Investigations of crystal-chemical variability in lead uranyl oxide hydrates. I. Curite

Structures have been refined for 14 single crystals of curite from various localities in the Democratic Republic of Congo, and for one synthetic crystal grown at 220 degrees C using hydrothermal techniques. Single-crystal diffraction data were collected using MoK alpha X-radiation and a CCD-based detector mounted on a Bruker three-circle diffractometer. The crystals have orthorhombic symmetry, space group Pnam, and have similar unit-cell parameters: a 12.53 - 12.58, b 13.01 - 13.03, c 8.39 - 8.40 Angstrom. The structures were refined to agreement indices (R) in the range 3.7 to 7.9%. The structures obtained are in good general agreement with earlier studies; they contain uranyl square bipyramids and uranyl pentagonal bipyramids that share edges and corners to form sheets oriented parallel to (100). There are two symmetrically distinct Pb2+ cations and one H2O group located in the interlayer. On the basis of the structure refinements, the site occupancy of Pb(1) is slightly deficient, ranging from 89 to 100%, whereas the Pb(2) site occupancy ranges from 57 to 63%. Earlier investigators suggested that hydroxyl occurs in the interlayer of the structure and provides the charge-balance mechanism that permits variation of the Pb content. However, the current study indicates that Pb variability in curite is limited, and supports minor variation in the hydroxyl content of the sheet of uranyl polyhedra as the charge-balancing mechanism. On the basis of the structure refinements, the structural formula for curite may be written as Pb3+x(H2O)(2)[(UO2)(4)O4+x(OH)(3-x)](2), Z = 2, with the constituents of the sheets of uranyl polyhedra enclosed in square braces.