mineral

Biotite / Black Mica

Biotite K(Mg,Fe)3AlSi3O10(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; Cleavage: Perfect basal (peels into thin 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 conditions; commonly found in igneous rocks like granite and pegmatites, as well as metamorphic rocks like schist and gneiss. It can also form through hydrothermal processes.

Uses & applications

Used historically as an insulator in electrical equipment and as a filler in paints and rubber. Its main value today is in geological research and as a collector's specimen.

Geological facts

Biotite is known as 'black mica' and is part of the mica group. It is unusual because it can contain radioactive elements like potassium-40, making it useful for dating the age of rocks through potassium-argon dating.

Field identification & locations

Identify in the field by its dark color and the way it flakes into flexible, elastic thin sheets (mica books). It is commonly found in mountainous regions globally, particularly in pegmatite veins.

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