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Open AccessArticle

Synchrotron Diffraction Study of the Crystal Structure of Ca(UO2)6(SO4)2O2(OH)6·12H2O, a Natural Phase Related to Uranopilite

1
Nanomaterials Research Centre, Kola Science Centre, Russian Academy of Sciences, Fersmana 14, 184209 Apatity, Russia
2
Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University, University Emb. 7/9, 199034 St. Petersburg, Russia
3
Musée cantonal de géologie, University of Lausanne, Anthropole, 1015 Lausanne, Switzerland
4
School of Earth, Atmosphere and the Environment, Monash University, Clayton, Vic 3168, Australia
5
Swiss-Norwegian Beamline, European Synchrotron Light Source, 38043 Grenoble CEDEX 9, France
6
Peter the Great St. Petersburg Polytechnic University, Polytekhnicheskaya 29, 195251 St. Petersburg, Russia
*
Author to whom correspondence should be addressed.
Minerals 2018, 8(12), 569; https://doi.org/10.3390/min8120569
Received: 27 October 2018 / Revised: 30 November 2018 / Accepted: 1 December 2018 / Published: 4 December 2018
(This article belongs to the Special Issue Actinide Mineralogy and Crystallography)
The crystal structure of a novel natural uranyl sulfate, Ca(UO2)6(SO4)2O2(OH)6·12H2O (CaUS), has been determined using data collected under ambient conditions at the Swiss–Norwegian beamline BM01 of the European Synchrotron Research Facility (ESRF). The compound is monoclinic, P21/c, a = 11.931(2), b = 14.246(6), c = 20.873(4) Å, β = 102.768(15), V = 3460.1(18) Å3, and R1 = 0.172 for 3805 unique observed reflections. The crystal structure contains six symmetrically independent U6+ atoms forming (UO7) pentagonal bipyramids that share OO edges to form hexamers oriented parallel to the (010) plane and extended along [1–20]. The hexamers are linked via (SO4) groups to form [(UO2)6(SO4)2O2(OH)6(H2O)4]2− chains running along the c-axis. The adjacent chains are arranged into sheets parallel to (010). The Ca2+ ions are coordinated by seven O atoms, and are located in between the sheets, providing their linkage into a three-dimensional structure. The crystal structure of CaUS is closely related to that of uranopilite, (UO2)6(SO4)O2(OH)6·14H2O, which is also based upon uranyl sulfate chains consisting of hexameric units formed by the polymerization of six (UO7) pentagonal bipyramids. However, in uranopilite, each (SO4) tetrahedron shares its four O atoms with (UO7) bipyramids, whereas in CaUS, each sulfate group is linked to three uranyl ions only, and has one O atom (O16) linked to the Ca2+ cation. The chains are also different in the U:S ratio, which is equal to 6:1 for uranopilite and 3:1 for CaUS. The information-based structural complexity parameters for CaUS were calculated taking into account H atoms show that the crystal structure of this phase should be described as very complex, possessing 6.304 bits/atom and 1991.995 bits/cell. The high structural complexity of CaUS can be explained by the high topological complexity of the uranyl sulfate chain based upon uranyl hydroxo/oxo hexamers and the high hydration character of the phase. View Full-Text
Keywords: uranium; crystal structure; sulfate; minerals; uranopilite; synchrotron radiation; structural complexity uranium; crystal structure; sulfate; minerals; uranopilite; synchrotron radiation; structural complexity
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Krivovichev, S.V.; Meisser, N.; Brugger, J.; Chernyshov, D.V.; Gurzhiy, V.V. Synchrotron Diffraction Study of the Crystal Structure of Ca(UO2)6(SO4)2O2(OH)6·12H2O, a Natural Phase Related to Uranopilite. Minerals 2018, 8, 569.

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