igneous
Biotite-bearing granite/pegmatite specimen
Granite or granitic pegmatite, chiefly quartz (SiO₂), feldspar, and biotite mica
AI-generated identification · may be incorrectCoarse-grained, irregular aggregate with reflective black-brown platy mica, pale translucent quartz, and likely feldspar. Quartz has Mohs hardness 7 and vitreous luster; feldspar is ~6–6.5 with two cleavages; biotite is ~2.5–3 and strongly cleavable. Exact identification is limited without a larger view or testing.
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Physical properties
Coarse-grained, irregular aggregate with reflective black-brown platy mica, pale translucent quartz, and likely feldspar. Quartz has Mohs hardness 7 and vitreous luster; feldspar is ~6–6.5 with two cleavages; biotite is ~2.5–3 and strongly cleavable. Exact identification is limited without a larger view or testing.
Formation & geological history
Slow cooling of silica-rich magma deep in the continental crust, allowing large interlocking crystals to grow; most such rocks formed in Precambrian to Phanerozoic continental intrusions. If unusually coarse, it may be pegmatitic.
Uses & applications
Used as decorative stone, aggregate, and building material; attractive coarse specimens are collected, though this small piece has little commercial value.
Geological facts
The dark reflective flakes are probably biotite, whose perfect basal cleavage produces thin elastic sheets. Pink or gray areas commonly represent potassium feldspar, while glassy areas are quartz.
Field identification & locations
Identify in the field by its hard, nonlayered interlocking grains, glassy quartz, blocky feldspar cleavage, and flaky black mica. Common in granitic terrains worldwide, including North America, Europe, India, and Brazil.
