mineral

Biotite (Black Mica)

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

Hardness: 2.5 - 3.0 on Mohs scale; Color: Black, dark green, or dark brown; Luster: Vitreous to pearly; Crystal structure: Monoclinic (tabular/platy); Cleavage: Perfect basal (peels in thin flexible sheets); Specific gravity: 2.7 - 3.3.

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

Identified April 8, 2026More mineral →

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Physical properties

Formation & geological history

Formed in a wide range of geological environments, primarily in igneous rocks like granite and metamorphic rocks like schist and gneiss. It crystallizes from magma or forms under high heat and pressure during metamorphism.

Uses & applications

Used as a filler in paints, as an additive in drilling muds, in some electrical insulators, and primarily as a geological indicator for age dating (argon-argon or potassium-argon dating).

Geological facts

Biotite is often called 'black mica' and is part of a large mineral group. It is very common in the Earth's crust and is unique because it can be used to determine the age of the rock it resides in through isotopic analysis.

Field identification & locations

Identify in the field by its dark color and 'books' of thin, flaky sheets that can be peeled off with a fingernail. It is common in mountain ranges and near riverbeds containing granitic sand.

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