igneous
Biotite granite
Granite; quartz–alkali feldspar–plagioclase rock with biotite mica
AI-generated identification · may be incorrectCoarse-grained, mottled tan, gray, black, and cream; interlocking crystals with vitreous quartz, feldspar, and dark biotite. Mohs hardness is approximately 6–7; no consistent cleavage as a whole, though feldspar and mica grains cleave; specific gravity about 2.63–2.75.
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Physical properties
Coarse-grained, mottled tan, gray, black, and cream; interlocking crystals with vitreous quartz, feldspar, and dark biotite. Mohs hardness is approximately 6–7; no consistent cleavage as a whole, though feldspar and mica grains cleave; specific gravity about 2.63–2.75.
Formation & geological history
Forms when silica-rich magma slowly crystallizes deep within the continental crust, producing visible crystals; individual specimens may range from Precambrian to Mesozoic or younger depending on locality.
Uses & applications
Widely used as dimension stone, countertops, monuments, flooring, aggregate, and crushed stone; attractive polished pieces are collected but are generally low value.
Geological facts
The black flakes are likely biotite, while pale grains are feldspar and translucent grains may be quartz. Its coarse texture distinguishes it from volcanic rhyolite, which cools too quickly to form large crystals.
Field identification & locations
Identify it by its visible interlocking grains and mottled appearance; a steel knife usually scratches some feldspar only weakly, while quartz resists scratching. Common worldwide in continental plutons, mountain belts, and granitic shield regions.
