mineral

Biotite Mica in Host Rock

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

Hardness: 2.5-3 (Mohs scale); Color: Black to dark brown; Luster: Vitreous/Pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); Specific gravity: 2.7-3.3

Hardness: 2
Color: Black to dark brown
Luster: Vitreous/Pearly

Identified March 7, 2026More mineral →

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Physical properties

Hardness: 2.5-3 (Mohs scale); Color: Black to dark brown; Luster: Vitreous/Pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); Specific gravity: 2.7-3.3

Formation & geological history

Formed in high-temperature environments, typically within igneous rocks like granite or metamorphic rocks like schist and gneiss. It is a common rock-forming mineral found globally in various geological age deposits.

Uses & applications

Used as an insulating material in the electronics industry due to its heat resistance and non-conductivity. Ground mica is used in paint, drywall joint compound, and drilling mud.

Geological facts

Biotite is often called 'Black Mica.' The thin, flexible sheets it produces can be peeled away with a fingernail. It is an important index mineral for determining the metamorphic grade of rocks.

Field identification & locations

Identify in the field by looking for shiny, black, 'plate-like' crystals that can be flaked off. Common in mountainous regions with granitic outcrops or metamorphic belts. Use a pocketknife or fingernail to test for soft, flexible cleavage.

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