The crystal structure of vesuvianite from Nakatatsu mine: reinvestigation of the cation site-populations and of the hydroxyl groups

Abstract

Refinement of the crystal structure of a vesuvianite [Ca_18.6 (Mn_0.3Fe_0.7) (OH)₂ (Mg_2.1Fe_0.7Ti_0.5Al_4.7) Al_4.2 (OH, O)₈ Si_18.2 O₆₈; a = 15.561(8) c = 11.777(7) Å; space group P4⁄nnc] from Nakatatsu mine, Fukui Prefecture, central Japan, was carried out by a least-squares method with X-ray single-crystal data. The final R index for 1901 observed independent reflections is 0.030. From site population refinement and observed interatomic distances of cation sites, the C-sites (square antiprism) and B-sites (tetragonal pyramid) are half occupied by Ca²⁺ and Fe²⁺ (+Mn²⁺), respectively, the A-sites (octahedron) are almost completely occupied by Al, and the AlFe-sites (octahedron) are occupied by Mn²⁺, Fe²⁺, Mg²⁺, Fe³⁺, Ti⁴⁺ and Al³⁺. From bond-valence calculations, the proton donor atoms are both O(10) and O(11). The cation site-populations and the hydroxyl groups of the structure models reported by ealier studies have been compared and reviewed.
Based on the structure analysis, the chemical formula of vesuvianite belonging to the space group P4⁄nnc should be written as Ca₁₉ (Mn, Fe²⁺) (F, OH)₂ (Mg, Fe²⁺, Mn, Al, Fe³⁺, Ti)₈Al₄ (OH, F, O)₈ (SiO₄)₁₀(Si₂O₇)₄.