The Geologic Importance of Graphite

Graphite most often occurs in metamorphic rocks formed from regional metamorphism or contact metamorphism of organic-rich sedimentary rocks, such as organic-rich marble, quartzite, schist, gneiss, and metamorphosed coal. The sediment's original organic component provides the source for the carbon from which graphite forms. Coal is almost completely composed of organic carbon, so its metamorphism can produce large amounts of graphite-bearing rock. To a lesser extent, graphite is also found in some igneous rocks, igneous veins and pegmatites that are associated with metamorphosed graphite-bearing sedimentary rocks. As with the metamorphic occurrences, the graphite component of these igneous bodies is assumed to have originated from the associated organic-rich sediments. Much more rarely, graphite also is found as small nodules in some iron meteorites. Graphite crystals are relatively rare and most often occur in marble, where they may occur with pyroxene and spinel - two other minerals that also form from the metamorphism of organic impurities in limestone.

Although graphite and diamond may appear to have little in common with one another, the two minerals are polymorphs. Polymorphs are minerals that have the same chemical composition (in this case, carbon) but different crystal structures. Although graphite and diamond have the same chemical composition, their different crystal structures give them very different physical characteristics. For example, diamond is one of the hardest known substances, while graphite is one of the softest. Diamonds are typically clear, make excellent abrasives, and are good electrical insulators.

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