title: Fluorite-related one-dimensional units in natural bismuth oxysulfates: the crystal structures of Bi14O16(SO4)5 and Bi30O33(SO4)9(AsO4)2.
abstract: 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) Å, β = 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) Å, α = 80.903 (5), β = 82.854 (14), γ = 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)(2-) and (AsO4)(3-) tetrahedra (new phase II). Owing to the effect of stereoactive lone pairs of Bi(3+), Bi-O units in both the structures 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)(2-) and (AsO4)(3-) tetrahedra 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.
authors.affiliation: Dipartimento di Scienze della Terra e Geoambientali, Università degli Studi di Bari A. Moro, Via E. Orabona, 4, I-70125 Bari, Italy.
authors.forename: Daniela
authors.initials: D
authors.lastname: Pinto
authors.affiliation: Dipartimento di Scienze della Terra e Geoambientali, Università degli Studi di Bari A. Moro, Via E. Orabona, 4, I-70125 Bari, Italy.
authors.forename: Anna
authors.initials: A
authors.lastname: Garavelli
authors.affiliation: Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via La Pira, 4, I-50121 Firenze, Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: Bindi
keywords: Bi oxysulfates; Vulcano; fluorite; oxo-centered tetrahedra; sublimates