Chemical variability in vyacheslavite, U(PO4)(OH): Crystal-chemical implications for hydrous and hydroxylated U4+, Ca, and REE phosphates
| Authors | |
|---|---|
| Year of publication | 2022 |
| Type | Article in Periodical |
| Magazine / Source | American Mineralogist |
| MU Faculty or unit | |
| Citation | |
| Web | https://pubs.geoscienceworld.org/msa/ammin/article-abstract/107/1/131/610326/Chemical-variability-in-vyacheslavite-U-PO4-OH?redirectedFrom=fulltext |
| Doi | http://dx.doi.org/10.2138/am-2021-7875 |
| Keywords | Vyacheslavite; crystal structure; chemical composition; electron-diffraction tomography; miscibility; rhabdophane; uranium deposits |
| Description | 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. |
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