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.
[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].