Abstract
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.
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