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Znucalite is a zinc uranyl-carbonate mineral that was until recently only partially characterized with a formula originally given as Zn12Ca(UO2)(CO3)3(OH)22·4H2O, with an unknown crystal structure and ambiguous symmetry determinations. We have reinvestigated this mineral using three-dimensional electron diffraction (3D ED) and powder X-ray diffraction and revealed for the first time its structural details. Znucalite is unambiguously monoclinic, P21/m, with a = 10.722(2) Å, b = 6.259(1) Å, c = 25.355(1) Å, β = 101.13(1)°, and V = 1669.54(9) Å3. The structure refinement of the 3D ED data using the dynamical approach (Robs = 0.1594 for 3579 observed reflections and 244 parameters) provided the following structure model. Znucalite possesses a layered structure, with a [Zn10(OH)14(CO3)2] double sheet (with Zn2+ both in octahedra and tetrahedra), which is connected to a thick interlayer that hosts U6+, Ca2+, and H2O molecules. The linkage between structural units and the interlayer occurs via the vertices of ZnO4 tetrahedra protruding from the sheet. In the interlayer, differences in ordering between U and Ca take place and likely cause the difficulties encountered during the attempts to solve the structure. The refined structural formula of znucalite, Zn10Ca0.828[UO2]0.828[CO3]4(OH)15.312(H2O)5.484, corresponds well to the composition obtained from the electron-microprobe analyses, (Zn9.84Al0.16)Σ10.00 Ca0.83(UO2)0.80[(CO3)3.96(SO4)0.04]Σ4.00(OH)15.42(H2O)5.48. Raman spectroscopy evidenced the presence of several non-equivalent CO3 groups, as well as OH and H2O. The U-O bond lengths obtained from the stretching frequencies of U O 2 2 + vibrations are in line with the structural model. A discussion on the environmental importance of znucalite is appended, based on geochemical calculations with an estimate of the solubility product for this mineral. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Particularly interesting chemical variability in the U4+ phosphate mineral vyacheslavite from Menzenschwand (Germany) has been discovered and investigated by means of electron-diffraction and micro-chemical methods. Suggested variability comprises the elevated contents of calcium and rare-earth elements (REEs or Ln). Based on the crystal structure refinement from 3D electron diffraction data, the structural formula of Ca-rich vyacheslavite studied is U0.895Ca0.105 PO4(OH)0.790(H2O)0.210. In general, such compositional variability involving Ca2+ can be expressed as U1–x CaxPO4(OH)1–2x(H2O)2x. Based on detailed electron-probe microanalysis, regions extremely enriched in Y and Ln have been discovered, characterized by the contents up to 11 wt% of Y2O3 and ~4.5 wt% of Ln2O3. In addition to the above-mentioned substitution mechanism, substitution involving Y and Ln can be expressed as U4+ + OH– → REE3+ + H2O. Although the structure refinement has not provided direct evidence of H2O in the studied nano-fragments of vyacheslavite, the presence of H2O and its substitution at OH– sites is a reasonable and necessary charge-balancing mechanism. One H atom site was located during structure refinements; however, an additional H-site is only partially occupied and thus was not revealed from the refinement despite the high-quality data. Substitutional trends observed here suggest possible miscibility or structural relationship between vyacheslavite, rhabdophane, and ningyoite that may depend strongly on OH/H2O content, considering that all crystallize under similar paragenetic conditions. [ABSTRACT FROM AUTHOR]