metamorphic

Biotite Schist

Biotite Gneissic Schist (K(Mg,Fe)3AlSi3O10(OH)2)

AI-generated identification · may be incorrect

Hardness: 2.5-3 (on mica flakes), Color: Dark brown to black with silvery sheen, Luster: Pearly to submetallic, Texture: Foliated with visible flaky mica grains

Hardness: 2

Identified July 14, 2026More metamorphic →

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Physical properties

Hardness: 2.5-3 (on mica flakes), Color: Dark brown to black with silvery sheen, Luster: Pearly to submetallic, Texture: Foliated with visible flaky mica grains

Formation & geological history

Formed through the regional metamorphism of shale or mudstone under high temperature and pressure, typically during mountain-building events. Geological age varies by location, often Proterozoic to Paleozoic.

Uses & applications

Primarily used as an indicator of metamorphic grade in geology. Historically used as an insulator and in ground form as a lubricant or filler in shingles and paints.

Geological facts

Biotite is often called black mica. The shiny flakes are actually very thin sheets that can be peeled away. It is a common constituent of many igneous and metamorphic rocks like granite and gneiss.

Field identification & locations

Identify by its sparkly, flaky appearance and dark color. It can be found in mountain ranges and areas with exposed crystalline basement rock. Collectors look for large, well-defined books of mica flakes.