#160
R3m
Example – Tourmaline, NaFe3Al6(BO3)3(Si6O18)(OH)4
The name comes from the Sinhalese word “Turmali” or “Thoramalli”, which applied to different gemstones found in Sri Lanka.
Schorl refinement showing composition dependence of the tourmaline structure
Fortier, S.; Donnay, G.
Canadian Mineralogist (1975) 13, p173-p177
http://www.mindat.org/min-4003.html
CIF
http://1drv.ms/1mbtrfK
Na (green)
FeO6 octahedra (purple)
AlO6 octahedra (blue)
SiO4 tetrahedra (orange)
BO3 trigonal planar units (blue)
The Fascination of Crystals and Symmetry 