mineral

Biotite or Phlogopite Mica

K(Mg,Fe)3(AlSi3O10)(F,OH)2

Hardness: 2.5–3.0 on Mohs scale; Color: Dark brown to black (Biotite) or golden-yellow/brown (Phlogopite); Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal (peels into thin flexible sheets); Specific Gravity: 2.7–3.3

Hardness: 2
Luster: Vitreous to pearly

Identified May 7, 2026More mineral →

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Physical properties

Formation & geological history

Forms in igneous rocks like granite and pegmatites, as well as metamorphic rocks like schist and gneiss. It is common in regional and contact metamorphic zones.

Uses & applications

Used primarily as an insulator in electrical equipment, in joint compound, as a filler in paints and plastics, and as a soil conditioner. High-quality sheets are used in optical instruments.

Geological facts

Commonly confused with gold flakes by novice prospectors due to its golden shimmer in water; however, mica flakes will crumble or shatter when poked with a needle, while gold is malleable.

Field identification & locations

Identify by its 'book-like' appearance and ability to peel into transparent, flexible flakes. Often found in coarse-grained igneous rocks. Check for the pearly luster and perfect cleavage.