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 (Fe(2+),Ba,Pb)(UO2)(2)(WO4)(OH)(4) · 12H2O 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 (Ba(0.35)Pb(0.27))(Σ0.62)[(U(6+)O2)(2)(W(0.98)(6+)Fe(0.26)(3+)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) Å, b = 7.388(9) Å, c = 13.71(4) Å, β = 99.04(13)°, and V = 632(2) Å(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 β-U3O8-type sheet of idealized composition [(UO2)(2)W(6+)Fe(0.25)(3+)square(0.75)O(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 β-U3O8 type of sheets.