An Appraisal of Endmember Energy and Mixing Properties of Rare-Earth Garnets

Energetics of rare Earth aluminum (REE3Al5O12), iron (REE3Fe5O12) and gallium (REE3Ga5O12) garnets are assessed through a critical evaluation of all the existing experimental data and a thermodynamic treatment of vibrational, static and volumetric properties of the various substances. Application of the developed thermodynamic data-base, coupled with the interionic static potential model previsouly developed for major silicate garnet end members [1] leads to establish the mixing properties of the various substances with the major isostructural silicate components and to determine the limits of Henry’s law behavior for REE in natural garnets. Based on calculations, mixing of REE granet components at trace level (i.e., below about 102 ppm) with major silicate components is virtually ideal and deviations from ideality become sensible at trace concentrations exceeding 103 ppm. Corresponding deviations from Nernst’s law behavior in garnet/fluid REE equilibria follows exponential trends whose nature is analogous to what has been experimentally observed in silicate/fluid equilibria involving other solid phases and other trace elements. It is finally stressed that the light REE–heavy REE (LREE/HREE) fractionation observed in natural garnet specimens is due to the intrinsic energy properties of the various REE-garnet end members, and not to a structural effect dictated by the carrier, as commonly assumed in literature. References: [1] Ottonello G. et al. (1996) Amer. Mineral, 81, 429–447. [2]