Abstract
The crystal structures of one high-symmetry (P4⁄nnc) vesuvianite from Japan and three low-symmetry (non-P4⁄nnc) vesuvianites from Japan, Pakistan and Norway were determined by single-crystal X-ray diffraction method. The four crystals have the following cell parameters; a=15.568(2), 15.564(2), 15.559(3), 15.572(2); c=11.790(1), 11.841(1), 11.826(2), 11.833(2) Å, respectively. The structure refinements were carried out in the space group P4⁄nnc, P4⁄n, P4⁄n and P4nc, respectively. The final R indices for 1821, 3024, 3743 and 2053 independent reflections are 0.038, 0.063, 0.082 and 0.038, respectively.
The chemical analyses of low-symmetry vesuvianites indicate a very small amount of F and almost negligible Cl while high-symmetry one contains a considerable amount of F and Cl. The high-symmetry vesuvianite can contain Cl− ions preferentially occupying the O(10) site which is split into two sites; one is occupied by Cl− and the other by F−.
The ordering of cation and vacancy in the two alternately and statistically occupied sites in a unit cell causes the lowering of the symmetry from the space group P4⁄nnc to P4⁄n or P4nc. The ordering scheme of P4nc structure is first confirmed. In the three low-symmetry vesuvianites, the refined site occupancy ratios for occupied (B(a) and C(a)) and vacant (B(b) and C(b)) sites are 63:37, 62:38 and 92:8(%), respectively, and the values suggest the ordering in these crystals is not complete.
In low-symmetry vesuvianite, the satisfaction of the local charge balance on O(10) anions requires alternate occupancy of oxygen and hydroxyl with an associated hydrogen bond and ordering sequences of cations. In Cl- and F-bearing high-symmetry vesuvianite, on the other hand, occupying of Cl and F in O(10) sites interrupts the sequences of cations and vacancies along the fourfold rotation axes.
