Vibrational spectra of magnesite (MgCO3) and calcite-III at high pressures

Raman spectra of magnesite (MgCO₃) and infrared spectra of the highest pressure meta-stable phase of calcite (designated calcite-III) are presented to pressures of 34 and 32 GPa, respectively. We document that both phases persist at 300 K to the highest pressures of these measurements. Our results for magnesite are similar to previous data on dolomite, in that the two lowest frequency Raman-active modes of magnesite (at 329 and 216 cm⁻¹ at zero pressure) diverge as a function of pressure and that the mode Grüneisen parameters of the internal vibrations of the carbonate group are between five and seven times smaller than those of the low-frequency lattice modes. The spectra of calcite-III are consistent with the presence of carbonate environments that are distorted from trigonal symmetry, and this phase plausibly contains more than one distinct crystallographic environment of the carbonate unit.