igneous
Quartz–feldspar–biotite pegmatite
Granitic pegmatite; chiefly quartz (SiO₂), feldspar, and biotite mica
AI-generated identification · may be incorrectCoarse, interlocking crystals with glassy to pearly luster; white/translucent quartz and feldspar contrast with black, platy biotite. Typical hardness is 6–7 for quartz, about 6 for feldspar, and 2.5–3 for biotite; feldspar shows cleavage, quartz generally does not.
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Physical properties
Coarse, interlocking crystals with glassy to pearly luster; white/translucent quartz and feldspar contrast with black, platy biotite. Typical hardness is 6–7 for quartz, about 6 for feldspar, and 2.5–3 for biotite; feldspar shows cleavage, quartz generally does not.
Formation & geological history
A very slowly cooled, water-rich granitic melt formed large crystals in a late-stage igneous vein or dike, commonly during Precambrian to Phanerozoic mountain-building events.
Uses & applications
Used mainly as a collector specimen; pegmatites can provide quartz, feldspar, mica, lithium minerals, and gem materials, while related granites are used for aggregate and dimension stone.
Geological facts
The dramatic crystal size results from volatile-rich melt conditions rather than simply exceptionally long cooling. Dark bands are likely biotite or other mafic minerals; exact identification requires testing.
Field identification & locations
Common in continental crust, especially Maine, New Hampshire, California, Brazil, Madagascar, and Scandinavia. Look for unusually large interlocking crystals and test hardness, cleavage, and streak to distinguish quartz from feldspar.
