Mineral Type - Tourmaline

Tourmaline (synonymous with tourmaline group) comprises trigonal borosilicates of the cyclosilicate superclass. The nomenclature was recently revised (Henry et al., 2011). Colour change, from deep-green to dark-red, with an increasing path length of light, occurring in Cr-bearing tourmalines, is known as the Usambara effect. Although similar to the alexandrite effect, the colour change in this case is due to (1) spectral positions of the spin-allowed bands of absorption of Cr3+ ions, (2) specific ratio of the transmission of light in two windows of transparency (green and red), and (3) exponential dependence of the light transmittance on the sample thickness. Other than Cr chromophores, like Fe, suppress the effect (Taran & Naumenko, 2016).

Name

Reported by Christianus-Fridericus Garmann in 1707. The name "tourmali" was a generic name used in Ceylon [Sri Lanka] for coloured gems, mostly zircons. About 1703, it had been discovered by Dutch lapidaries that some of the "zircons" arriving in the Netherlands were actually a previously undescribed mineral. Several names were given to the new mineral including "Pierre de Ceylan, by Lemery in 1717. Tourmalin, as a more or less specific mineral name, was used by Rinmann in 1766. Hill called it Tourmaline Garnet in 1771 and Richard Kirwan shortened the name to "Tourmaline" in 1794.

Physical Properties

Hardness: 7 on Mohs scale
Fracture: Irregular/Uneven, Conchoidal
Density: 2.9 - 3.1 g/cm3 (Measured)

Chemical Properties

Formula: A(D3)G6(T6O18)(BO3)3X3Z
A = Ca, Na, K, or is vacant (large cations);
D = Al, Fe2+, Fe3+, Li, Mg2+, Mn2+ (intermediate to small cations - in valence balancing combinations when the A site is vacant);
G = Al, Cr3+, Fe3+, V3+ (small cations);
T = Si (and sometimes minor Al, B3+);
X = O and/or OH; and
Z = F, O and/or OH.

Crystallography

Crystal System: Trigonal

Synonyms

Aphrizite, Aphrysite, Aphryzite, Ash Drawer, Blue Schorl, Ceylonese Peridot, Chameleonite, Iochroite, Taltalite, Tourmaline Group, Tourmaline Supergroup, Tourmalinite, Turmaline, Xeuxite, and Zeuxite

Varieties

Adachiite, Bosiite, Chromium-dravite, Chromo-alumino-povondraite, Darrellhenryite, Dravite, Elbaite, Feruvite, Fluor-buergerite, Fluor-dravite, Fluor-elbaite, Fluor-feruvite, Fluor-liddicoatite, Fluor-schorl, Fluor-tsilaisite, Fluor-uvite, Foitite, Liddicoatite, Lucchesiite, Luinaite-(OH), Magnesio-foitite, Maruyamaite, Olenite, Oxy-hromium-dravite, Oxy-dravite, Oxy-foitite, Oxy-rossmanite, Oxy-schorl, Oxy-uvite, Oxy-vanadium-dravite, Povondraite, Rossmanite, Schorl, Tsilaisite, UM2000-64-SiO:BFeHKMg, Unnamed (Pb-dominant Tourmaline), Uvite Series, Vanadio-oxy-chromium-dravite, and Vanadio-oxy-dravite