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 P21cn, 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.