Znucalite, the only known zinc uranyl carbonate: Its crystal structure and environmental implications
| Authors | |
|---|---|
| Year of publication | 2024 |
| Type | Article in Periodical |
| Magazine / Source | American Mineralogist |
| MU Faculty or unit | |
| Citation | |
| web | https://doi.org/10.2138/am-2023-8956 |
| Doi | http://dx.doi.org/10.2138/am-2023-8956 |
| Keywords | Znucalite; uranyl carbonate; crystal structure; 3D electron diffraction; Rietveld refinement; conditions of formation; uranium immobilization |
| Description | 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)4H(2)O, 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) & Aring;, b = 6.259(1) & Aring;, c = 25.355(1) & Aring;, beta = 101.13(1)degrees, and V = 1669.54(9) & Aring;(3). 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 [Zn-10(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)(Sigma 10.00) Ca-0.83(UO2)(0.80)[(CO3)(3.96)(SO4)(0.04)](Sigma 4.00)(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 UO22+ 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. |
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