Crystal chemistry of uranyl molybdates. I. The structure and formula of umohoite

The structure of umohoite, [(UO2)MoO4(H2O)](H2O), triclinic, space group P (1) over bar, a 6.3748(4),b 7.5287(5), c 14.628(1) Angstrom, alpha 82.64(1), beta 85.95(1), gamma 89.91(1)degrees, V 694.52(8) Angstrom(3), has been solved by direct methods and refined by full-matrix least-squares to an agreement index (R1) of 4.6% and a goodness-of-fit of 1.02 for 2456 unique observed reflections collected using MoK alpha X-radiation and a CCD-based detector. The structure differs substantially from models previously proposed in both the coordination of the cations and the connectivity of the structure. The structure contains two non-equivalent U sites that are occupied by U6+ cations. Each is part of a nearly linear (UO2)(2+) uranyl ion (Ur) that is further coordinated by five O atoms arranged at the equatorial corners of pentagonal bipyramids capped by the O-Ur atoms. Two non-equivalent Mo6+ cations are each coordinated by five O atoms and one H2O group; the O atoms are arranged at the apices of a square bipyramid, with Mo6+-O bond-lengths in the range 1.69 to 1.96 Angstrom. The H2O groups are located at the apical positions of highly distorted octahedra, with Mo6+-H2O bond-lengths in the range 2.44 to 2.46 Angstrom. The cation polyhedra link by sharing edges, forming sheets with the uranophane anion-topology, in which the pentagons are populated by uranyl ions and the squares are populated by Mo6+. This is the first mineral structure known that is based upon the uranophane anion-topology with the squares populated by octahedra. The interlayer at z = 0 contains H2O groups, whereas the interlayer at z = 0.5 is not occupied. Sheets are linked together by hydrogen bonding, with bonds bridging between sheets where H2O is absent in the interlayer, and to H2O groups in the interlayer where they are present.