The Crystal Structure of Ianthinite, a Mixed-Valence Uranium Oxide Hydrate

Abstract

ABSTRACTIanthinite, [U4+2(UO2)4O6(OH)4(H2O)4](H2O)5, is the only known uranyl oxide hydrate mineral that contains U4+, and it has been proposed that ianthinite may be an important Pu4+-bearing phase during the oxidative dissolution of spent nuclear fuel. The crystal structure of ianthinite, orthorhombic,a7.178(2),b11.473(2),c.30.39(1) Å,V2502.7 Å3,Z= 4, space groupP21cn, has been solved by direct methods and refined by least-squares methods to anRindex of 9.7 % and awRindex of 12.6 % using 888 unique observed [ | F | ≥ 5σ | F | ] reflections. The structure contains both U6+and U4+. The U6+cations are present as roughly linear (U6+O2)2+uranyl ions (Ur) that are in turn coordinated by five O2-and OH located at the equatorial positions of pentagonal bipyramids. The U4+cations are coordinated by O2-, OH and H2O in a distorted octahedral arrangement. TheUrφ5and U4+φ6(φ: O2-, OH, H2O) polyhedra link by sharing edges to form two symmetrically distinct sheets atzz ≈ 0.0 and z ≈ 0.25 that are parallel to (001). The sheets have the β-U3O8sheet anion-topology. There are five symmetrically distinct H2O groups located at z ≈ 0.125 between the sheets of Uφnpolyhedra, and the sheets of Uφnpolyhedra are linked together only by hydrogen bonding to the intersheet H2O groups. The crystal-chemical requirements of U4+and Pu4+are very similar, indicating that extensive Pu4+↔ U4+substitution can occur within the sheets of Uφnpolyhedra in the structure of ianthinite.