EXCESS Y-GROUP CATIONS IN THE CRYSTAL-STRUCTURE OF VESUVIANITE

Some vesuvianite crystals show sums of Y-group cations [Al, Mg, Fe3+, Fe2+, Ti4+, Cu2+, Zn, Mn] significantly in excess of 13, the number of sites available to accommodate these cations in the current model of the vesuvianite structure. The structures of four such crystals were r efined in the space group P4/nnc to R indices of 3-4% using single-cry stal MoK alpha X-ray intensity data. The same crystals were subsequent ly examined by wavelength-dispersion electron-microprobe analysis; the mean Y-group sum is 13.57 apfu. The crystals show an additional site, T(1), that is tetrahedrally coordinated by four oxygen atoms. This si te is occupied by (Al,Fe), which replaces two H atoms at adjacent H(1) positions. Local bond-valence arguments show that the substitution is accompanied by the incorporation of a vacancy at an adjacent X(3) sit e according to the mechanism nism(T(1))(Al,Fe3+)(2) + (H(1))square(4) + (X(3))square --> (T(1))square(2) + (H(1))H-4 + (X(3))Ca, or by the s ubstitution (T(1))(Al,Fe3+) + (H(1))square(2) + (Y)(Mg,Fe2+) --> (T(1) )square + (H(1))H-2 + (Y)(Al,Fe3+).