igneous
Pegmatitic quartz–mica rock
Quartz-rich granitic pegmatite with biotite (possibly muscovite) mica
AI-generated identification · may be incorrectVisible white to translucent, glassy quartz is intergrown with dark bronze-brown, platy mica; coarse, irregular crystals and no obvious layering. Quartz is Mohs 7, mica about 2–3, and both have vitreous to pearly luster; exact composition cannot be confirmed from the photo.
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Physical properties
Visible white to translucent, glassy quartz is intergrown with dark bronze-brown, platy mica; coarse, irregular crystals and no obvious layering. Quartz is Mohs 7, mica about 2–3, and both have vitreous to pearly luster; exact composition cannot be confirmed from the photo.
Formation & geological history
Likely crystallized slowly from water-rich, silica-rich late-stage granite magma, producing unusually coarse crystals. Pegmatites commonly formed during Precambrian to younger intrusive events; the specimen’s age cannot be determined visually.
Uses & applications
Quartz and feldspar from pegmatites are used in glass, ceramics, and industrial applications; mica has electrical and thermal uses. This small piece is mainly of collecting interest.
Geological facts
The reflective bronze flakes are characteristic of mica, whose sheets split readily along one perfect cleavage plane; the pale bulk is likely quartz with possible feldspar.
Field identification & locations
Pegmatites occur in the Pacific Northwest, so the reported northwest Oregon location is plausible, though not diagnostic. Check for flaky sheets that peel apart (mica) and glassy, hard grains that scratch glass (quartz); a hand lens or mineral tests would refine the identification.
