The crystal structure and crystal chemistry of uranosphaerite, Bi(UO2)O2OH

The crystal structure of uranosphaerite, Bi(UO2)O2OH, has been solved and refined using single-crystal X-ray-diffraction data collected from synthetic and natural crystals. The structure of natural uranosphaerite from the Clara barite and fluorite mine, Black Forest, Germany, was refined to an agreement index (RI) of 2.86%, calculated for 1194 unique observed reflections (\F-o\ > 4sigma (F)). Uranosphaerite is monoclinic, space group P2(1)/n, a 7.559(2), b 7.811(2), c 7.693(2) Angstrom, beta 92.88(3)degrees, V 453.6(2) Angstrom(3). Crystals of synthetic uranosphaerite were obtained by mild hydrothermal techniques; the structure was refined to an R-1 of 5.84%, calculated for 1590 unique observed reflections (\F-o\ > 4sigma (F)). The synthetic crystal has slightly smaller unit-cell dimensions, a 7.540(3), b 7.801(3), c 7.674(3) Angstrom, beta 92.948(7)degrees, V450.75(1) Angstrom(3). The structure consists of one symmetrically distinct U6+ cation that is bonded to two atoms of oxygen, forming a nearly linear uranyl ion [(UO2)(2+), designated Ur]. The uranyl ion is further coordinated by five O and OH anions, resulting in a pentagonal bipyramid capped by the oxygen atoms of the uranyl ion. The single symmetrically distinct Bill cation in the structure is coordinated by four anions arranged at the base of a pyramid that is capped by Bi3+, consistent with the presence of a tone pair of electrons on Bi3+. The Bi3+ cation is weakly bonded to two additional O-Ur atoms. Urphi(5) (phi: O, OH) pentagonal bipyramids share an equatorial edge, giving a dimer. Pairs of Biphi(4) polyhedra also share an edge, and the resulting dimers are linked to the dimers of uranyl polyhedra, forming a heteropolyhedral sheet that is parallel to (101). Adjacent sheets are linked by weak Bi-O bonds and by weak hydrogen bonds. The sheet is based upon the anion-topology of the francevillite sheet.