Muscovite

Physical Properties
Chemical formula KAl2(AlSi3O10)(OH)2
Class Phyllosilicate
Sheets of linked tetrahedra
Crystal system Monoclinic
Habit Disseminated grains

Books with or without
pseudohexagonal outline

Color White, grey-brown
Hardness 2 to 2.5
Specific gravity 2.8
Cleavage Perfect basal (001)
Fracture Elastic
Luster Vitreous
Transparency Translucent
Streak White
Optical Properties
PPL Colorless
Basal cleavage may be visible as parallel lines
XPL Birdseye extinction
High 2nd order, low 3rd order
δ 0.035
Extinction Birdseye
after Perkins, 322

Muscovite in Hand Sample

Hand sample

Scanning electron micrograph of muscovite (Note: it always does that. Every single time. Everything looks great until the image finishes stitching, and then WHAM! Great big black holes. The important thing here is that the black holes aren’t totally obscuring the micaceous cleavage.)

Muscovite in Thin Section

Thin Section GigaPans

Fine-grained muscovite with chlorite (green), plane polars

Fine-grained muscovite with chlorite, crossed polars

Fine-grained muscovite with chlorite (green), PPL
Fine-grained muscovite with chlorite, XPL

Further Reading

Muscovite at webmineral.com

Muscovite at mindat.org