metamorphic
Biotite mica schist fragment
Biotite-rich mica schist; chiefly biotite K(Mg,Fe)3AlSi3O10(F,OH)2 with quartz and feldspar
AI-generated identification · may be incorrectMohs hardness 2.5–3 for biotite; dark brown to black, with bright bronze-gold reflections and a vitreous to pearly luster. Platy crystals show perfect basal cleavage, separating into thin flexible sheets; composition and texture appear schistose.
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Physical properties
Mohs hardness 2.5–3 for biotite; dark brown to black, with bright bronze-gold reflections and a vitreous to pearly luster. Platy crystals show perfect basal cleavage, separating into thin flexible sheets; composition and texture appear schistose.
Formation & geological history
Formed when clay-rich sedimentary rock underwent medium-grade regional metamorphism, typically during mountain building; its exact geological age cannot be determined from the image. Commonly associated with quartz, feldspar, garnet, and other mica minerals.
Uses & applications
Used locally as decorative stone, aggregate, or a collectible specimen; mica has industrial uses in insulation, fillers, paints, and cosmetics, though this rough fragment has little commercial value.
Geological facts
The glittering flakes are biotite mica reflecting light from cleavage surfaces; weathering can alter biotite to chlorite or iron oxides, producing rusty colors.
Field identification & locations
Identify by its platy, peelable sheets, strong cleavage, dark bronze sheen, and foliated texture; common in metamorphic belts worldwide, including the Appalachians, New England, Scandinavia, and the Rocky Mountains.
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