igneous
Quartz-rich pegmatite (likely quartz–feldspar–mica specimen)
Granitic pegmatite; chiefly quartz (SiO₂), feldspar, and mica
AI-generated identification · may be incorrectCoarse interlocking crystals with white to translucent quartz, cream feldspar, and dark bronze-brown mica or iron staining; vitreous to pearly luster. Quartz hardness is 7 Mohs, feldspar 6–6.5, and mica 2–3; feldspar and mica show cleavage, while quartz generally lacks cleavage.
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Physical properties
Coarse interlocking crystals with white to translucent quartz, cream feldspar, and dark bronze-brown mica or iron staining; vitreous to pearly luster. Quartz hardness is 7 Mohs, feldspar 6–6.5, and mica 2–3; feldspar and mica show cleavage, while quartz generally lacks cleavage.
Formation & geological history
Formed when water-rich granitic magma cooled very slowly in late-stage veins, allowing unusually large crystals to grow, typically during Precambrian to Phanerozoic mountain-building events.
Uses & applications
Used mainly as a teaching, decorative, or collecting specimen; pegmatites can also supply feldspar, quartz, mica, lithium, beryl, and tourmaline for ceramics, glass, electronics, and gemstones.
Geological facts
The image shows a rough, heterogeneous aggregate rather than a single mineral; brown coloration may be weathered biotite or iron oxides. Individual crystals and mineral boundaries are more diagnostic than overall color.
Field identification & locations
Common in granite terrains and exposed pegmatite dikes worldwide, including Maine, California, Brazil, Madagascar, and Norway. Identify it by large visible crystals, glassy quartz, blocky feldspar, and platy dark mica; acid testing is unnecessary.
