Stable isotope signatures of hydration water in secondary mineralization on UO₂

Hydrated secondary mineralization readily forms on the surface of UO₂ particles exposed to humidity in an oxidizing environment. The oxygen stable isotope composition of the secondary uranium oxide may reflect that of the water vapor, as well as the hydrogen and oxygen stable isotopic composition of the mineral hydration water. The geospatial organization of δ²H and δ¹⁸O values of atmospheric humidity and precipitation is increasingly well understood, which suggests that the hydrogen and oxygen stable isotopes in secondary mineral hydration water may yield information on the environment in which the mineralization formed. UO₂ powders were exposed to air with constant 30%, 61%, and 91% relative humidity, and constant H and O stable isotope composition. Aliquots were sampled from the UO₂ materials at intervals of 1-10 days through the total humidity exposure duration of 180 days. Scanning electron microscopy, transmission electron microscopy, and x-ray diffraction analysis of the humidity-exposed UO₂ indicates that schoepite/metaschoepite [(UO₃)•2H₂O] secondary phases had formed on the underlying UO₂. The δ²H and δ¹⁸O values of mineral hydration waters were determined by thermogravimetry-enabled isotope ratio infrared spectroscopy (TGA-IRIS). Results indicate that hydrogen in the surface sorbed and mineral hydration waters is exchangeable and thus their δ²H values are difficult to interpret. However, oxygen in these waters is less exchangeable, and thus the oxygen stable isotope composition of the schoepite/metaschoepite hydration water is likely to be related to that of the exposure water vapor. After formation of schoepite/metaschoepite, the δ¹⁸O values of the hydration water in schoepite/metaschoepite does not change in response to changes in exposure vapor δ¹⁸O values, which suggests that the δ¹⁸O values of the hydration water is relatively durable. These findings suggest that information about the origin and storage history of a UO₂ sample may be discernable from δ¹⁸O values of schoepite/metaschoepite hydration water.