mineral

Biotite on Matrix

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

Hardness: 2.5-3.0 Mohs; Color: Dark brown to black (biotite) on tan/white matrix; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal cleavage (easily peels into thin sheets).

Hardness: 2
Color: Dark brown to black (biotite) on tan/white matrix
Luster: Vitreous to pearly

Identified April 6, 2026More mineral →

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Physical properties

Formation & geological history

Forms in igneous and metamorphic rocks under a wide range of temperature and pressure conditions. It is common in granites, pegmatites, and schists.

Uses & applications

Used as an insulating material in electrical equipment, as a ground filler in paints and rubber, and as a popular specimen for mineral collectors.

Geological facts

Biotite is often referred to as 'Black Mica' to distinguish it from White Mica (Muscovite). It is a major constituent of the Earth's crust and helps geologists determine the age of rocks through argon-argon or potassium-argon dating.

Field identification & locations

Identify in the field by its dark color and the way it flakes into flexible, elastic thin sheets (books). It is commonly found in granitic outcrops and metamorphic belts worldwide.

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