make the pots or crucibles that hold molten metals.
Grossularite. _See_ Garnet.
Gypsite. _See_ Gypsum.