CRYSTAL CHEMISTRY OF A SECTOR-ZONED VESUVIANITE FROM JEFFREY MINE, QUEBEC

A transparent, clove-brown vesuvianite crystal from the Jeffrey mine in Quebec shows sector zoning when sectioned parallel to (001). The most prominent sectors are those associated with the {001}, {101}, and {100} faces; these are referred to as the core, intermediate, and rim zones, respectively. The 2V angles for the zones are 0-10, 36, and 60-62°, respectively. The structures of crystals extracted from the intermediate and rim zones were refined in the normal tetragonal P4/nnc space group as well as several tetragonal, orthorhombic, monoclinc and triclinic subgroups. The best result for the intermediate zone crystal was R = 0.023 for P4/nnc [a = 15.5353(4), c = 11.8177(4) Å]. The structure of the rim zone crystal refined best (R = 0.033) in the monoclinic space group P2/n [a = 15.5367(4), b = 11.8272(3), c = 15.5604(4) Å, β = 90.020(1)°], which in this orientation is a subgroup of P4/n. The crystal structure of the rim crystal shows the same X4/Y1 ordering seen in P4/n vesuvianites, suggesting formation below 500 °C. Refined site occupancies suggest that in the rim zone there is some ordering of substituents for Al at the octahedral sites; work continues to determine the cause of the lower symmetry in this vesuvianite. An unusual feature of both the intermediate and rim zone crystals is.the splitting of the Y1 cation site into two positions such that one site has a comparatively short Y1-O10 distance, and the cation has the usual square pyramidal coordination, whereas the second position has a very long Y1-O10 distance such that the cation is in square planar coordination.