igneous
Quartz with Mica (Pegmatite)
Silicon Dioxide (SiO2) with Potassium Aluminum Silicate Hydroxide (KAl2(AlSi3O10)(OH,F)2)
Hardness: 7 (Quartz) / 2.5 (Mica), Color: White, gray, and silvery-brown, Luster: Vitreous to pearly, Crystal structure: Trigonal (quartz) and Monoclinic (mica), Cleavage: Perfect basal on mica flakes/none on quartz
- Hardness
- 7 (Quartz) / 2
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Physical properties
Hardness: 7 (Quartz) / 2.5 (Mica), Color: White, gray, and silvery-brown, Luster: Vitreous to pearly, Crystal structure: Trigonal (quartz) and Monoclinic (mica), Cleavage: Perfect basal on mica flakes/none on quartz
Formation & geological history
Formed from the slow cooling of magma in a pegmatite dike, typically during the final stages of a granitic intrusion where high water content allows for large crystal growth.
Uses & applications
Quartz is used in glassmaking, abrasives, and electronics. Mica is used as an insulator in electrical equipment and for glitter in cosmetics.
Geological facts
Pegmatites are known for containing exceptionally large crystals; mica was historically used as 'Muscovy glass' for windows in Russia before modern glass was common.
Field identification & locations
Identify by looking for the vitreous, glass-like quartz matrix combined with shiny, flaky, peelable layers of mica (books). Often found in mountainous granitic regions.
