A unique structure of uranyl-carbonate mineral sharpite: a derivative of the rutherfordine topology

摘要

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) ?, V=1691.2 (2) ?3, 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.