High–temperature Raman spectroscopic study of CO₂–containing melanophlogite

Abstract

CO₂–containing melanophlogite from Fortunillo, Italy was studied using a micro–Raman spectrometer with the ability to measure the low–frequency region. A very intense and broad feature was found below 100 cm⁻¹. To clarify the origin of this feature in relation with CO₂, heat treatment experiments and in–situ high–temperature Raman measurements were conducted up to 1100 °C. As a result of the heat treatment experiments, nearly CO₂–free melanophlogite was obtained at 950 °C for 6 h. For shorter time duration or lower treatment temperature, CO₂ vibrational Raman peaks (Fermi diad) were still observed, and those peaks were split. The low–frequency feature also reduced its intensity in these degassed samples. For the in–situ study, the intensity of CO₂ Raman peaks started to drop at around 450 °C, and simultaneously the low–frequency feature intensity decreased. The splitting of the CO₂ Raman peaks started from 450 °C, and it was interpreted as redistribution of CO₂ molecules in two distinct cages in the structure. The low–frequency feature completely disappeared at 1100 °C. It was concluded that the low–frequency feature is originated from CO₂ molecules. Librational and translational modes of CO₂ molecules in the cages of melanophlogite would be responsible for the low–frequency feature. The high–temperature Raman spectroscopic study thus provides us insight into CO₂ diffusion in melanophlogite structure.