mineral

Biotite (Black Mica)

Biotite - K(Mg,Fe)3(AlSi3O10)(F,OH)2

AI-generated identification · may be incorrect

Hardness: 2.5-3.0 (Mohs); Color: Dark brown to greenish black; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); Specific Gravity: 2.7-3.3

Hardness: 2
Color: Dark brown to greenish black
Luster: Vitreous to pearly

Identified July 12, 2026More mineral →

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Physical properties

Hardness: 2.5-3.0 (Mohs); Color: Dark brown to greenish black; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); Specific Gravity: 2.7-3.3

Formation & geological history

Formed in igneous and metamorphic environments, specifically in granite, diorite, and schists. It is a common rock-forming mineral found in the Earth's crust.

Uses & applications

Used as an insulating material in electrical equipment, as a filler in paints and plastics, and as an additive in oil-well drilling muds. It is also significant for geological dating (Potassium-Argon dating).

Geological facts

Biotite is known as 'black mica' and is part of the mica group. Its distinctive perfect cleavage allows it to be split into extremely thin, flexible, and elastic sheets.

Field identification & locations

Identified in the field by its dark color and the ability to flake off thin, flexible sheets with a fingernail or pocketknife. Common in mountainous regions with exposed granite.