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, Lode `ve, France, was solved for the first time. Arsenuranospathite is orthorhombic, space group Pnn2, Z ¼ 2, a ¼ 29.9262(7), b ¼ 7.1323(1), c ¼ 7.1864(1) A ˚, and V ¼ 1533.9(1) A ˚3. The crystal structure was refined from single-crystal X-ray diffraction data to R 1 ¼ 0.0373 for 3348 unique observed reflections, and to wR 2 ¼ 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 (Ascations 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 .

Supergene Uranyl Mineralization of the Rabejac Deposit, Lodève, France

“…These unit-cell parameters are in good agreement with those that were previously reported for arsenuranospathite [45,46] and for uranospathite [43,46]. The crystal structure was solved for the first time, thus confirming the presence of 20 water molecules pfu, and the isomorphism with uranospathite [42]. O), occurs in two distinct habitus.…”

“…Chemical analyses point out that a part of As is replaced by P, thus confirming the common substitution between these cations in tetrahedral coordination, and the solid solution between arsenuranospathite and uranospathite (Table 9) [42,43]. The F content reaches 0.77 apfu, the U content is close to 2 apfu, and the Al content is close to 1 apfu.…”

“…Energy-dispersive X-ray spectrometric analysis shows that this compound contains high amounts of U, S, and Ca, as well as a low amount of Fe; it may correspond to rabejacite. Several crystals of arsenuranospathite are completely coated by these yellowish crystals, indicating that this mineral crystallized during a later stage [42].…”

Crystal structure of bassetite and saléeite: new insight into autunite-group minerals

“…The crystal structures have been determined for approximately twenty of these minerals, and new species belonging to the autunite and meta-autunite groups continue to be reported, as shown by the recent discovery of metarauchite Ni[(UO 2 )(AsO 4 )] 2 (H 2 O) 8 (Pla ´s ˇil et al, 2010a) and rauchite Ni[(UO 2 )(AsO 4 )] 2 (H 2 O) 10 (Pekov et al, 2012). Recent structural investigations have also been performed on sale ´eite Mg[(UO 2 )(PO 4 )] 2 (H 2 O) 10 (Yakubovich et al, 2008) and on arsenuranospathite Al[(UO 2 )(AsO 4 )] 2 F(H 2 O) 20 (Dal Bo et al, 2015). The investigation of these minerals is of interest because they control the mobility and solubility of uranium in many geological environments.…”