OPTICAL VIBRATIONAL SPECTRA OF IANTHINITE [U24+(UO2)4O6(OH)4(H2O)4]·5H2O, A RARE MIXED-VALENCE URANIUM OXIDE HYDRATE

Ianthinite ([U₂⁴⁺(UO₂)₄O₆(OH)₄(H₂O)₄]·5H₂O) is a rare mineral phase possessing U in both tetravalent and hexavalent oxidation states in an identical anion topology to technogenic phase U₃O₈, which appears throughout the nuclear fuel cycle. The structural accommodation of multiple U valence states as seen in ianthinite is exceedingly uncommon, and ianthinite is the only known mineral species that contains both U⁴⁺ and U⁶⁺. To gain a more complete understanding of the structural and spectroscopic features associated with this mixed-valence phase and apply this knowledge to materials encountered in the nuclear fuel cycle, we have reexamined the crystal structure of ianthinite, collected Raman and infrared spectra for this material, and assessed its structural and spectroscopic similarity with its oxidation product, schoepite and the topologically identical β-U₃O₈. We find that despite variability in intensity observed in several spectroscopic features, the overall spectra collected for several points in a crystal agglomerate of ianthinite are consistent from a novel data processing and analysis method. We also provide potential spectroscopic assignments for features observed in the Raman in the context of topologically related U minerals and technogenic phases.