Tourmaline

Elbaite

Rosina vein, San Piero in Campo, Campo nell'Elba, Elba Island, Livorno Province, Tuscany, Italy

Schorl, etc.

Bulochi, Astor valley, Astor District, Gilgit-Baltistan, Pakistan

Uvite

Pomba pit, Serra das Éguas, Brumado, Bahia, Brazil

Dravite

Power's Farm, Pierrepont, St. Lawrence Co., New York, USA

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;

Z = F, O and/or OH.

Note: In the formulas of the group members, we have put the D site cations in parentheses in order to facilitate the assignment of the different cations to the crystallographic sites.

System:

Trigonal

Hardness:

7

Name:

Reported by Christianus-Fridericus Garmann in 1707. The name "tourmali" was a generic name used in Ceylon [Sri Lanka] for colored 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.

Tourmaline (synonymous with tourmaline group) comprises trigonal borosilicates of the cyclosilicate superclass. The nomenclature was recently revised (Henry et al., 2011).


Notes on the general formula: 
- Additional G and D site cations are reported substituting in minor amounts for the principal cations of these sites.
- The X and Z sites are usually combined in the simplified formula for the Group as Z4.

The structure for the group is one in which SiO₄ tetrahedra are linked into six-membered rings having a hexagonal pattern and are stacked up with intervening distorted triangular BO₃ groups, linked by D site cations; the SiO₄ tetrahedra are linked vertically by G site cations and X site anions, while the columns are linked horizontally by both D site cations and X site anions. The A site cations and the Z site anions occupy the channels down the centre of the columns (or the A site may be vacant.)

Luinaite-(OH) represents a monoclinic (pseudo-rhombohedral) distorted variant (space group Cm) of the tourmaline structure. Unnamed triclinic variants have also been reported.