Biotite
K(Mg,Fe,Mn)3[(OH,F)2|(Al,Fe,Ti)Si3O10]
Chemical name
Biotite
Mineral name
Biotite
Chemical formula
K(Mg,Fe,Mn)3[(OH,F)2|(Al,Fe,Ti)Si3O10]
Crystal system
monoclinic (pseudo-hexagonal)
Crystal Class
monoclinic-prismatic (2/m)
Space Group
C2/m
Unit cell
a [Å]:5.31
b [Å]:9.23
c [Å]:10.18
Formula units
Z=2
Twinning
common on {310}, less common on {001}
Mohs hardness
2.5 - 3
Density [g/cm3]
2.7 - 3.3
Cleavage
perfect on {001}
Fracture / Tenacity
brittle to flexible, elastic, micaceous
Color
dark brown, greenish brown, blackish brown, yellow, white
Streak
white
Transparency
transparent to translucent to opaque
Optical character
biaxial (-)
Refractive Index
nα=1.565-1.625; nβ=1.605-1.675; nΥ=1.605-1.675
Birefringence δ
0.030 - 0.070
Pleochroism δ
strong