The Mg₂GeO₄ olivine-spinel phase transition

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, ΔH₉₇₃ and ΔS₉₇₃ 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 for ΔH₉₇₃. 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 for ΔS₁₀₀₀. The excellent agreement between these three independent determinations of ΔH and ΔS 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.