mineral

Biotite (Mica)

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

Hardness: 2.5-3.0 Mohs; Color: Dark brown to 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 black
Luster: Vitreous to pearly

Identified May 8, 2026More mineral →

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Physical properties

Hardness: 2.5-3.0 Mohs; Color: Dark brown to 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 primarily in igneous rocks such as granite and diorite, and metamorphic rocks such as schist and gneiss. It crystallizes from silicate melts or during the metamorphism of clay-rich sediments.

Uses & applications

Used in insulation, as an additive in drilling mud, as a filler in paints and rubber, and as a component in construction materials like plasterboard.

Geological facts

Biotite was named in honor of Jean-Baptiste Biot, a French physicist who researched the optical properties of the mica group. It is sometimes called 'black mica' and is part of the phyllosilicate mineral group.

Field identification & locations

Identified in the field by its dark color, perfect cleavage that allows it to be peeled into flexible, tiny sheets with a fingernail, and its metallic-like shine when hit by light. Found globally in continental crust.