Raman spectroscopic study of non-stoichiometry in cerianite from critical zone

Cerianite (CeO2) is one of the key minerals controlling the behavior of rare earth elements (REE) in supergene environment, yet little is known about the composition and structure of the mineral. We present Raman spectroscopy and compositional data on regolith cerianite from carbonatite-related supergene REE deposits. Cerianite showed a low content of impurities, distinguishing from high-temperature environments, where the mineral is characterized by elevated concentrations of lanthanides other than Ce. The Raman spectra are characterized by the shift of the dominant peak located at 464-465 cm(-1) in stoichiometric CeO2 to the lower wavenumbers of 450-455 cm(-1), the widening of this peak with the increasing shift, and the presence of additional peaks at 225-230, 284, and 604-613 cm(-1). The independence of the main spectral characteristics from the laser wavelength (tested by three lasers: 488, 532, and 633 nm) suggests that the features are genuine and not produced by REE luminescence. The anhydrous composition of supergene cerianite is indicated by the absence of Raman bands in the water-bearing region of the spectra (2700-3200 cm(-1)). The spectral features of supergene cerianite, combined with the compositional data, suggest non-stoichiometry in the mineral and the presence of Ce3+ with oxygen vacancies in the mineral structure. Our observations demonstrate the efficiency of Raman spectroscopy for the identification of the non-stoichiometric composition of cerianite and its common occurrence in the critical zone.