Fluorite-related one-dimensional units in natural bismuth oxysulfates: The crystal structures of Bi14O16(SO4)5 and Bi30O33(SO4)9(AsO4)2

The crystal structures of two new natural Bi oxysulfates with the formula Bi14O16(SO4)5 [labelled new phase I; monoclinic, space group C2, a = 21.658 (4), b = 5.6648 (9), c = 15.092 (3) A ˚ , = 119.433 (11) and Z = 2] and Bi30O33(SO4)9(AsO4)2 [labelled new phase II; triclinic, space group P1, a = 5.670 (3), b = 13.9408 (9), c = 22.7908 (18) A ˚ , = 80.903 (5), = 82.854 (14), = 78.27 (2) and Z = 1] from the high-temperature fumarole deposit of the La Fossa crater at Vulcano (Aeolian Islands, Italy) are reported. The structures are built up by a combination of fluorite-related Bi—O units and isolated (SO4)2 tetrahedra (new phase I) or both (SO4)2and (AsO4)3tetrahedra (new phase II). Owing to the effect of stereoactive lone pairs of Bi3+, Bi—O units in both thestructures can be suitably described in terms of oxo-centered OBi4 tetrahedra. The structure of Bi14O16(SO4)5 is based upon one-dimensional [O16Bi14]10+ribbons formed by six chains of edge-sharing OBi4 tetrahedra extending along [010]. In the structure of Bi30O33(SO4)9(AsO4)2 the same ribbon type coexists with another one-dimensional ribbon formed by seven chains of edge-sharing OBi4 tetrahedra and with the composition [O17Bi16]14+. Ribbons of the same type are joined by (SO4)2and (AsO4)3tetrahedra along [010] – if a reduced triclinic unit-cell setting is considered – so forming two different (001) slabs which alternate to each other along [001] and are joined by additional (SO4)2 tetrahedra. New phase I represents the natural analogues of synthetic Bi14O16(SO4)5, but with an ordered structure model.