Cannonite [Bi_2O(SO_4)(OH)_2] from Alfenza (Crodo, Italy): crystal structure and morphology

摘要

Canonite from Alfenza grows as crowded, radiating, acicular aggregates covering bismuthinite crystals. Individual crystals have a lozenge-shaped habit on {010}, the presumed cleavage plane of cannonite. Crystal structure refinements in the P2_(1/C) space group of two single crystals led to the following cell parameters: a = 7.7196(5) A, b = 13.8856(9), c = 5.6980(4), β = 109.174(1)° (R_1 = 0,0424); and a = 7.7100(8), b = 13.8717(14), c = 5.6939(6), β = 109.155(2)° (R_1 = 0.0438). Hydrogen atoms were also localized in the density-difference Fourier map and refined with soft restraints on the bond distances. Raman and IR spectroscopy confirm the presence of OH groups and the absence of molecular water, and deliver OH…O geometry wholly comparable with the structure refinement. Electron microprobe analyses revealed no significant levels of elements other than those expected in the ideal formula except fluorine which was present up to 0.14 a.p.f.u. The crystal structure can be described in terms of anion-centred OBi_4 edge-sharing tetrahedra forming chains running parallel to z and strongly cemented along x by isolated SO_4 tetrahedra. Each OBi_4 tetrahedron is further connected along y by OH groups, making walls of composition Bi_4O_2(SO_4)_2(OH)_4 parallel to (010). These walls are tied to each other along y by fewer Bi-O-S bridges and weaker OH…O bonds.