metamorphic
Biotite mica schist (probable)
Mica schist, dominated by biotite mica (K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂) with quartz and feldspar
AI-generated identification · may be incorrectDark gray to black, sparkly, foliated, and platy with a micaceous luster; biotite has Mohs hardness 2.5–3 and perfect basal cleavage, while quartz/feldspar are harder (6–7). The apparent thin layering and reflective flakes support schist rather than a massive igneous rock.
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Physical properties
Dark gray to black, sparkly, foliated, and platy with a micaceous luster; biotite has Mohs hardness 2.5–3 and perfect basal cleavage, while quartz/feldspar are harder (6–7). The apparent thin layering and reflective flakes support schist rather than a massive igneous rock.
Formation & geological history
Formed when shale or another clay-rich sediment was subjected to medium-grade regional metamorphism, commonly during mountain building; its exact age cannot be determined from the image and depends on locality.
Uses & applications
Used locally as decorative stone, landscaping material, and teaching or collection specimens; its strong foliation limits structural construction use. Attractive mica-rich pieces have modest collector appeal.
Geological facts
The glitter comes from aligned mica flakes reflecting light, and the rock may split along foliation. It could also be a mica-rich gneiss or phyllite if the texture is less distinctly schistose than it appears.
Field identification & locations
Look for visible aligned mica plates, wavy foliation, and easy splitting; a glass scratch test can separate quartz-rich grains from softer mica. Common in regional metamorphic belts worldwide, including the Appalachians, Alps, and Precambrian shields.
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