Refinement of the crystal structure of calcium-lithium-aluminum tourmaline from the pegmatite vein in the Sangilen Upland (Tuva Republic)

The crystal structure of a natural calcium-lithium-aluminum tourmaline, which has the unique composition (Ca_0.62Na_0.32□_0.06)(Al_1.08Li_0.99Fe _0.66 ²⁺ Mg_0.24Ti_0.03)Al₆Si₆O₁₈](BO₃)₃(OH_2.28O_0.72) · (F_0.84O_0.16), is refined (R = 0.019, R _w = 0.022, S = 1.47). It is found that the O(1)(W) site is split into two sites, O(1) and O(11), which are incompletely occupied by fluorine and oxygen anions, respectively, and that the O(3)(V) site contains bivalent oxygen anions. The solid solution studied is close in composition to the liddicoatite mineral species and differs from the latter one by the Li: Al ratio in the Y octahedra and the presence of bivalent oxygen anions in the O(3) site. The tourmaline studied differs from the hypothetical oxyliddicoatite by the population of the O(1)(W) site by fluorine and accommodation of additional oxygen anions in the O(3)(V) site.