Tourmaline
(Ca,K,Na)(Al,Fe,Li,Mg,Mn)3(Al,Cr,Fe,V)6(BO3)3(Si,Al,B)6O18(OH,F)4
Chemical name
Tourmaline
Mineral name
Tourmaline
Chemical formula
(Ca,K,Na)(Al,Fe,Li,Mg,Mn)3(Al,Cr,Fe,V)6(BO3)3(Si,Al,B)6O18(OH,F)4
Crystal system
trigonal
Crystal Class
ditrigonal-pyramidal (3m)
Space Group
R3m
Unit cell
a [Å]:15.84 - 16.03
b [Å]:15.84 - 16.03
c [Å]:7.10 - 7.15
Formula units
Z=3
Mohs hardness
7 - 7.5
Density [g/cm3]
2.82 - 3.32
Cleavage
indistinct
Fracture / Tenacity
brittle, uneven, small conchoidal
Color
black (common), colorless to brown, red, orange, yellow, green, blue, violet, pink, can be bi-colored or even tri-colored in between
Streak
white
Transparency
translucent to opaque
Optical character
uniaxial (-)
Refractive Index
nω=1.635-1.675; nε=1.610-1.650
Birefringence δ
0.018 - 0.040
Pleochroism δ
moderate to strong