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The structure of hügelite, an arsenate of the phosphuranylite group, and its relationship to dumontite

Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana, 46556, USA

^* E-mail: alocock{at}nd.edu

The crystal structure of hügelite, Pb₂[(UO₂)₃O₂(AsO₄)₂](H₂O)₅, monoclinic, space group P2₁/m, a = 31.066(3) Å, b = 17.303(2) Å, c = 7.043(1) Å, ß = 96.492(2)°, V = 3761.6(1) ų, Z = 8, D_calc = 5.74 g/cm³, was solved by direct methods using data from a crystal twinned by pseudo-merohedry, and was refined by full-matrix least-squares techniques on the basis of F² to agreement indices R1 of 3.3% calculated for 5519 unique observed reflections (|F_o| 4_F), and wR₂ of 6.7% for all data. Intensity data were collected at room temperature using Mo-K radiation and a CCD-based area detector. Hügelite is a member of the phosphuranylite group and is the arsenate counterpart of dumontite. The sheets of uranyl pentagonal and hexagonal bipyramids and arsenate tetrahedra in hügelite are oriented parallel to (100), and the interlayer contains four symmetrically independent Pb atoms, each of which is coordinated by two oxygen atoms from uranyl ions, two oxygen atoms from arsenate tetrahedra, and three symmetrically distinct H₂O groups. The unit-cell volume is four times larger than that previously reported for hügelite, or expected by comparison to dumontite, Pb₂[(UO₂)₃O₂(PO₄)₂](H₂O)₅; the larger cell probably results from the accommodation of the larger As atoms (relative to P) in the structure, and consequent subtle variations in the coordination geometries of the U and Pb positions.

KEYWORDS: hügelite, phosphuranylite, uranyl arsenate, twin, crystal structure

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