Cubic cation framework in trigonal structures of tourmalins: The core of structural type stability

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 F_cub (a_к ≈ 4.40 Å, α_к ≈ 93°) with 40% vacant positions in it. The anion ordering is less regular.