igneous
Quartz–feldspar pegmatite specimen
Granitic pegmatite, chiefly quartz (SiO₂), potassium feldspar (KAlSi₃O₈), and mica-group minerals
AI-generated identification · may be incorrectCoarse interlocking crystals with white to translucent quartz/feldspar and dark, platy mica; vitreous to pearly luster. Quartz hardness is 7, feldspar 6–6.5, mica 2–3; feldspar shows cleavage, quartz lacks cleavage.
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Physical properties
Coarse interlocking crystals with white to translucent quartz/feldspar and dark, platy mica; vitreous to pearly luster. Quartz hardness is 7, feldspar 6–6.5, mica 2–3; feldspar shows cleavage, quartz lacks cleavage.
Formation & geological history
Formed when water-rich granitic magma cooled slowly in veins, allowing exceptionally large crystals to grow, generally during the Precambrian or Phanerozoic. The exact age and locality cannot be determined from the image.
Uses & applications
Collected as a mineral specimen; pegmatites may supply feldspar, quartz, mica, lithium, beryllium, tourmaline, and other gem minerals. Attractive crystals can be used for display, but this rough piece has little construction value.
Geological facts
The pale massive material may be quartz or feldspar, while the dark reflective layers are likely biotite or muscovite mica. Identification would be improved by a hardness, streak, and cleavage test.
Field identification & locations
Common in granitic terrains worldwide, including Brazil, the United States, Madagascar, and Scandinavia. Look for very coarse crystals, mica books, and sharp feldspar cleavage; avoid acid testing unless necessary.
