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

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)₄·12H₂O 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_0.35Pb_0.27)_Σ0.62[(U⁶⁺O²)₂(W⁶_0.98+ Fe³_0.26⁺ ◻_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, P21/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) Å₃ (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)₂W⁶⁺Fe³_0.25⁺ ◻_0.75O_4.75(OH)_1.5(H₂O)_1.75]^0.25– is composed of UO₇ polyhedra linked by (W,Fe)O₅ 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. [ABSTRACT FROM AUTHOR]