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