Biotite (Black Mica)

Physical Properties

Hardness: 2.5–3.0 on Mohs scale; Color: Black, dark brown, or greenish black; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal (peels into thin, flexible sheets); Specific Gravity: 2.7–3.3.

Formation & Geological History

Formed through the cooling of molten rock (igneous) like granite and pegmatite, or through the metamorphosis of sedimentary rocks like shale into schist and gneiss. It is a common rock-forming mineral found across various geological ages.

Uses & Applications

Used primarily as an insulator in the electrical industry, as a filler in paints and plastics, and as an additive in drilling muds. It is also highly valued as an educational specimen for studying cleavage.

Geological Facts

Biotite is often called 'Black Mica' to distinguish it from its lighter cousin, Muscovite. While it can be peeled into thin sheets, unlike Muscovite, Biotite is rarely used for windows because the high iron content makes it opaque.

Field Identification & Locations

Identify in the field by its dark color and 'book-like' appearance; if you can peel a thin, flexible, translucent sheet off with a fingernail or pocketknife, it is likely mica. Common in the Appalachian Mountains, the Rockies, and Scandinavian shield areas.

Identified on: 4/24/2026

Mode: Standard