Crystal Chemistry and Structure of Vesuvianite

OHKAWA Makio

Journal of science of the Hiroshima University. Series C, Earth and planetary sciences

The chemical variations of vesuvianite samples from 16 different localities, eleven in Japan, two in Norway, one in Canada, Pakistan, and Russia, have been examined using electron-microprobe analyses. For some of selected samples boron content was also examined. Results of analyses were reduced to formula unit on the basis of 50 cations excluding boron.
The crystal structures of six high-symmetry (P4/nnc) vesuvianite from Japan and three low-symmetry (non-P4/nnc) vesuvianitei from Japan, Pakistan and Norway were determined by single-crystal X-ray diffraction method. The nine crystals have the following cell parameters; a = 15.568(2), 15.559(3), 15.528(3), 15.472(2), 15.564(2), 15.559(3), 15.572(2), 15.759(1), 15.563(2); c = 11.790(1), 11.797(2), 11.755(2), 11.754(1), 11.841(1), 11.826(2), 11.833(2), 11.727(1), 11.818(1) Å, respectively. The structure refinements were carried out in the space group P4/nnc, P4/n and P4nc. The final R indices for 1821, 1493, 1357, 1415, 3024, 3743, 2053, 1745 and 1873 independent reflections are 0.038, 0.031, 0.040, 0.032, 0.063, 0.082, 0.038, 0.039 and 0.060, respectively. The EXAFS method was also used to investigate the local environment of the Cu and Mn ions in the crystal structure of vesuvianites from Norway and Japan.
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 chemical compositions of high-symmetry vesuvianites are more variable than those of low-symmetry vesuvianites.
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 firstly confirmed in this paper. In the three low-symmetry vesuvianites, the refined site occupancy ratios for occupied (B(a) and C(a)) and vacant sites (B(b) and C(b)) are 63:37, 62:38 and 92:8 (%), respectively, and the values suggest that 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.
Boron-bearing vesuvianite from Russia contain more Mg and less Al than those of boron-free vesuvianite. Boron occupies the additional cation sites, tetrahedrally coordinated site at the 8h position and triangularly coordinated site at the 2a position. The excess of the scattering power at these sites derived from the measured amounts of boron indicates that some elements other than boron should occupy in boron positions.

eng

Hiroshima University

[ISSN] 0075-4374

[NCID] AA11095875