igneous
Porphyritic granite
Granite; chiefly quartz (SiO₂), feldspar, and mica/amphibole
AI-generated identification · may be incorrectCoarse-grained, dark gray to brown rock with conspicuous pale feldspar/quartz crystals and possible biotite; Mohs hardness about 6–7, vitreous to dull luster, interlocking crystalline texture, with no regular cleavage as a whole.
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Physical properties
Coarse-grained, dark gray to brown rock with conspicuous pale feldspar/quartz crystals and possible biotite; Mohs hardness about 6–7, vitreous to dull luster, interlocking crystalline texture, with no regular cleavage as a whole.
Formation & geological history
An intrusive igneous rock formed by slow cooling of silica-rich magma deep in the crust, commonly during Precambrian to Phanerozoic mountain-building events; the specimen may also show later weathering or veining.
Uses & applications
Used extensively as dimension stone, countertops, monuments, aggregate, and building material; attractive polished pieces are collected but usually have modest value.
Geological facts
The contrasting large crystals indicate porphyritic growth, where some minerals crystallized before the remaining magma cooled. Dark patches may represent biotite, hornblende, or mafic inclusions rather than a separate rock type.
Field identification & locations
Identify it by its visible interlocking crystals, pale feldspar/quartz, and dark mica or amphibole; a steel file generally fails to scratch fresh surfaces. Common worldwide in continental crust, especially mountain belts and shield regions.
