mineral

Biotite Mica in Quartz

Biotite (K(Mg,Fe)3AlSi3O10(F,OH)2) in Quartz (SiO2)

Hardness: 2.5-3 (Biotite), 7 (Quartz); Color: Dark brown to black (Biotite), milky white (Quartz); Luster: Vitreous to pearly; Crystal Structure: Monoclinic sheets; Cleavage: Perfect basal cleavage (flakes easily).

Hardness: 2
Color: Dark brown to black (Biotite), milky white (Quartz)
Luster: Vitreous to pearly

Identified February 27, 2026More mineral →

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Physical properties

Formation & geological history

Mainly formed in igneous rocks such as granite or pegmatites and metamorphic rocks like schist and gneiss during the cooling of magma or under high pressure and temperature conditions.

Uses & applications

Biotite is used as a filler in paints, as an additive in drilling muds, and for potassium-argon dating in geology. Quartz is widely used in construction and electronics. Small specimens are primarily for education and beginner collections.

Geological facts

Biotite is often called black mica. It is distinguished from other micas by its dark color and the presence of iron and magnesium. Individual sheets are flexible but will eventually break if bent too far.

Field identification & locations

Identify by the distinct 'books' of dark, flaky layers that can be peeled with a fingernail, embedded in a hard, glassy white matrix. Commonly found in mountainous regions or glacial till.

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Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

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Epidote

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Nephrite Jade

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Mineral/Rock

Banded Gneiss (River Rock/Pebble)

Gneiss (specifically a banded variety often found as a river rock)

Metamorphic Rock

Green Apatite on Albite

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

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