Muscovite

Chemical Formula : KAl₂(AlSi₃O₁₀)(OH)₂
Hardness : 2.5 on cleavage, 4 across cleavage
Cleavage : One, perfect
Luster : Vitreous to pearly
Color : Colorless or pale shades of green, gray, or brown.
Streak : Whtie to gray
Density : 2.8-2.9
Diagnostic Features and Trivia : This member of the mica group
is easily identified by its stereotypical color and cleavage.
Crystal System and Habit : Monoclinic; common crystals are pseudo-hexagonal prisms.
Twinning : None
Polymorphs : None
Index of Refraction : alpha=1.552-1.574; beta=1.582-1.610; gamma=1.587-1.610