igneous
Quartz-rich pegmatite with mica
Granitic pegmatite; chiefly quartz (SiO₂), feldspar, and muscovite/biotite mica
AI-generated identification · may be incorrectCoarse-grained, irregular translucent gray-white quartz with silvery mica and probable feldspar; vitreous to pearly luster, uneven fracture, and Mohs hardness about 2–7 depending on the mineral. Quartz is hexagonal in structure and lacks cleavage; mica shows prominent basal cleavage.
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Physical properties
Coarse-grained, irregular translucent gray-white quartz with silvery mica and probable feldspar; vitreous to pearly luster, uneven fracture, and Mohs hardness about 2–7 depending on the mineral. Quartz is hexagonal in structure and lacks cleavage; mica shows prominent basal cleavage.
Formation & geological history
Formed when volatile-rich granitic magma cooled slowly in a late-stage vein or pocket, allowing exceptionally large crystals to grow. Its exact geological age and locality cannot be determined from the photograph.
Uses & applications
Common pegmatite is mainly a collector specimen; feldspar and quartz may have industrial uses, while attractive varieties can yield gem minerals such as tourmaline, beryl, or spodumene.
Geological facts
The blue-gray platy material may be mica or another accessory mineral, while rusty patches are likely iron-oxide weathering. Pegmatites are important sources of lithium, tantalum, beryllium, and rare-earth minerals.
Field identification & locations
Identify by very coarse interlocking crystals, abundant mica sheets, hard glassy quartz, and possible feldspar cleavage; a streak, hardness, and locality test would improve identification. Common in granitic regions worldwide, including Maine, California, Brazil, and Madagascar.
