igneous
Pegmatitic quartz–feldspar rock with mica
Granitic pegmatite; mainly quartz (SiO₂), feldspar, and biotite/muscovite mica
AI-generated identification · may be incorrectVery coarse-grained, with white to translucent quartz and feldspar plus dark, platy mica; vitreous to pearly luster, uneven fracture, and locally visible cleavage. Quartz hardness is 7, feldspar 6–6.5, and mica 2–3; overall specific gravity is roughly 2.6–2.8.
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Physical properties
Very coarse-grained, with white to translucent quartz and feldspar plus dark, platy mica; vitreous to pearly luster, uneven fracture, and locally visible cleavage. Quartz hardness is 7, feldspar 6–6.5, and mica 2–3; overall specific gravity is roughly 2.6–2.8.
Formation & geological history
Formed when water-rich granitic magma cooled slowly in a late-stage vein or pocket, allowing exceptionally large crystals to grow. Pegmatites are generally Precambrian to Phanerozoic and occur within or near granite bodies.
Uses & applications
Used as a source of feldspar, quartz, mica, and sometimes rare minerals or gemstones; attractive pieces are collected as specimens, though this example appears chiefly ornamental.
Geological facts
The blocky white crystals are likely feldspar, glassy zones are quartz, and shiny dark flakes are biotite mica. Pegmatites can host tourmaline, beryl, garnet, lithium minerals, and other rare elements.
Field identification & locations
Identify it by its unusually coarse interlocking crystals and abundant mica, unlike ordinary granite. Common worldwide in granitic terrains, including Brazil, the United States, Canada, Madagascar, and Scandinavia; a simple glass-scratch test can distinguish quartz from softer feldspar.
