| The role of growth steps in inducing disequilibrium is investigated in crystals of vesuvianite from the Jeffrey mine, Asbestos, Quebec, using optical microscopy, atomic force microscopy, electron microprobe analysis, and single-crystal X-ray diffraction. The selective uptake of elements Fe and Al by asymmetric growth-steps on three crystallographic forms, {100}, {110}, and {121}, is documented. The prisms {100} and {110} show hillocks that display kinetically controlled oscillatory zoning along growth steps parallel to <010> and <11¯1>, but not on vicinal faces defined by [001] steps. Sector-specific zoning of extinction angles and 2V angles indicate different degrees of optical dissymmetrization in crystals spanning a range of growth habits. Unit-cell parameters and the presence of violating reflections confirm sectoral deviations from P4/nnc symmetry in the prismatic sectors. The partial loss of three glide planes follows the pattern expected from order of the cations Al and Fe induced by tangential selectivity at the edge of non-equivalent steps during layer-by-layer growth. |
