mineral

Biotite (Black Mica) in Host Rock

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

Hardness: 2.5–3.0; Color: Dark brown to black; Luster: Vitreous to pearly; Crystal Structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); Specific Gravity: 2.7–3.3

Hardness: 2
Color: Dark brown to black
Luster: Vitreous to pearly

Identified June 28, 2026More mineral →

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Physical properties

Hardness: 2.5–3.0; Color: Dark brown to black; Luster: Vitreous to pearly; Crystal Structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); Specific Gravity: 2.7–3.3

Formation & geological history

Forms in igneous rocks like granite and pegmatite, or in metamorphic rocks like schist and gneiss. Most biotite is millions of years old, often dating to the Precambrian or Paleozoic eras depending on the source formation.

Uses & applications

Used primarily for geological research and as an insulator in electrical equipment. In pulverized form, it is used as a filler in paints, rubber, and plastics.

Geological facts

Biotite is known as 'black mica' and is a member of a large mica group. Unlike white mica (muscovite), biotite contains iron and magnesium, which gives it its dark color. Large crystals are often called mica books.

Field identification & locations

Identify by its shiny, black, plate-like appearance and its ability to be flaked off with a fingernail or needle. Found worldwide in mountain ranges with exposed granite or metamorphic bedrock.

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