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

  • Peter C. Burns (a1), Robert J. Finck (a2), Frank C. Hawthorne (a3), Mark L. Miller (a4) and Rodney C. Ewing (a4)...

Abstract

Ianthinite, [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, a 7.178(2), b 11.473(2), c. 30.39(1) Å, V 2502.7 Å3, Z = 4, space group P2 1 cn, has been solved by direct methods and refined by least-squares methods to an R index of 9.7 % and a wR index 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. The Urφ5 and U4+φ6 (φ: O2-, OH, H2O) polyhedra link by sharing edges to form two symmetrically distinct sheets at z z ≈ 0.0 and z ≈ 0.25 that are parallel to (001). The sheets have the β-U3O8 sheet anion-topology. There are five symmetrically distinct H2O groups located at z ≈ 0.125 between the sheets of Uφn polyhedra, and the sheets of Uφn polyhedra 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 Uφn polyhedra in the structure of ianthinite.

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