Vibrational Analysis of Natural Rhodochrosites by Raman Spectroscopy

Abstract

This study investigates an unusual behavior observed in the Raman spectra of natural rhodochrosites under ambient conditions and varying temperature and pressure. While the typical MnCO3 spectrum is maintained, the relative intensities of libration (L) and translation (T) bands exhibit significant variations, potentially linked to changes in the Mn2+ state within the crystal lattice. Mapping experiments under ambient conditions revealed intensity inversions in lattice modes depending on the sampling region and excitation wavelength. Initially attributed to laser-induced degradation, this phenomenon was ruled out by temperature-dependent Raman spectroscopy, which showed no significant intensity changes even under conditions promoting sample degradation. Further investigations under hydrostatic pressure revealed a decrease in L band intensity, potentially indicating crystal compaction and modified cation–anion interactions. Thermodynamic parameters were calculated using both linear and quadratic fitting methods, demonstrating that while linear fitting provides acceptable results for temperature-dependent analysis, a quadratic approach is crucial for pressure-induced changes, influencing the value of calculated Gruneisen and anharmonicity parameters. This work highlights the sensitivity of Raman spectroscopy to subtle changes within the rhodochrosite structure and provides valuable insights into the intricate interplay of temperature, pressure, and Mn ion configuration in this mineral.