Refinement of the crystal structures of three fluorine-bearing elbaites

Abstract

The crystal structures of three Li-Al natural tourmalines (elbaites) containing 0.88–1.39 wt % F are refined to R= 0.0294, 0.0308, and 0.0417. It is revealed that the W threefold anion site is split into two sites, namely, the W1 threefold site and W2 ninefold site (W1–W2 ∼ 0.4 A, Y-W1 ≥ 1.94 A, Y-W2 ≥ 1.75 A). The following hypothesis is proposed and justified: the W1 and W2 sites are partially occupied by OH groups and fluorine anions, respectively. The ratio of the [YO4(OH)2] octahedra to the [YO4(OH)F] octahedra depends on the fluorine content and varies from structure to structure. The fact that the W site is more than 50% occupied by fluorine in the structures of two tourmalines under investigation allows the conclusion that fluor-elbaite with the ideal formula Na(Li1.5A11.5)A16(Si6O18)(BO3)3(OH)3F is a new mineral species and that elbaite can be considered a superspecies.