igneous
Muscovite Mica (Granitic Pegmatite)
Muscovite - KAl2(AlSi3O10)(OH)2
Hardness: 2.5–3 on Mohs scale; Color: Silvery-white, gray, or faint green; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); Specific gravity: 2.8
- Hardness
- 2
- Color
- Silvery-white, gray, or faint green
- Luster
- Vitreous to pearly
Identified More igneous →
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Physical properties
Hardness: 2.5–3 on Mohs scale; Color: Silvery-white, gray, or faint green; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); Specific gravity: 2.8
Formation & geological history
Formed in granitic pegmatites during the final stages of magma crystallization where mineral-rich fluids cool slowly, allowing for large crystal growth. Common in mountain belts of various ages from Precambrian to Mesozoic.
Uses & applications
Used as an electrical insulator in capacitors, a filler in paints and plastics, and as pearlescent flakes in cosmetics. Historically used for oven windows (isinglass) and cold-weather windows.
Geological facts
Muscovite is the most common mica and can form 'books' that are several feet across. Its name comes from 'Muscovy-glass,' a term used in Elizabethan England because it was used as cheap glass in Russia.
Field identification & locations
Identify by its 'peelable' sheets and silvery reflection. Commonly found in coarse-grained igneous rocks like granite. Collectors should look for intact 'books' or hexagonal crystals without edge fraying.
