mineral

Biotite (Black Mica)

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

Hardness: 2.5-3.0; Color: Black, dark green, or dark brown; Luster: Pearly to vitreous; Crystal Structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); SG: 2.7-3.3.

Hardness: 2
Color: Black, dark green, or dark brown
Luster: Pearly to vitreous

Identified April 9, 2026More mineral →

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Physical properties

Hardness: 2.5-3.0; Color: Black, dark green, or dark brown; Luster: Pearly to vitreous; Crystal Structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); SG: 2.7-3.3.

Formation & geological history

Forms primarily in igneous rocks such as granite and pegmatites, and metamorphic rocks like schist and gneiss. It is a common rock-forming mineral crystallizing from cooling magma.

Uses & applications

Used as an insulating material in the electronics industry (though muscovite is preferred), as a filler in paints and rubber, and in specialized optical equipment.

Geological facts

Unlike muscovite, which is clear or silver, biotite is dark due to high iron and magnesium content. It is often nicknamed 'iron mica' and can weather into 'fool's gold' under certain conditions.

Field identification & locations

Identified by its dark color and the ability to peel off paper-thin, flexible sheets from the specimen. Found globally in granitic terrains.

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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)]

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Epidote

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

mineral