Pegmatite with Muscovite

Physical Properties

Hardness: 6-7 (feldspar/quartz) and 2-2.5 (mica); Color: White, tan, with silvery-green mica books; Luster: Vitreous to pearly; Crystal structure: Monoclinic (mica); Cleavage: Perfect basal cleavage in mica flakes.

Formation & Geological History

Formed during the final stages of magma crystallization where high water content allows for the growth of exceptionally large crystals. Typically Precambrian to Cenozoic in age.

Uses & Applications

Source of sheet mica for electrical insulators, quartz for glass, and feldspar for ceramics. Also a source of rare earth elements and gemstones.

Geological Facts

The large silvery-green flakes are Muscovite, often called 'Isinglass'. In the past, large sheets of this mica were used as oven windows because they are heat-resistant and transparent.

Field Identification & Locations

Identify by the presence of very large (over 1cm) crystals of quartz, feldspar, and mica. Commonly found in mountain belts and shield areas like the Black Hills (USA) or Minas Gerais (Brazil).

Identified on: 5/3/2026

Mode: Standard