AN APPRAISAL OF ENDMEMBER ENERGY AND MIXING PROPERTIES OF RARE EARTH GARNETS - CORRELATION ALGORITHMS FOR IONIC SPECIES AND EQUATIONS OF STATE PREDICTIONS TO 5KB AND 1000°C

Abstract

The thermodynamic properties of rare earth aluminum (REE3Al5O12), iron (REE3Fe5O12), and gallium (REE3Ga5O12) garnets are assessed by means of critical evaluation of the existing experimental data and thermodynamic treatment of their vibrational, static and volumetric properties.The mixing properties of the various REE garnet components are calculated from these endmember thermodynamic data and the interionic static potential model developed earlier for major silicate garnet components. These results permit evaluation of the solubility behavior for REE in natural phases. The results suggest that the mixing of REE garnet components at trace level (i.e., below about 102 ppm) with major silicate garnet components is virtually ideal, but strong positive deviations, associated with generalized polyhedral distortion of the host phase at higher concentration level of the solute, prevent bulk REE solubility from exceeding a few hundreds ppm.The computed rare earth element fractionations between garnet and supercritical fluid at various P, T conditions put in evidence the main control operated by the intrinsic energy properties of REE-garnet end members in determining the light REE/heavy REE (LREE/HREE) fractionation observed in natural garnet specimens.