High-temperature Raman and FTIR study of aragonite-group carbonates

In situ high-temperature Raman and Fourier transform infrared spectra were measured for aragonite, strontianite, cerussite, and witherite at ambient pressure. The orthorhombic to trigonal phase transitions were observed by the vibrational spectra for aragonite and witherite, at the temperatures of 773 and 1150 K, respectively. The isobaric mode Gruneisen parameters (γiP), derived from this study, are compared with the isothermal mode Gruneisen parameters (γiT), calculated from the reported high-pressure measurements. The γiP and γiT parameters range from 0.46 to 3.43 for the lattice vibrational modes, whereas they are smaller than 0.4 for the internal vibrational modes of the CO3 group, consistent with the CO3 group serving as rigid bodies in the crystal structure. At high temperatures, the γiP parameters for in-plane and out-of-plane bending modes are systematically smaller than those for asymmetric and symmetric stretching modes of CO3, implying that the O–C–O angles are even less sensitive to temperature than the C–O bond lengths. The intrinsic anharmonicities are also evaluated. The averaged anharmonic modes (ai_avg) are positive for cerussite, but negative for aragonite, strontianite and witherite. The intrinsic anharmonicity has quite different contributions to the equation of state and thermodynamic properties of cerussite, compared with other carbonate minerals, at the high temperatures and high pressures of mantle conditions.