Abstract
The structure of hydrated calcium uranyl carbonate, mineral sharpite, has been solved for the first time. The structure is orthorhombic, space group Cmcm, with the unit-cell parameters a=4.9032 (4), b=15.6489 (11), c=22.0414 (18) Å, V=1691.2 (2) Å3, Z=4. The structure was refined from 858 unique observed reflections to a final R=5.48% (GOF=1.48). The structure is based upon infinite Ca2+-uranyl-carbonate sheets, which are unique among uranyl minerals and inorganic compounds. They result from edge-sharing of triangles, occupied by CO3, and hexagons that are occupied both by UO8 and Ca2+(H2O)3O5. Thus, topology of the sheets is identical to rutherfordine, (UO2)(CO3). Nevertheless Ca2+ is occupying one of the hexagons of the topology in sharpite. Sheets are extremely corrugated, stacked perpendicular to b and linked by H-bonds and Van Der Waals interactions, only. The ideal chemical formula of sharpite is {Ca(H2[3]O)3[(UO2)3(CO3)3.6O0.2]}0, Z=4; however, one of the C sites was found to be only partially occupied. The character of sharpite structure is probably a key to understand its scarcity of occurrences in Nature.
Acknowledgements
Dr. Florias Mees (The Royal Museum for Central Africa, Tervuren, Belgium) is acknowledged for providing access to the sharpite specimen stored in the museum’s collections. Eddy Van Der Meersche (Gent, Belgium) and Pavel Škácha (Příbram, Czech Republic) are thanked for microphotographs of the minerals. This research was supported by the project No. LO1603 under the Ministry of Education, Youth and SportsNational sustainability program I of the Czech Republic.
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The online version of this article offers supplementary material (https://doi.org/10.1515/zkri-2018-2056).
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