Crystal structure of arsenuranospathite from Rabejac, Lodève, France

Abstract

The crystal structure of the natural aluminium uranyl arsenate hydrate arsenuranospathite, ideally Al[(UO 2 )(AsO 4 )] 2 F(H 2 O) 20 , from the Rabejac deposit, Lodeve, France, was solved for the first time. Arsenuranospathite is orthorhombic, space group Pnn 2, Z = 2, a = 29.9262(7), b = 7.1323(1), c = 7.1864(1)A, and V = 1533.9(1)A 3 . 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[(UO 2 )(PO 4 )] 2 F(H 2 O) 20 ; both minerals contain the autunite-type sheet [(UO 2 )((As,P)O 4 )]– constituted by corner-sharing UO 6 square bipyramids and (As,P)O 4 tetrahedra. Aluminium is linked with water molecules to form an Al(H 2 O) 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 H 2 O molecules in the Al(H 2 O) 6 octahedron. The crystal-chemical formula, calculated on the basis of (As + P + Si + S) = 2 cations per formula unit is (Al 1.02 Cu 0.01 )[(U 0.98 O 2 )(As 0.89 P 0.08 Si 0.02 S 0.01 )O 4 ] 2 (F 0.77 OH 0.05 )(H 2 O) 20.18 .