Raman Spectroscopic Investigation of the Structure-Property Relationships of Uranyl Phosphate and Arsenate Minerals and Synthetic Analogues

Abstract

Uranyl phosphates and arsenates are important materials in environmental remediation and nuclear waste due to their low solubility and are also constituents of economically significant uranium ore deposits. Interestingly, many of the structures in which uranyl phosphates and arsenate minerals crystallize display topological isomerism. Despite the structural similarity often adopted by these mineral groups, significant differences are observed in the Raman spectral responses of uranyl phosphates and arsenates. Recently, a survey of Raman spectroscopic features of all known uranium minerals was conducted which revealed limitations in the available Raman spectra for uranyl minerals with phosphate and arsenate oxyanions relative to other mineral groups. In that report, all spectra within secondary oxyanion groups (SO42-, CO32-, AsO43-, etc.) were used to generate an average spectrum to understand how common spectroscopic features corresponded to crystal chemistry. Here, we include an additional four uranyl arsenate and seven uranyl phosphate minerals or mineral analogues in the spectral averages and discuss differences in spectral features due to secondary oxyanion chemistry (AsO43-, PO43-). Spectral differences between structurally analogous uranyl phosphate and arsenate minerals are discussed to highlight trends in spectroscopic features between the two oxyanion groups. Group averages for structure types exhibited by uranyl phosphate and arsenate minerals—autunite, meta-autunite, phosphuranylite, uranophane, and walpurgite—are compared and their spectroscopic features are interpreted with respect to structural properties from which they originate. Unique features that can be used for identification of unknown minerals are described for these structural groups of uranyl phosphates and arsenates. The presence of fluorescence in uranyl phosphate spectra is discussed and the importance of excitation wavelength in data collection is highlighted. Our work emphasizes the importance of incorporation of diverse structure types in average spectra for mineral groups based on secondary oxyanion chemistry and identifies future directions towards a more complete understanding of the link between the crystallographic features of a mineral and its spectral response.