Crystal structure of arsenuranospathite from Rabejac, Lodève, France
Dal Bo, Fabrice; Hatert, Frédéric; Baijot, Maxime; Philippo, Simon
European Journal of Mineralogy Volume 27 Number 4 (2015), p. 589 - 597
published: Aug 1, 2015
The crystal structure of the natural aluminium uranyl arsenate hydrate arsenuranospathite, ideally Al[(UO2)(AsO4)]2F(H2O)20, from the Rabejac deposit, Lodève, France, was solved for the first time. Arsenuranospathite is orthorhombic, space group Pnn2, Z = 2, a = 29.9262(7), b = 7.1323(1), c = 7.1864(1) Å, and V = 1533.9(1) Å3. The crystal structure was refined from single-crystal X-ray diffraction data to R 1 = 0.0373 for 3348 unique observed reflections, and to wR 2 = 0.0768 for all 3833 unique reflections. The mineral is isostructural with its P-analogue uranospathite, Al[(UO2)(PO4)]2F(H2O)20; both minerals contain the autunite-type sheet [(UO2)((As,P)O4)]– constituted by corner-sharing UO6 square bipyramids and (As,P)O4 tetrahedra. Aluminium is linked with water molecules to form an Al(H2O)6 octahedron located in the interlayer of the structure. Eight independent and isolated water molecules are also located in the interlayer. A very complex network of H-bonds links the water molecules together, to the Al octahedra and to the uranyl-arsenate sheets. Fluorine was not accurately located by the current structure model and is supposed to replace some of the H2O molecules in the Al(H2O)6 octahedron. The crystal-chemical formula, calculated on the basis of (As + P + Si + S) = 2 cations per formula unit is (Al1.02Cu0.01)[(U0.98O2)(As0.89P0.08Si0.02S0.01)O4]2(F0.77OH0.05)(H2O)20.18.