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Biotite vs Graphite: Sheets, Streak and Greasy Feel

Biotite and graphite can both appear as dark reflective flakes in metamorphic rocks. Biotite splits into thin flexible, elastic sheets and commonly has vitreous to pearly reflections. Graphite is softer, feels greasy, and readily leaves a black mark on paper or fingers. Flex a loose flake only if the specimen permits. Biotite mica forms thin elastic sheets that tend to spring back; graphite is extremely soft and greasy and readily writes a black mark. Graphite has a black streak, while biotite’s streak is white to gray. Use a hand lens to determine whether the reflective material occurs as discrete mica flakes or a dark smearing mass. On a loose expendable flake, gently test flexibility. Elastic sheet behaviour supports biotite. Make a light mark on clean paper. An easy black line and greasy residue support graphite. Use streak and host-rock context to confirm, especially when graphite is fine-grained or biotite is altered. Do not peel flakes from a valuable specimen or generate dust from graphite-bearing rock. Fine dark material may be a mixture of graphite, mica, sulfides, or alteration minerals; microscopy may be required. Wash hands after testing unknown mineralized material and keep test residues away from food areas.

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