igneous
Biotite-bearing granite or granitic gneiss
Quartz–feldspar–biotite rock; commonly granite (quartz, K-feldspar, plagioclase, biotite)
AI-generated identification · may be incorrectLight gray, coarse granular matrix with conspicuous black, platy biotite; quartz is glassy and feldspar white. Typical hardness is 6–7 Mohs, nonmetallic luster, interlocking crystals, and weak or absent cleavage overall; the dark fragment shows biotite cleavage.
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Physical properties
Light gray, coarse granular matrix with conspicuous black, platy biotite; quartz is glassy and feldspar white. Typical hardness is 6–7 Mohs, nonmetallic luster, interlocking crystals, and weak or absent cleavage overall; the dark fragment shows biotite cleavage.
Formation & geological history
Crystallized slowly from silica-rich magma deep in the continental crust, generally during Precambrian to Phanerozoic plutonic events; the apparent layering may reflect weathering or gneissic fabric. The image may show a weathered surface or rock aggregate containing a dark biotite-rich fragment.
Uses & applications
Granite is widely used for building stone, countertops, monuments, paving, and crushed aggregate; attractive pieces are collected, though this specimen has little jewelry value.
Geological facts
Biotite commonly gives granite its black flecks and can weather to vermiculite or chlorite. A hand lens should reveal distinct quartz, feldspar, and shiny mica flakes.
Field identification & locations
Identify it by coarse interlocking white, gray, and black grains; a scratch test should show quartz and feldspar resisting steel. Common worldwide in continental batholiths, especially India, Canada, Scandinavia, and the United States.
