mineral

Biotite on Quartz

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

Hardness: 2.5-3.0 (Mohs). Color: Black, dark brown. Luster: Vitreous to pearly. Crystal Structure: Monoclinic (often tabular or platy). Cleavage: Perfect basal cleavage, allowing it to be peeled into thin sheets. Specific Gravity: 2.7-3.3.

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

Identified June 16, 2026More mineral →

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Physical properties

Formation & geological history

Commonly forms in metamorphic rocks such as schist and gneiss, and in igneous rocks like granite. It forms across a wide range of geological ages in high-temperature, high-pressure environments.

Uses & applications

Used as an insulating material in the electronics and electrical industries due to its thermal stability. It is also used as a filler in paints and as a component in drilling mud. Some specimens are kept by collectors for educational or aesthetic value.

Geological facts

Biotite is often called black mica. It is part of a solid-solution series between phlogopite and annite. It contains significant amounts of iron and magnesium which give it its distinctive dark color unlike transparent muscovite.

Field identification & locations

Identify in the field by its dark color, metallic or pearly luster, and the ability to flake off paper-thin sheets with a fingernail or blade. Often found as small shiny black flecks in granite countertops.

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