CRYSTAL CHEMISTRY OF URANYL MOLYBDATES. I. THE STRUCTURE AND FORMULA OF UMOHOITE

Abstract

The structure of umohoite, [(UO 2 )MoO 4 (H 2 O)](H 2 O), triclinic, space group P 1, a 6.3748(4), b 7.5287(5), c 14.628(1) A, α 82.64(1), β 85.95(1), γ 89.91(1)°, V 694.52(8) A 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 Mo K α 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 U 6+ cations. Each is part of a nearly linear (UO 2 ) 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 Mo 6+ cations are each coordinated by five O atoms and one H 2 O group; the O atoms are arranged at the apices of a square bipyramid, with Mo 6+ –O bond-lengths in the range 1.69 to 1.96 A. The H 2 O groups are located at the apical positions of highly distorted octahedra, with Mo 6+ –H 2 O bond-lengths in the range 2.44 to 2.46 A. 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 Mo 6+ . 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 H 2 O groups, whereas the interlayer at z = 0.5 is not occupied. Sheets are linked together by hydrogen bonding, with bonds bridging between sheets where H 2 O is absent in the interlayer, and to H 2 O groups in the interlayer where they are present.