Tourmaline is a common cyclosilicate (6-membered Si rings) and is the chief mineral source of boron in soils. It has a unit formula of (Na,Ca)(Li,Mg,Al)-(Al, Fe, Mn)6(BO3)3(Si6O18)(OH)4 wherein the first group of elements (Na,Ca) is located along the ring channels; the second group (Li,Mg,Al) is in octahedral coordination and links the silicate rings to the BO3 groups; the third group (Al, Fe, Mn) is in octahedral coordination and links the silicate rings together; and the last group is the 6-membered silicate backbone.
Highlighting FeaturesBorate (BO3) groups (note 3-fold planar configuration) Silicon (Si) atoms 6-membered cyclosilicate ring Aluminum (Al) atoms Sodium (Na) atoms Oxygen (O) atoms Hydrogen (H) atoms Single Unit Cell All atoms
Tourmaline is commonly found in granites and often occurs as elongated prismatic crystals, and is commonly black or brown, but may also be green, yellow, red-pink, and blue. Some gem-quality stones show several colors, varying in color either lengthwise or concentrically around the c axis.
Foit Jr., F.F. 1989. Crystal chemistry of alkali-deficient schorl and tourmaline structural relationships. American Mineralogist 74:422-431.