Investigations of Crystal-Chemical Variability in Lead Uranyl Oxide Hydrates. Ii. Fourmarierite

Structures have been refined for twelve crystals of fourmarierite from various localities in the Democratic Republic of Congo, and one synthetic crystal. Single-crystal diffraction data were collected using MoKa X-radiation and a CCD-based detector mounted on a Bruker three-circle diffractometer. All crystals have orthorhombic symmetry, space group Bb21m. The natural crystals have similar unit-cell parameters: a 14.00–14.03, b 16.40–16.48, c 14.32–14.38 Å. The parameters for the synthetic crystal, which contains less Pb than the natural crystals, are a 13.938(2), b 16.638(3), c 14.672(2) Å. The structures refined to agreement indices (R1) in the range of 3.6 to 6.2%. The structure of fourmarierite contains uranyl pentagonal bipyramids that share edges and corners to form sheets oriented parallel to (001). There are two distinct Pb2+ cations and eight H2O groups located in the interlayer. On the basis of the structure refinements, the site occupancy of Pb(1) in the natural crystals is typically deficient, ranging from 71 to 100%, whereas the site occupancy of Pb(2) is ~100 % in the crystals studied. In the synthetic crystal the Pb(1) and Pb(2) occupancies are 13 % and 87%, respectively. Increase of the Pb content is correlated with a minor increase in the a unit-cell parameter, as well as large decreases in the b and c unit-cell parameters, and the unit-cell volume. The substitution O2 – $ (OH) – at two anion sites [O(12) and O(15)] in the sheet of uranyl polyhedra provides the charge-balance mechanism associated with interlayer cation variation. The structural formula for fourmarierite may be written as Pb1–x[(UO2)4O3–2x(OH)4+2x](H2O)4, Z = 8, with the constituents of the sheet of uranyl polyhedra enclosed in square braces.