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Biotite

K(Mg,Fe,Mn)3[(OH,F)2|(Al,Fe,Ti)Si3O10]

Chemical 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

Molecular weight [g/mol]

433.53

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

Luster

vitreous to pearly

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