Abstract
Enthalpies and entropies of transition for the Mg2GeO4 olivine-spinel transformation have been determined from self-consistency analyses of Dachille and Roy's (1960), Hensen's (1977) and Shiota et al.'s (1981) phase boundary studies. When all three data sets are analyzed simultaneously,ΔH 973 andΔS 973 are constrained between −14000 to −15300 J mol−1 and −13.0 to −14.1·J mol−1 K−1, respectively. High-temperature solution calorimetric experiments completed on both polymorpha yield a value of −14046±1366 J mol−1 forΔH 973. Kieffer-type lattice vibrational models of Mg2GeO4 olivine and spinel based on newly-measured infrared and Raman spectra predict a value of −13.3±0.6 J mol−1 K−1 forΔS 1000. The excellent agreement between these three independent determinations ofΔH andΔS suggests that the synthesis runs of Shiota et al. (1981) at high pressures and temperatures bracket equilibrium conditions. In addition, no configurational disorder of Mg and Ge was needed to obtain the consistent parameters quoted. The Raman spectrum and X-ray diffractogram show that little disorder, if any, is present in Mg2GeO4 spinel synthesized at 0.2 GPa and 973–1048 K.
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Ross, N.L., Navrotsky, A. The Mg2GeO4 olivine-spinel phase transition. Phys Chem Minerals 14, 473–481 (1987). https://doi.org/10.1007/BF00628825
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DOI: https://doi.org/10.1007/BF00628825