Abstract
Enthalpy of solution measurements of synthetic garnet solid solutions on the joins Fe 3Al 2Si 3O 12 (almandine)-Ca 3Al 2Si 3O 12 (grossular) and Fe 3Al 2Si 3O 12-Mg 3Al 2Si 3O 12 (pyrope) have been made in eutectic (Li, Na)BO 2 at 760°C. Garnets were prepared by high pressure, high temperature crystallization of homogeneous glasses and mechanical mixtures of end-member glasses and characterized by X-ray diffraction, microprobe analysis and Mössbauer resonance spectroscopy. Less than one percent of the total iron is ferric. The calorimetry shows that Ca, Fe 2+ enthalpy of mixing in garnet is virtually zero and that Mg, Fe 2+ mixing is substantially non-ideal, with largest positive deviations near the almandine composition. Leastsquares fits to the midpoints of the experimental brackets, including the end-members, give, for (Fe, Ca) 3Al 2Si 3O 12 and (Fe, Mg) 3Al 2Si 3O 12 solid solutions: W H Gr = −9.08 KJ, W H A1 = 13.68 KJ , W H Py = 36.17 KJ, W H A1 = −15.76 KJ where ΔH ex = X 2 Gr X A1 W H A1 + X 2 A1 X Gr W H Gr and similarly for the almandine-pyrope excess enthalpy. These calorimetric results are not predictable from ionic size considerations, which would suggest ideality for Mg, Fe 2+ mixing and considerable non-ideality for Ca, Fe 2+ mixing. They are, however, consistent with predictions based on analysis of natural garnet occurrences. Apparently, subtle effects of X-site volume and distortion in garnet solid solutions produce important energetic effects not currently predictable from crystal chemistry. The present calorimetry, in conjunction with experimental phase equilibrium work, indicates that excess entropy in both solid solutions is very small.
Published Version
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