mineral

Biotite Mica

Biotite (K(Mg,Fe)3AlSi3O10(F,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 (peels into thin, flexible sheets); Specific Gravity: 2.8–3.4.

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

Identified February 10, 2026More mineral →

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Physical properties

Formation & geological history

Formed in a wide range of geological environments, including regional metamorphic rocks (schists and gneisses) and igneous rocks like granite. It forms through the crystallization of magma or the metamorphism of clay-rich sedimentary rocks.

Uses & applications

Used as a filler in paints, as an additive to drilling muds, in the manufacture of rubber and plastic, and in electronics as an insulator (though Muscovite is more common for this). In jewelry, it is occasionally used as 'books' or inclusions in quartz.

Geological facts

Biotite is often referred to as 'black mica.' It is an indicator mineral for geologists to determine the temperature and pressure conditions under which a rock formed. It is also used in potassium-argon dating to determine the age of rocks.

Field identification & locations

Identify in the field by its dark color and the ability to peel off thin, elastic flakes with a fingernail or needle. It is commonly found in granitic terrains and metamorphic belts globally.