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THE STRUCTURE AND COMPOSITION OF URANOSPATHITE, Al_1–xx[(UO₂)(PO₄)]₂(H₂O)_20+3xF_1–3x, 0 < x < 0.33, A NON-CENTROSYMMETRIC FLUORINE-BEARING MINERAL OF THE AUTUNITE GROUP, AND OF A RELATED SYNTHETIC LOWER HYDRATE, Al_0.670.33[(UO₂)(PO₄)]₂(H₂O)_15.5

Andrew J. Locock^1,, William S. Kinman² and Peter C. Burns²

¹ Mineralogy, Department of Natural History, Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario M5S 2C6, Canada
² Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556, U.S.A.

E-mail address: andrewl{at}rom.on.ca

The crystal structure of uranospathite, ideal formula Al_1–xx[(UO₂)(PO₄)]₂(H₂O)_20+3xF_1–3x, 0 < x < 0.33, orthorhombic, space group Pnn2, a 30.020(4), b 7.0084(9), c 7.0492(9) Å, V 1483.1(3) ų, Z = 2, D_calc 2.54 g/mL, and that of synthetic Al_0.670.33 [(UO₂)(PO₄)]₂(H₂O)_15.5, AlUP, triclinic, space group P, a 7.0020(6), b 13.7120(11), c 14.0243(11) Å, 78.418(2), ß 89.676(2), 81.863(2)°, V 1305.4(2) ų, Z = 2, D_calc 2.61 g/mL, were refined by full-matrix least-squares techniques on the basis of F² to agreement indices R1 (uranospathite and AlUP) of 4.0 and 4.9%, calculated for 2980 and 4545 unique observed reflections (|F_o| 4_F), and wR₂ of 9.0 and 14.1% for all data, respectively. Intensity data were collected at room temperature using MoK radiation and a CCD-based area detector. Uranospathite and AlUP contain the autunite-type sheet with composition [(UO₂)(PO₄)]^–, which involves the sharing of equatorial vertices of uranyl square bipyramids with phosphate tetrahedra. In both uranospathite and AlUP, aluminum occurs as isolated Al(H₂O)₆ octahedra, which are held in the interlayer by a complex network of H-bonding involving an additional eight and ten symmetrically independent H₂O groups, respectively, some of which form square-planar sets both above and below the Al octahedra. The arrangement of the interlayer H₂O groups in uranospathite leads to tilting of the Al octahedra, and prevents the structure from being centrosymmetric. The presence of F in both uranospathite and its As analogue, arsenuranospathite, was confirmed by qualitative wavelength-dispersion X-ray emission spectroscopy and appears to be necessary to maintain electroneutrality in the crystals studied. Charge balance is maintained in the synthetic aluminum uranyl phosphate hydrate AlUP by partial occupancy of the Al position.

Keywords: uranospathite, arsenuranospathite, autunite, uranyl phosphate, uranyl arsenate, fluorine, crystal structure.