Raman Study of Barite and Celestine at Various Temperatures

The Raman spectra of barite and celestine were recorded from 25 to 600 °C at ambient pressure and both minerals were stable over the entire temperature range. Most of the Raman bands of barite decreased in wavenumber with increasing temperature with the exception of the ν2 modes and the ν4 band at 616 cm−1, which did not exhibit a significant temperature dependence. These vibrations may be constrained by the lower thermal expansion along the a-axis and b-axis of barite. Similar to barite, most of the Raman bands of celestine also decreased in wavenumber with increasing temperature, with the exception of the ν2 modes and the ν4 band at 622 cm−1, which showed very little variation with increasing temperature. Variations of Raman shift as a function of temperature and FWHM (full width at half maximum) as a function of Raman shift for the main, ν1 modes of barite and celestine show that both minerals have almost identical linear trends with a slope of −0.02 cm−1/°C and −0.5, respectively, which allows for the prediction of Raman shifts and FWHM up to much higher temperatures. The calculated isobaric and isothermal mode Gruneisen parameters and the anharmonicity parameters show that the M–O modes (M = Ba2+ and Sr2+) in barite and celestine exhibit much higher values of both mode Gruneisen parameters and anharmonicity than the SO4 tetrahedra. This indicates that the S–O distances and S–O–S angles are less sensitive to pressure and temperature increase than the M–O distances in the structure. Furthermore, the generally higher anharmonicity in celestine is due to the smaller size of the Sr2+ cation, which causes the celestine structure to be more distorted than the barite structure.