igneous
Biotite granite (likely granitic gneiss)
Granite dominated by quartz, feldspar, and biotite mica; possible gneissic equivalent
AI-generated identification · may be incorrectHardness about 6–7; coarse-grained, mottled gray, white, and black with vitreous quartz, feldspar, and shiny dark mica. No obvious cleavage overall, though biotite has perfect basal cleavage; specific gravity about 2.6–2.8.
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Physical properties
Hardness about 6–7; coarse-grained, mottled gray, white, and black with vitreous quartz, feldspar, and shiny dark mica. No obvious cleavage overall, though biotite has perfect basal cleavage; specific gravity about 2.6–2.8.
Formation & geological history
Slow cooling of silica-rich magma deep underground produced interlocking crystals, commonly during Precambrian to Phanerozoic continental crust formation. If foliated, it may have been recrystallized into gneiss during regional metamorphism.
Uses & applications
Commonly used as dimension stone, countertops, paving, aggregate, and monuments; attractive hand specimens have modest collector value.
Geological facts
The dark reflective flakes are likely biotite, while translucent gray grains are quartz and pale grains are feldspar. Grain size indicates slow cooling rather than volcanic eruption.
Field identification & locations
Identify it by its coarse interlocking texture, abundant quartz/feldspar, and black mica; unlike basalt, it is light-colored and visibly crystalline. Common worldwide in continental shields and mountain belts.
