igneous
Pegmatitic granite (quartz–feldspar–mica rock)
Granitic pegmatite; primarily quartz (SiO₂), K-feldspar/plagioclase, and biotite or muscovite mica
AI-generated identification · may be incorrectCoarse, interlocking crystals with white to gray quartz and feldspar, plus bronze-brown mica or iron-stained surfaces; vitreous quartz and pearly mica luster. Hardness is approximately 6–7 for quartz/feldspar and 2–3 for mica; no single cleavage, though feldspar and mica cleave distinctly.
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Physical properties
Coarse, interlocking crystals with white to gray quartz and feldspar, plus bronze-brown mica or iron-stained surfaces; vitreous quartz and pearly mica luster. Hardness is approximately 6–7 for quartz/feldspar and 2–3 for mica; no single cleavage, though feldspar and mica cleave distinctly.
Formation & geological history
Forms when water-rich granitic magma cools slowly in late-stage veins, allowing exceptionally large crystals; commonly Precambrian to Phanerozoic in age. The orange-brown coloration likely reflects weathering or iron-oxide staining.
Uses & applications
Common material is mainly a collector specimen or educational sample; pegmatites can supply feldspar, quartz, mica, lithium, beryl, and tourmaline, while related granite is used for dimension stone and aggregate.
Geological facts
The visible sparkly, platy grains are likely mica, whereas translucent gray grains are quartz. A glass scratch test and strong feldspar cleavage help distinguish it from ordinary sandstone.
Field identification & locations
Common in granitic terrains and pegmatite districts worldwide, including Brazil, Madagascar, the United States, and Scandinavia. Identify by very coarse grain size, glassy quartz, blocky feldspar, and flaky mica.
