igneous
Quartz-rich pegmatite (weathered feldspar–quartz rock)
Granitic pegmatite; primarily quartz (SiO₂), feldspar, and minor biotite/iron-oxide minerals
AI-generated identification · may be incorrectCoarse, irregular interlocking crystals; white to cream quartz and feldspar with brown iron-oxide staining and dark mica inclusions. Quartz hardness 7, feldspar 6–6.5; vitreous quartz luster, feldspar cleavage, low-to-moderate specific gravity (~2.6).
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Physical properties
Coarse, irregular interlocking crystals; white to cream quartz and feldspar with brown iron-oxide staining and dark mica inclusions. Quartz hardness 7, feldspar 6–6.5; vitreous quartz luster, feldspar cleavage, low-to-moderate specific gravity (~2.6).
Formation & geological history
Forms when water-rich, silica-rich magma cools slowly in veins or pockets, allowing exceptionally large crystals to grow. Pegmatites are commonly Precambrian to Mesozoic, depending on the host intrusion.
Uses & applications
Used as decorative stone, aggregate, silica, and a collector specimen; some pegmatites contain valuable beryl, tourmaline, lithium minerals, or rare-element ores, though none are clearly evident here.
Geological facts
The rusty coloration is likely limonite/goethite from weathering of iron-bearing minerals. Quartz can be distinguished from feldspar by its lack of cleavage and greater hardness.
Field identification & locations
Common in granitic terrains and exposed pegmatite dikes worldwide, including Brazil, Madagascar, the United States, and Scandinavia. Look for very coarse crystals, glassy quartz, blocky feldspar, and zoned veins.
