Tourmaline

Mineral Description

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(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 link the silicate rings together; and the last group is the 6-membered silicate backbone.

Highlighting Features

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Borate (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

Distribution and uses

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.

Crystallographic Data

Reference

Foit Jr., F.F. 1989. Crystal chemistry of alkali-deficient schorl and tourmaline structural relationships. American Mineralogist 74:422-431.

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Original release: 1 Oct 1998;