metamorphic
Biotite-bearing gneiss (probable)
Foliated quartz–feldspar gneiss with biotite mica
AI-generated identification · may be incorrectDark gray, coarse-grained, irregularly foliated rock with contrasting pale mineral bands and reflective dark mica. Typical hardness is about 6–7 Mohs; no obvious cleavage at the specimen scale, though biotite has perfect sheet cleavage; luster is dull to vitreous.
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Physical properties
Dark gray, coarse-grained, irregularly foliated rock with contrasting pale mineral bands and reflective dark mica. Typical hardness is about 6–7 Mohs; no obvious cleavage at the specimen scale, though biotite has perfect sheet cleavage; luster is dull to vitreous.
Formation & geological history
Formed when granite or another felsic protolith underwent intense heat and pressure, recrystallizing and segregating minerals into bands. Such rocks commonly formed during Precambrian to Paleozoic mountain-building events, though age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, building stone, decorative slabs, and dimension stone; attractive pieces are collected as representative metamorphic specimens.
Geological facts
The banding reflects mineral segregation during deformation rather than simple sedimentary layering. Visible mica flakes can produce a subtle sparkle when the specimen is rotated.
Field identification & locations
Look for alternating light quartz/feldspar and dark biotite-rich bands, coarse interlocking grains, and mica sheen; common in ancient continental shields and metamorphic mountain belts. A hand lens and fresh broken surface would improve identification.
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