Graphite Occurrence and Composition Graphite is a native element, having composition very close to 100% carbon. It is a widespread accessory mineral in schists, marbles, gneisses, and other metamorphic rocks. It is also, much less commonly, found in some igneous rocks. Identification Graphite is one of several opaque minerals that can be difficult to distinguish. In reflected light it appears silvery and metallic, occasionally brownish gray. It may appear similar to magnitite or other silver-colored oxides, but generally has lower reflectance. Perhaps the best way to distinguish it from other opaques is that is sometimes has a sheety habit and cleavage. Similar minerals: ·Graphite can be mistaken for any of a number of silver-colored opaque minerals

Biotite-Sillimanite Schist with Graphite, from the Beartooth Mountains, Montana This photomicrograph, about 2 mm across, is of graphite in a biotite-sillimanite schist. The graphite is opaque (black in both PP and XP), flakey, and is intimately associated with the biotite. The biotite is brown (PP) and shows second order interference colors in the XP view. The sillimanite is relatively fine grained, showing high relief. The largest "patch" of sillimanite is in the upper left of the photo. Other, coarser sillimanite grains are along the bottom edge, and show up to second order blue interference colors in the XP view. The clear material that is not sillimanite, is quartz and K-feldspar. However, it is difficult to distinguish the two minerals in these views, and some of the quartz appears isotropic due to grain orientation (although it is not). The field of view is about 2 mm across.

Biotite-Sillimanite Schist with Graphite, from the Beartooth Mountains, Montana This photomicrograph, about 1 mm across, is of the same biotite-sillimanite schist shown above. The PP view shows abundant fine grained, clear, sillimanite needles and blades. Brown biotite, some associated with graphite, and clear quartz are also present. In the XP view, the biotite shows typical second order interference colors and the quartz shows first order interference colors. The fine needly sillimanite also shows first order colors, but a few larger sillimanite grains (oriented so the long dimension is not parallel to the slide) show second order colors.

Grraphite in a Biotite Schist from Western Massachusetts Here we see mostly biotite flakes. They are spotted with small grains of graphite, which is opaque and shows a somewhat "flakey" habit. The biotite is pleochroic from nearly clear to medium brown (PP) and shows mica's characteristic cleavage. It also displays many halos around radiation burns, probably centered on included zircon grains. The clear mineral (PP) is quartz. The field of view is about 2.5 mm.