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

The infrared and Raman spectra of CaCO3 calcite substituted with 80% O -18 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 s tates (VDOS) from which the specific heat, the entropy, and the high-t emperature equation of state are calculated. Excellent agreement betwe en calculated and measured values of the thermodynamic properties is o btained when measured vibrational mode anharmonicity is included in th e calculations. The model can be used to infer the properties of calci te at high pressures (less than or equal to 3 Gpa) and temperatures (l ess than or equal to 1200 K). The observed frequency shifts induced by O-18 substitution in both IR and Raman spectra are used to construct the VDOS of (CaCO3)-O-18 calcite. The reduced partition function of ca lcite is then calculated and the effects of anharmonicity are discusse d. Finally the effect of pressure on the reduced partition function is calculated and is shown to be appreciable even at high temperatures.