Biotite - Phlogopite
Chemical Formula : Biotite - K(Mg,Fe)3(Al,Fe)Si3O10(OH,F)2
Phlogopite - KMg3(AlSi3O10)(F,OH)2
Hardness : 2.5 on cleavage, 4 across cleavage
Cleavage : One, perfect
Luster : Vitreous to pearly
Color : Pale yelow to brown to black(biotite) to bronze(phlogopite)
Streak : Whtie to gray
Density : 2.8-3.4
Diagnostic Features and Trivia : These two members of the mica group
are easily identified by their stereotypical colors and cleavage.
Crystal System and Habit : Monoclinic; common crystals are pseudo-hexagonal prisms.
Twinning : None
Polymorphs : None
Index of Refraction : alpha=1.565-1.625; beta=1.605-1.696; gamma=1.605-1.696