mineral

Biotite (Black Mica)

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

Hardness: 2.5-3.0 (Mohs scale), Color: Black/Dark Brown, Luster: Vitreous/Pearly, Crystal Structure: Monoclinic, Cleavage: Perfect basal (peels into thin sheets), Specific Gravity: 2.7-3.3

Hardness: 2

Identified June 16, 2026More mineral →

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Physical properties

Hardness: 2.5-3.0 (Mohs scale), Color: Black/Dark Brown, Luster: Vitreous/Pearly, Crystal Structure: Monoclinic, Cleavage: Perfect basal (peels into thin sheets), Specific Gravity: 2.7-3.3

Formation & geological history

Formed during the crystallization of igneous rocks (granite, diorite) or during the regional metamorphism of clay-rich sedimentary rocks.

Uses & applications

Used as an insulating material in electrical devices, a filler in paints and rubber, and occasionally as a marker mineral in geological dating (argon-argon dating).

Geological facts

Biotite is known as 'black mica' and belongs to a phyllosilicate group. It is often referred to as a 'garbage can mineral' because it can contain many different trace elements within its structure.

Field identification & locations

Recognized in the field by its dark color and the ability to flake off thin, flexible, but brittle sheets with a fingernail or pocket knife. Common in granitic terrains worldwide.

Other mineral specimens

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Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Schist

Metamorphic

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