mineral

Biotite (Mica)

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

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

Hardness: 2
Color: Dark brown to black, brassy
Luster: Vitreous to pearly

Identified May 7, 2026More mineral →

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Physical properties

Formation & geological history

Forms primarily in igneous rocks such as granite and pegmatite, and in metamorphic rocks like schist and gneiss. It crystallizes from silicate melts or during regional metamorphism.

Uses & applications

Used as an insulating material in electrical devices, as a filler in paints and plastics, and as an additive in oil-well drilling fluids. Also valued by mineral collectors.

Geological facts

Biotite is often referred to as 'Black Mica'. It is a 'trash mineral' in some industries because it interferes with the processing of other minerals, but it is essential for radiometric dating of rocks.

Field identification & locations

Identified in the field by its dark color and the ability to flake off into thin, flexible, transparent sheets using a fingernail or needle. Found worldwide in mountainous terrains.