Crystal structure of the uranyl-molybdate mineral calcurmolite Ca[(UO2)(3)(MoO4)(2)(OH)(4)](H2O)(similar to 5.0): insights from a precession electron-diffraction tomography study
| Authors | |
|---|---|
| Year of publication | 2020 |
| Type | Article in Periodical |
| Magazine / Source | Journal of Geosciences |
| MU Faculty or unit | |
| Citation | |
| Web | http://www.jgeosci.org/content/jgeosci.297_Steciuk.pdf |
| Doi | http://dx.doi.org/10.3190/jgeosci.297 |
| Keywords | calcurmolite; uranyl-motybdate; crystal structure; modulation; baumoite topology; precession electron diffraction |
| Description | Calcurmolite is a rare supergene U mineral formed during the alteration-hydration weathering of uraninite and hypogene Mo minerals; its structure has remained unsolved owing to a lack of crystal material suitable for conventional structure analysis. Here, single-crystal precession electron-diffraction tomography shows the calcurmolite (Rabejac, France) structure to be modulated; it is triclinic, crystallizing in the super-space group P1(alpha 00)0, with a = 3.938 angstrom, b = 11.26 angstrom, c = 14.195 angstrom, alpha = 84.4 degrees, = 112.5 degrees, gamma = 133.95 degrees and has a modulation vector q = 0.4 a*. Due to the poor quality of diffraction data, only a kinematical refinement was undertaken, although final results were reasonable: R-obs/R-all = 0.3825/0.3834 for 3953/17442 observed/all reflections. The structure of calcurmolite is based upon the infinite uranyl-molybdate sheets with baumoite topology (U : Mo ratio = 1.5) and an intcrlayer of 6-coordinated Ca2+ cations with interstitial H2O (ligands arc apical uranyl O atoms and molecular H2O). Adjacent sheets are linked via Ca-O, as well as H-bonds. The structure formula, based on assumed occupancies in the supercell 5a x b x c, is Ca[(UO2)(3) (MoO4)(2)(OH)(4)](H2O)(similar to 0.5), (for Z = 4). |
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