igneous
Pegmatitic quartz–feldspar–mica rock
Granitic pegmatite, composed mainly of quartz (SiO₂), feldspar, and black mica (biotite); possible accessory tourmaline or schorl
AI-generated identification · may be incorrectCoarse, interlocking crystals with translucent gray to tan quartz, pale feldspar, and platy black mica. Typical hardness is about 6–7 Mohs; vitreous quartz luster, feldspar cleavage, and uneven granular fracture are expected.
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Physical properties
Coarse, interlocking crystals with translucent gray to tan quartz, pale feldspar, and platy black mica. Typical hardness is about 6–7 Mohs; vitreous quartz luster, feldspar cleavage, and uneven granular fracture are expected.
Formation & geological history
Formed when water-rich granitic magma cooled slowly in a late-stage intrusive vein, allowing exceptionally large crystals to grow. Most pegmatites are Precambrian to Mesozoic, though age depends on locality.
Uses & applications
Used as decorative stone and a collector specimen; pegmatites are important sources of quartz, feldspar, mica, lithium minerals, beryl, and tourmaline, though this small specimen has mainly ornamental value.
Geological facts
The large crystals and contrasting black mica are characteristic of pegmatites. Exact identification requires a fresh surface, hardness testing, and examination for diagnostic accessory minerals.
Field identification & locations
Common in granitic terrains worldwide, including Brazil, Madagascar, the United States, and Scandinavia. Quartz scratches glass, feldspar shows two cleavage planes, and mica splits into thin elastic sheets; acid testing is generally unnecessary.
