mineral

Biotite mica

Biotite, a potassium-rich mica in the annite–phlogopite series; approximate formula K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂

AI-generated identification · may be incorrect

Dark brown, platy specimen with mica’s sheet-like cleavage and vitreous to pearly luster; Mohs hardness 2.5–3, monoclinic crystal structure. Identification from this image is uncertain because the specimen is small and partly obscured.

Identified October 4, 2026More mineral →

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Physical properties

Formation & geological history

Forms in igneous and metamorphic rocks as magma cools or during metamorphism; found in rocks spanning much of Earth’s geological history.

Uses & applications

Used as an indicator mineral in geology and occasionally in educational collections; mica has broader industrial uses, though biotite is less commonly used than lighter-colored mica.

Geological facts

Mica splits into thin, flexible sheets because its crystal structure contains strongly bonded layers with weaker bonds between them.

Field identification & locations

Look for dark brown-to-black flakes that cleave into thin sheets; common in granite, schist, and gneiss worldwide. A small loose specimen typically sells for about $1–$10.