Mineral - Schorl
Schorl
Chemical Properties
Mineral Class
Silicates
Toxicity
none
Formula
NaFe²⁺₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH)
Unit Cell
a=16.03 c=7.19 Z=3
Physical Properties
Hardness
7 - 7.5
Streak
white
Density
Normal (2-3,5)
Cleavage
Indistinct after {11 2 0} and {10 1 1}
Fracture
uneven
Tenacity
Brittle
Optical Properties
Color
black
Birefringence
0.025
Luster
Vitreous
Transparency
translucent,
opaque
Pleochroism
Geomineralogical Properties
Crystal System
trigonal
Point Group
ditrigonal-pyramidal - 3m
Space Group
R3m (160)
Habit
blocky, acicular, prismatic, radial
Formation
(1) Magmatic, resulting from the fractional crystallization of boron-rich residual melts in acidic plutonic rocks, particularly in granitic pegmatites.
(2) Metasomatic, resulting from the introduction of boron from magmatic fluids into the host rock (metasomatism), e.g., leading to the formation of greisen.
Paragenesis
Twinning
Rarity
Very common
Synonyms
Schorl
Composition
Elements
Mass
SiO₂
36.04
TiO₂
0.54
B₂O₃
10.43
Al₂O₃
30.83
FeO
17.59
MnO
0.11
MgO
0.42
CaO
1.01
Na₂O
2.72
K₂O
0.05
Germany (Dana 1895)
Groups and Members
Tourmaline-Group