CUBIC CATION FRAMEWORK IN TRIGONAL STRUCTURES OF TOURMALINS: THE CORE OF STRUCTURAL TYPE STABILITY
S. V. Borisov, N. V. Pervukhina, S. A. Magarill
Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia
Keywords: кристаллографический анализ, кубический катионный каркас, структурный тип турмалина, катионная подрешетка, феномен "биений", crystallographic analysis, cubic cation framework, tourmaline structure type, cation sublattice, beating phenomenon
Abstract
The crystallographic analysis of the tourmaline structure type with the general formula XY3Z6(T6O18)(BO3)3V3W (R3m, Z = 3, a ≈ 16 Å, c ≈ 7.2 Å) shows that tetrahedrally coordinated T (Si, Al, …) cations, octahedrally coordinated Z (Al, Fe, Mg), and Y (Fe, Mg, Mn, …) form a joint cation framework with a sublattice whose geometry is close to Fcub (ak ≈ 4.40 Å, ak ≈ 93°) with 40% vacant positions in it. The anion ordering is less regular.
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