mineral

Biotite on Quartz Matrix

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

Hardness: 2.5-3 (Biotite), 7 (Quartz). Color: Black to dark brown with white/clear matrix. Luster: Vitreous to pearly. Crystal structure: Monoclinic (sheets). Cleavage: Perfect basal cleavage producing thin, flexible flakes.

Hardness: 2
Color: Black to dark brown with white/clear matrix
Luster: Vitreous to pearly

Identified June 21, 2026More mineral →

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Physical properties

Formation & geological history

Formed in igneous or metamorphic environments, primarily within granitic pegmatites or schists through the cooling of silica-rich magma or regional metamorphism of clay-rich rocks.

Uses & applications

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

Geological facts

Biotite is often called 'Black Mica.' It is a member of the phyllosilicate group and is significant to geologists for K-Ar (Potassium-Argon) dating to determine the age of host rocks.

Field identification & locations

Identify by its shiny black color and 'book-like' layers that can be peeled into thin, flexible sheets. Commonly found in mountainous regions with granitic outcrops.

Other mineral specimens

See all

Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Schist

Metamorphic

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