Synchrotron diffraction study of the crystal structure of Ca(Uo 2 ) 6 (SO 4 ) 2 O 2 (OH) 2 O, a natural phase related to uranopilite

Sergey V. Krivovichev, Nicolas Meisser, Joel Brugger, Dmitry V. Chernyshov, Vladislav V. Gurzhiy

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Abstract

The crystal structure of a novel natural uranyl sulfate, Ca(UO 2 ) 6 (SO 4 ) 2 O 2 (OH) 6 ·12H 2 O (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, P2 1 /c, a = 11.931(2), b = 14.246(6), c = 20.873(4) Å, β = 102.768(15), V = 3460.1(18) Å 3 , and R 1 = 0.172 for 3805 unique observed reflections. The crystal structure contains six symmetrically independent U 6+ atoms forming (UO 7 ) pentagonal bipyramids that share O O edges to form hexamers oriented parallel to the (010) plane and extended along [1–20]. The hexamers are linked via (SO 4 ) groups to form [(UO 2 ) 6 (SO 4 ) 2 O 2 (OH) 6 (H 2 O) 4 ] 2− chains running along the c-axis. The adjacent chains are arranged into sheets parallel to (010). The Ca 2+ 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, (UO 2 ) 6 (SO 4 )O 2 (OH) 6 ·14H 2 O, which is also based upon uranyl sulfate chains consisting of hexameric units formed by the polymerization of six (UO 7 ) pentagonal bipyramids. However, in uranopilite, each (SO 4 ) tetrahedron shares its four O atoms with (UO 7 ) bipyramids, whereas in CaUS, each sulfate group is linked to three uranyl ions only, and has one O atom (O16) linked to the Ca 2+ 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.

Original languageEnglish
Article number569
Number of pages11
JournalMinerals
Volume8
Issue number12
DOIs
Publication statusPublished - 1 Jan 2018

Keywords

  • Crystal structure
  • Minerals
  • Structural complexity
  • Sulfate
  • Synchrotron radiation
  • Uranium
  • Uranopilite

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