mineral

Biotite (Black Mica)

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

Hardness: 2.5–3.0 on Mohs scale; Color: Black, dark brown, or dark green; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal cleavage (splits into thin, flexible sheets); Specific gravity: 2.7–3.3

Hardness: 2
Color: Black, dark brown, or dark green
Luster: Vitreous to pearly

Identified January 25, 2026More mineral →

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Physical properties

Formation & geological history

Formed in a wide range of geological environments, including igneous rocks (like granite and diorite) and metamorphic rocks (like schist and gneiss). It crystallizes from magma or forms through metamorphism of clay-rich sedimentary rocks.

Uses & applications

Used as an insulating material in electrical equipment, as a filler in paints and rubber, and occasionally in the production of drilling mud for the oil industry. It is also highly valued by mineral collectors for its unique platelet structure.

Geological facts

Biotite is often referred to as 'black mica' and is a member of the mica group. It is named after the French physicist Jean-Baptiste Biot, who researched the optical properties of micas in the early 19th century.

Field identification & locations

Identify in the field by its dark color and the way it peels into thin, elastic sheets with a fingernail or knife. It is commonly found in mountainous regions or areas with exposed granite and metamorphic bedrock. Collectors should look for its characteristic 'books' or stacks of plates.

Other mineral specimens

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Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Epidote

Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)

metamorphic

Nephrite Jade

Nephrite

Mineral/Rock

Banded Gneiss (River Rock/Pebble)

Gneiss (specifically a banded variety often found as a river rock)

Metamorphic Rock

Green Apatite on Albite

Fluorapatite [Ca5(PO4)3F] on Albite [Na(AlSi3O8)]

mineral

Epidote

Epidote - Ca2(Al2,Fe3+)3(SiO4)3(OH)

mineral