igneous
Pegmatitic quartz–feldspar rock with dark mica
Granitic pegmatite; quartz (SiO₂), feldspar, and biotite mica
AI-generated identification · may be incorrectCoarse, visibly interlocking crystals; white to pink feldspar and glassy gray quartz with black/brown platy mica. Quartz hardness 7, feldspar 6–6.5, biotite 2.5–3; generally nonmetallic luster, uneven fracture, and feldspar cleavage.
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Physical properties
Coarse, visibly interlocking crystals; white to pink feldspar and glassy gray quartz with black/brown platy mica. Quartz hardness 7, feldspar 6–6.5, biotite 2.5–3; generally nonmetallic luster, uneven fracture, and feldspar cleavage.
Formation & geological history
Formed when water-rich granitic magma cooled slowly in veins or pockets, allowing exceptionally large crystals to grow. Pegmatites are commonly Precambrian to younger and occur near granite intrusions.
Uses & applications
Used as an ornamental and decorative stone, source of quartz and feldspar for glass and ceramics, and as a collector specimen; associated pegmatites may contain tourmaline, beryl, or rare-element minerals.
Geological facts
Large crystals and graphic quartz–feldspar textures are characteristic of pegmatites. The dark flakes are likely biotite, though amphibole or another dark silicate is possible from the photograph.
Field identification & locations
Identify by the unusually coarse grain size, interlocking quartz–feldspar masses, and flaky dark mica; common in granitic terrains worldwide, including Maine, California, Brazil, Madagascar, and Scandinavia. A fresh surface and streak, hardness, and cleavage tests improve identification.
