THERMODYNAMIC PROPERTIES AND ISOTOPIC FRACTIONATION OF CALCITE FROM VIBRATIONAL SPECTROSCOPY OF 18O-SUBSTITUTED CALCITE

Abstract

The infrared and Raman spectra of CaCO3 calcite substituted with 80% 18O have been recorded. A detailed mode assignment is proposed for all the observed bands, including combinations and overtones. These data are used to propose a simplified model of the vibrational density of states (VDOS) from which the specific heat, the entropy, and the light-temperature equation of state are calculated. Excellent agreement between calculated and measured values of the thermodynamic properties is obtained when measured vibrational mode anharmonicity is included in the calculations. The model can be used to infer the properties of calcite at high pressures (≤3 GPa) and temperatures (≤ 1200 K). The observed frequency shifts induced by 18O substitution in both IR and Raman spectra are used to construct the VDOS of CaC18O3 calcite. The reduced partition function of calcite is then calculated and the effects of anharmonicity are discussed. Finally the effect of pressure on the reduced partition function is calculated and is shown to be appreciable even at high temperatures.