mineral

Biotite (Mica)

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

Hardness: 2.5-3.0 Mohs; Color: Black, dark brown, or dark green; Luster: Pearly to vitreous; Crystal Structure: Monoclinic (pseudo-hexagonal); Cleavage: Perfect basal (peels into thin sheets); Specific Gravity: 2.7-3.3

Hardness: 2
Color: Black, dark brown, or dark green
Luster: Pearly to vitreous

Identified March 2, 2026More mineral →

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Physical properties

Formation & geological history

Forms in a wide range of geological environments, primarily igneous rocks like granite and metamorphic rocks like schist and gneiss. It is a common rock-forming mineral formed through the cooling of magma or the metamorphism of clay-rich sedimentary rocks.

Uses & applications

Used as an insulating material in the electrical industry (due to heat and electrical resistance), as a filler in paints and rubber, and as an additive in drilling mud. It is also highly valued as a geological specimen for students and collectors.

Geological facts

Biotite is often referred to as 'black mica' and is part of the phyllosilicate mineral group. It is a key index mineral used by geologists to determine the temperature and pressure history of metamorphic rocks. Unlike muscovite, it is rarely transparent when in thick books.

Field identification & locations

Identify in the field by its dark color and 'books' of thin, flexible, elastic sheets that can be peeled away with a fingernail. It is common in the Appalachian Mountains, Scandinavia, and Canada. Collectors look for large, intact 'books' or crystals showing well-defined hexagonal outlines.

Other mineral specimens

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Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

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

Epidote

Epidote - Ca2(Al2,Fe3+)3(SiO4)3(OH)

mineral