igneous
Quartz-rich pegmatitic rock
Quartz–feldspar pegmatite; chiefly SiO₂ quartz with K-feldspar/plagioclase
AI-generated identification · may be incorrectCoarse, interlocking translucent white-to-beige crystals with vitreous to pearly luster and irregular fractures; quartz hardness 7, feldspar 6–6.5, with no obvious cleavage in quartz but prominent cleavage in feldspar. The specimen appears weathered and may contain minor mica or iron-oxide staining.
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Physical properties
Coarse, interlocking translucent white-to-beige crystals with vitreous to pearly luster and irregular fractures; quartz hardness 7, feldspar 6–6.5, with no obvious cleavage in quartz but prominent cleavage in feldspar. The specimen appears weathered and may contain minor mica or iron-oxide staining.
Formation & geological history
Forms when silica- and volatile-rich magma cools slowly in late-stage veins, allowing exceptionally large crystals to grow; commonly Precambrian to Phanerozoic, depending on the host intrusion. Its exact age cannot be determined from the photograph.
Uses & applications
Used as a source of quartz and feldspar for glass, ceramics, and fillers; attractive pieces are sold as mineral specimens, though this example is mainly a teaching or cabinet specimen.
Geological facts
Pegmatites can host rare minerals and gemstones such as tourmaline, beryl, spodumene, and garnet. Crystal size reflects slow cooling and concentration of water and other volatiles.
Field identification & locations
Identify it by unusually coarse quartz and feldspar crystals, glassy quartz, and feldspar’s blocky cleavage; acid testing is unnecessary. Common in granitic terrains worldwide, including Brazil, Madagascar, the United States, and Scandinavia.
