igneous
Quartz-rich pegmatite
Granitoid pegmatite; predominantly quartz (SiO₂), with abundant feldspar and probable mica
AI-generated identification · may be incorrectCoarse-grained, interlocking crystals; white to gray quartz with pale feldspar and silvery mica, vitreous to pearly luster, irregular fracture, and no obvious cleavage in quartz. Quartz hardness is 7 Mohs; feldspar is 6–6.5 and mica about 2–3.
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Physical properties
Coarse-grained, interlocking crystals; white to gray quartz with pale feldspar and silvery mica, vitreous to pearly luster, irregular fracture, and no obvious cleavage in quartz. Quartz hardness is 7 Mohs; feldspar is 6–6.5 and mica about 2–3.
Formation & geological history
Formed when late-stage, water-rich magma cooled slowly in a granitic intrusion or dike, allowing exceptionally large crystals to grow. Most pegmatites formed during Precambrian to Mesozoic tectonic events, depending on locality.
Uses & applications
Used as a source of quartz and feldspar for glass, ceramics, and occasionally lithium, beryllium, or tantalum minerals; attractive pieces are collected as cabinet specimens.
Geological facts
The blocky white crystals are likely feldspar, while translucent gray areas are quartz; mica may occur as reflective flakes. Exact identification of accessory minerals requires testing or microscopy.
Field identification & locations
Common in granitic terrains worldwide, including Brazil, the United States, Madagascar, and Scandinavia. Quartz remains unfazed by a steel knife, whereas feldspar can often be scratched; acid testing is generally unnecessary.
