metamorphic
Biotite-rich schist
Mica schist; predominantly biotite mica (K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂), commonly with quartz and feldspar
AI-generated identification · may be incorrectTypically dark brown to black, silver-bronze, and strongly sparkly, with a foliated, flaky texture. Biotite has Mohs hardness 2.5–3, perfect basal cleavage, and vitreous to pearly luster; schist overall is variable and anisotropic.
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Physical properties
Typically dark brown to black, silver-bronze, and strongly sparkly, with a foliated, flaky texture. Biotite has Mohs hardness 2.5–3, perfect basal cleavage, and vitreous to pearly luster; schist overall is variable and anisotropic.
Formation & geological history
Formed when shale or other clay-rich sediment is regionally metamorphosed under elevated pressure and temperature, aligning mica into schistosity. Most examples formed during ancient mountain-building events, commonly hundreds of millions to over a billion years ago.
Uses & applications
Used locally as decorative stone, landscaping material, wall facing, and aggregate, though its pronounced foliation limits structural use. Attractive pieces are collected but are rarely used as jewelry because mica flakes readily.
Geological facts
The glitter comes from reflective cleavage surfaces of abundant mica rather than metallic metal. Foliation causes the rock to split along parallel, mica-rich planes.
Field identification & locations
Identify it by abundant aligned, peelable mica flakes, strong foliation, and glittering surfaces; it is commonly found in regional metamorphic belts worldwide, including New England, the Appalachians, India, Brazil, and Scandinavia.
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