igneous
Biotite granite (weathered granitic rock)
Granite; predominantly quartz (SiO₂), feldspar, and biotite mica
AI-generated identification · may be incorrectCoarse-grained, speckled gray, cream, and black; vitreous quartz, blocky feldspar, and platy dark mica are visible. Hardness is about 6–7 overall, with irregular fracture, little cleavage as a rock, and specific gravity roughly 2.6–2.8.
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Physical properties
Coarse-grained, speckled gray, cream, and black; vitreous quartz, blocky feldspar, and platy dark mica are visible. Hardness is about 6–7 overall, with irregular fracture, little cleavage as a rock, and specific gravity roughly 2.6–2.8.
Formation & geological history
A plutonic igneous rock formed when silica-rich magma cooled slowly deep underground, allowing large crystals to grow. Its exact geological age cannot be determined from the image; granites range from Archean to modern geologic ages.
Uses & applications
Widely used as dimension stone, countertops, monuments, paving, aggregate, and building material; attractive polished varieties are also used decoratively.
Geological facts
The contrasting black grains are likely biotite or hornblende, while translucent grains are quartz and pale grains are feldspar. Weathering commonly produces rusty-brown staining from iron-bearing minerals.
Field identification & locations
Identify it by its coarse interlocking crystals and absence of layering or fossils; a glassy quartz grain, feldspar cleavage, and dark mica flakes are diagnostic. Common worldwide in continental crust and exposed plutons.
