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

Author(s)
Gwladys Steciuk, Uwe Kolitsch, Viktor Goliáš, Radek Skoda, Jakub Plasil, Franz Xaver Schmidt
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)4·12H2O 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 (Ba0.35Pb0.27)ς0.62[(U6+O2)2(W60.98+ Fe30.26+ □0.75)O4.7(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 (Robs = 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+Fe30.25+ □0.75O4.75(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.

Organisation(s)
Department of Mineralogy and Crystallography
External organisation(s)
Czech Academy of Sciences, Naturhistorisches Museum Wien (NHM), Charles University Prague, Staatliches Museum für Naturkunde Baden-Württemberg, Masaryk University
Journal
American Mineralogist
Volume
107
Pages
1709-1716
No. of pages
8
ISSN
0003-004X
DOI
https://doi.org/10.2138/am-2022-8112
Publication date
09-2022
Peer reviewed
Yes
Austrian Fields of Science 2012
105116 Mineralogy, 105113 Crystallography
Keywords
ASJC Scopus subject areas
Geochemistry and Petrology, Geophysics
Portal url
https://ucris.univie.ac.at/portal/en/publications/uranotungstite-the-only-natural-uranyl-tungstate-crystal-structure-revealed-from-3d-electron-diffraction(3de38e7e-a6e5-4c36-b86e-e200a192f2dc).html