igneous
Weathered granitic rock with feldspar-rich vein
Granite or granitic pegmatite; predominantly quartz, potassium feldspar, and plagioclase
AI-generated identification · may be incorrectCoarse-grained, pale tan and gray, with a vitreous-to-dull luster and irregular fracture; feldspar hardness ~6, quartz ~7 Mohs. Individual crystals are blocky or granular, but strong weathering obscures boundaries.
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Physical properties
Coarse-grained, pale tan and gray, with a vitreous-to-dull luster and irregular fracture; feldspar hardness ~6, quartz ~7 Mohs. Individual crystals are blocky or granular, but strong weathering obscures boundaries.
Formation & geological history
Formed by slow cooling of silica-rich magma deep in the crust, likely during Precambrian or younger continental plutonism; the tan material may be altered feldspar or a pegmatitic vein. Surface weathering produces the crumbly, stained appearance.
Uses & applications
Used as dimension stone, aggregate, monuments, and countertops when sound; ordinary weathered pieces have little commercial value but may be useful as teaching or reference specimens.
Geological facts
The pale tan regions likely represent feldspar altered to clay minerals, while darker grains may be biotite or amphibole. Iron oxidation commonly creates the brown-yellow staining.
Field identification & locations
Identify by interlocking visible grains, abundant pale feldspar, translucent gray quartz, and occasional dark mica; it is common in continental mountain belts and exposed basement terrains. A fresh broken surface is more diagnostic than the weathered exterior.
