igneous
Biotite-rich granite or granitic gneiss
Granite; quartz–feldspar–biotite assemblage
AI-generated identification · may be incorrectCoarse-grained, tan-gray rock with visible quartz, feldspar, and dark biotite; granular texture, vitreous quartz luster, Mohs hardness about 6–7, poor cleavage overall, and specific gravity roughly 2.6–2.7. The irregular foliation-like surface may indicate weak gneissic deformation or weathering.
Identified More igneous →
Explore Biotite-rich granite or granitic gneiss in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Coarse-grained, tan-gray rock with visible quartz, feldspar, and dark biotite; granular texture, vitreous quartz luster, Mohs hardness about 6–7, poor cleavage overall, and specific gravity roughly 2.6–2.7. The irregular foliation-like surface may indicate weak gneissic deformation or weathering.
Formation & geological history
Likely crystallized slowly from silica-rich magma deep in the continental crust, commonly during Precambrian to Phanerozoic plutonic events; if gneissic, it was later subjected to high-temperature regional metamorphism.
Uses & applications
Used as dimension stone, aggregate, paving, monuments, and building material; attractive polished pieces may be sold as decorative stone or geological specimens.
Geological facts
Quartz appears glassy and gray, feldspar is pale and blocky, and biotite forms dark reflective flakes. The specimen’s strongly weathered, rounded surface can obscure its original texture.
Field identification & locations
Identify it by interlocking visible grains, abundant pale feldspar, glassy quartz, and black mica; it should not fizz with dilute hydrochloric acid. Common worldwide in continental shields, mountain belts, and granite plutons.
