igneous
Quartz-rich pegmatite (likely milky quartz with feldspar)
Pegmatitic quartz–feldspar rock; primarily SiO₂ quartz with K-feldspar/plagioclase
AI-generated identification · may be incorrectHardness about 6–7 Mohs; white to gray, glassy to pearly luster, coarse interlocking crystals, and poor or indistinct cleavage in quartz with feldspar cleavage. The specimen appears massive and weathered, with reflective crystal faces.
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Physical properties
Hardness about 6–7 Mohs; white to gray, glassy to pearly luster, coarse interlocking crystals, and poor or indistinct cleavage in quartz with feldspar cleavage. The specimen appears massive and weathered, with reflective crystal faces.
Formation & geological history
Forms from the final, water-rich fluids of slowly cooling felsic magma, producing exceptionally large crystals in igneous pegmatite dikes. Most pegmatites are Precambrian to Phanerozoic, depending on the deposit.
Uses & applications
Used as a source of quartz and feldspar for glass, ceramics, abrasives, and aggregate; attractive pieces are collected as mineral specimens. It is generally not gem-quality material.
Geological facts
Pegmatites may also contain tourmaline, beryl, mica, and rare-element minerals such as lithium-bearing spodumene. Strongly reflective white patches commonly indicate fresh feldspar or quartz cleavage surfaces.
Field identification & locations
Identify it by its very coarse grain size, hard glassy quartz, and blocky feldspar crystals; a steel nail usually will not scratch quartz. Common in granitic terrains worldwide, including Brazil, the United States, Madagascar, and Scandinavia.
