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
FeO6 octahedra (purple)
AlO6 octahedra (blue)
SiO4 tetrahedra (orange)
BO3 trigonal planar units (blue)