Crystal structure of richetite revisited: Crystallographic evidence for the presence of pentavalent uranium

Revision of crystal structure of the rare U-oxide mineral richetite provided crystallographic evidence for the presence of pentavalent U. The structure of richetite, space group P1, a = 12.0919(2), b = 16.3364(4), c = 20.2881(4) Å, a = 68.800(2), b = 78.679(2), g = 76.118(2)°, with V = 3600.65(14) Å3 and Z = 1, was solved by charge-flipping algorithm and refined to an agreement index (R) of 5.6% for 9955 unique reflections collected using microfocus X-ray source. The refined structure, in line with the previous structure determination, contains U-O-OH sheets of the a-U3O8 type (protasite topology) and an interstitial complex comprising Pb2+, Fe2+, Mg2+ cations and molecular H2O. However, the polyhedral geometry, the bond-valence sum incident at one U site within the sheet (U17) together with charge-balance requirements, indicate that U17 site is occupied by U5+. The U17F7 (F: O, OH) polyhedra is distorted, with two shorter U–O bond-lengths (~2.01 Å), four longer U–O bond-lengths (~2.2 Å) and one, very long U–O bond (2.9 Å). The color of richetite also supports the presence of U5+ in the structure The current results show that a-U3O8 type of sheet can incorporate U5+. Richetite is the third mineral containing pentavalent uranium that occurs in nature.