Rapid Cation Depletion During Electron Microprobe Analysis of Uranium Phosphates

Alkali cation loss during electron microprobe microanalysis (EPMA) is a well-known phenomenon. Our recent work on uranyl phosphates, however, has revealed extraordinarily rapid and extensive depletion of interlayer Cu in metatorbernite [Cu(UO2)2(PO4)2·8H2O]. Minerals in the autunite and meta- autunite groups, e.g. metatorbernite, are important sinks for uranium in oxidizing environments such as contaminated soils at Hanford, WA and Oak Ridge, TN. EPMA is a valuable tool for investigating the fine-grained minerals in soils and sediments, allowing for imaging and chemical analysis of the same grains. However, the autunites and meta-autunites are hydrous and beam sensitive, making EPMA challenging. Upon exposure to vacuum and the electron beam, water is lost. In addition, exposure to the electron beam results in the loss of interlayer cations from the analysis volume. We present results from the Cu, Ca, and Ba-bearing meta-autunite minerals, metatorbernite, meta-autunite, and meta-uranocircite, respectively. Copper is lost much more rapidly than calcium, which is, in turn, lost more rapidly than barium. While the migration of alkalis during EPMA was first described more than forty years ago, to our knowledge this is the first report of the migration of a transition metal. Volatile element correction schemes that use simple exponential functions to fit dropping count rates through time are insufficient for the accurate calculation of metatorbernite compositions. This result has implications for the analysis of metatorbernite found at contaminated sites such as Hanford. More fundamentally, it has the potential to enhance our understanding of the bonding environment of Cu in metatorbernite.