The crystal structure of ianthinite, [U-2(4+)(UO2)(4)O-6(OH)(4)(H2O)(4)](H2O)(5): a possible phase for Pu4+ incorporation during the oxidation of spent nuclear fuel

Ianthinite, [U-2(4+)(UO2)(4)O-6(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 = 0.7178(2), b = 1.1473(2), c = 3.039(1) nm, V = 2.5027 nm(3), 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\greater than or equal to 5 sigma\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 Ur phi(5) and U4+ phi(6) (phi: O2-, OH-, H2O) polyhedra link by sharing edges to form two symmetrically distinct sheets at z approximate to 0.0 and z approximate to 0.25 that are parallel to (001). The sheets have the beta-U3O8 sheet anion-topology. There are five symmetrically distinct H2O groups located at z approximate to 0.125 between the sheets of U phi(n) polyhedra, and the sheets of U phi(n) polyhedra are linked together only by hydrogen bonding to the intersheet H2O groups. The crystal-chemical requirements of U4+ and Pu4+ are very similar, suggesting that extensive Pu4+ <-> U4+ substitution may occur within the sheets of U phi(n) polyhedra in the structure of ianthinite. (C) 1997 Published by Elsevier Science B.V.