Uranotungstite, the only natural uranyl tungstate: crystal structure revealed from 3D electron diffraction

Abstract

Uranotungstite is an uranyl-tungstate mineral that was until recently only partially characterized with a formula originally given as (Fe2+,Ba,Pb)(UO2)(2)(WO4)(OH)(4)center dot 12H(2)O and an unknown crystal structure. This mineral has been reinvestigated by electron microprobe analysis coupled with three-dimensional electron diffraction. According to the electron microprobe data, the holotype material from the Menzenschwand uranium deposit (Black Forest, Germany) has the empirical formula (Ba0.35Pb0.27)(Sigma 0.62)[(U6+O2)(2)(W0.986+Fe0.263+square(0.75))O-4.7(OH)(2.5)(H2O)(1.75)](H2O)(1.67) (average of 8 points calculated on the basis of 2U apfu; H2O content derived from the structure). According to the precession-assisted 3D ED data, holotype uranotungstite from Menzenschwand is monoclinic, P2(1)/m, with a = 6.318(5) angstrom, b = 7.388(9) angstrom, c = 13.71(4) angstrom, beta = 99.04(13)degrees, and V = 632(2) angstrom(3) (Z = 2). The structure refinement of the 3D ED data using the dynamical approach (R-obs = 0.0846 for 3287 independent observed reflections) provided a structure model composed of heteropolyhedral sheets. A beta-U3O8-type sheet of idealized composition [(UO2)(2)W6+Fe0.253+square O-0.75(4.75)(OH)(1.5)(H2O)(1.75)](0.25-) is composed of UO7 polyhedra linked by (W,Fe)O-5 polyhedra in which the W:Fe ratio is variable as well as the bulk occupancy of this site; the W site may also host a minor proportion of Cu, Mg, or V. In uranotungstite, the interlayer spaces between adjacent U-W-O sheets host water on one side and, on the other side, a partially occupied cation site mostly occupied by Ba and, to a lesser extent, Pb, as well as a partially occupied H2O site. This work is the first structural description of a natural uranyl-tungstate mineral and confirms the great structural and chemical flexibility of beta-U3O8 type of sheets.