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Tourmaline
Tourmaline is the only common mineral of which boron is an essential constituent. Its presence in a rock, therefore, indicates that the rock contains significant boron.
Formula
(Na,Ca)(Mg,Fe,Mn,Li,Al)3(Al,Mg,Fe3+)6[Si6O18](BO3)3(O,OH)3(OH,F)
Structure
Ring silicate
Crystal System
Crystal system: trigonal (-)
Physical Properties
Colour in hand specimen: black
Hardness: 7
Optical Properties
Colour in plane polarised light: typically brown-green or green
Pleochroism: strongly pleochroic in many colours
Relief: moderate
Birefringence: typically 2nd order
Rock Specimens
Mineral Name
tourmaline