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Uranotungstite is an uranyl-tungstate mineral that was until recently only partially characterized with a formula originally given as (Fe 2+ ,Ba,Pb)(UO 2 ) 2 (WO 4 )(OH) 4 ·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 (Ba 0.35 Pb 0.27 ) Σ0.62 [(U 6+ O 2 ) 2 (W 6  0.98+  Fe 3  0.26  +  ◻ 0.75 )O 4.7 (OH) 2.5 (H 2 O) 1.75 ](H 2 O) 1.67  (average of 8 points calculated on the basis of 2U apfu; H 2 O content derived from the structure). According to the precession-assisted 3D ED data, holotype uranotungstite from Menzenschwand is monoclinic,  P 21/ 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-U 3 O 8 -type sheet of idealized composition [(UO 2 ) 2 W 6+ Fe 3  0.25  +  ◻ 0.75 O 4.75 (OH) 1.5 (H 2 O) 1.75 ] 0.25–  is composed of UO 7  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 H 2 O site. This work is the first structural description of a natural uranyl-tungstate mineral and confirms the great structural and chemical flexibility of β-U 3 O 8  type of sheets.

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