Rocks, Metamorphic Rocks, and Metamorphic Environments
Most graphite is formed in metamorphic environments. With this in mind, a basic description of metamorphism and metamorphic environments is consistent with the material presented in other sections of this article. The following information is provided to give the reader a basic and general introduction to geological metamorphism.
From the point of view of composition rocks are generally classified as either "monomineralic," i.e., composed of one mineral only, or "polymineralic," i.e., mixtures of two or more minerals. An example of a monomineralic rock is quartzite, which is composed of only quartz. An example of a polymineralic rock is granite, which is typically composed of quartz, feldspar, and mica.
The three basic rock types are igneous, metamorphic, and sedimentary. Rocks are considered igneous if they result from solidification of a significant melt, i.e., melting other than "point melting" as the result of localized melting resulting from metamorphic reactions. Igneous rocks are considered intrusive or extrusive. Intrusive igneous rocks are those igneous rocks that occur below the surface as in rocks that solidify from magma. Extrusive igneous rocks are those igneous rocks that are released above the surface as in rocks that solidify from lava. Granite, basalt, and andesite are types of igneous rocks.
Rocks are considered metamorphic if the "proto-lith" is exposed to enough heat and pressure to change the mineralogy and/or structure and/or chemical composition, without significant melting. Any rock type can be transformed into a metamorphic rock (for that matter any rock type can be transformed into any other rock type given the right environment and enough time). Metamorphism must occur below the surface since it occurs as the result of a rock "adjusting" to heat and pressure. For this reason there is no "intrusive/extrusive" distinction required. Some examples of metamorphic rocks are meta-quartzite, gneiss, and schist.
Rocks are considered sedimentary if they are formed from the consolidation and compaction of the weathering products of any other type of rock, or the deposition and consolidation of biological materials (as in carbon from ancient plant life), or from aqueous precipitation (as in salt or carbonate deposits). Sedimentary rocks may be formed under relatively high pressures as may be realized from the burial pressure of a few kilometers of crust but typically form at temperatures below 100 C. Shale, halite, and limestone are examples of sedimentary rocks.
Metamorphic environments are categorized by geologists and geochemists into logical groupings based on the temperature and pressure of the enclosing rocks. Basic divisions of metamorphic environments more familiar to non-geologists would be for example, low-grade, medium-grade, and high-grade metamorphic environments. Gradations from low to high indicate increased levels of geological energy, in other words higher temperature and higher pressure with increasing degree of metamorphism. These terms are acceptable for generalized descriptions of metamorphic environments but leave much to the imagination. Using this generalized system flake graphite, for example, can be said to be "a high grade metamorphic mineral." However, using this terminology how does one distinguish between high grade graphite forming environments and some other "grade" which may be only 100 C lower in temperature but the same pressure? To solve this problem geologists define a series of what are known as "metamorphic facies." A "facies" is basically a set of pressure and temperature conditions used to define a metamorphic environment. There are about 12 described facies that grade from about 1 kilobar (15,000 PSI) and 100 C, to over 17 kilobar (255,000 PSI) and 1000 C.
The "low" end of the metamorphism energy scale contains facies groups such as zeolite and pumpellyite (grouped together by some into the Subgreenschist facies). These facies cover temperature and pressure environments from 1-6 kilobar and 100-400 C. The "middle grade" of metamorphic rocks is included in the greenshist and amphibolite facies groups. These cover pressures from about 3-12 kilobar and 400-700 C. Finally the high-grade metamorphic environments are placed into the facies group known as granulite facies. Granulite facies environments range from about 4-15 kilobar pressure and 700-1000 C.
Each metamorphic facies can be further subdivided to better describe a specific geologic environment. Natural flake graphite is considered a granulite facies metamorphic mineral. Typical conditions under which flake graphite may form are 750 C and 5 kilobar (75,000 PSI). Those interested in an in depth description of metamorphic facies are urged to review "Petrogenisis of Metamorphic Rocks," Bucher & Frey, Springer-Verlag Books, 1994, which is an excellent text on metamorphic petrology.