igneous
Weathered biotite granite or granitic gneiss
Felsic granitoid, chiefly quartz, K-feldspar, plagioclase, and biotite; possible gneissic deformation
AI-generated identification · may be incorrectCoarse, crystalline, brown-gray and strongly weathered, with granular texture and dark mica-rich patches; constituent minerals are mostly Mohs 6–7, with no single overall hardness or cleavage. Luster ranges from vitreous quartz to pearly mica and dull weathered feldspar; specific gravity is typically 2.63–2.75.
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Physical properties
Coarse, crystalline, brown-gray and strongly weathered, with granular texture and dark mica-rich patches; constituent minerals are mostly Mohs 6–7, with no single overall hardness or cleavage. Luster ranges from vitreous quartz to pearly mica and dull weathered feldspar; specific gravity is typically 2.63–2.75.
Formation & geological history
Likely crystallized slowly from silica-rich magma deep in the crust during a Precambrian–Phanerozoic intrusive event, then was uplifted and chemically weathered. If foliation is genuine rather than surface weathering, it may be metamorphosed granite (gneiss).
Uses & applications
Used as dimension stone, aggregate, curbing, monuments, and building material; attractive polished pieces may be used for countertops and architectural cladding.
Geological facts
Rusty brown coloration commonly comes from oxidation of iron-bearing biotite and feldspar. Granite is composed of interlocking crystals, so it generally lacks vesicles and fossils.
Field identification & locations
Identify by visible quartz, pink/gray feldspar, and black mica in an interlocking texture; a fresh broken surface and hand lens would distinguish granite from gneiss. Common worldwide in continental shields and mountain belts.
