Vibrational spectroscopy of synthetic analogues of ankoleite, chernikovite and intermediate solid solution

• Synthetic analogues of ankoleite, chernikovite and intermediate sample were characterized by Raman and FTIR spectroscopies
• Uranyl stretching modes wavenumbers were correlated to bond length using empirical model
• Local symmetry breaks of PO4 groups were evidenced and linked to the alternation of H2O, K+ and H3O+ in the interlayer space
• Along with H3O+, the presence of complex H5O2+ units was pointed out in FTIR spectra

Ankoleite (K(UO2)PO4·nH2O), chernikovite (H3O(UO2)PO4·nH2O) and intermediate solid solutions are frequently encountered in the uranium ores that result from the alteration of uranium primary minerals. This paper reports a thorough FTIR and Raman study related to synthetic analogues for these minerals. First, the vibration bands associated to the UO22 + uranyl ion were used to calculate the U = O bond length which appeared in good agreement with the data coming from PXRD. Then, the examination of the phosphate vibration modes in both sets of spectra confirmed the general formulation of the samples and ruled out the presence of hydrogenphosphate groups. Finally, the presence of H2O as well as protonated H3O+ and/or H5O2+ species was also pointed out, and could be used to clearly differentiate the various phases prepared. Vibrational spectroscopy then appeared as an efficient method for the investigation of such analogues of natural samples. It should be particularly relevant when identifying these phases in mineral ores or assemblies.

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