Hiroshi Kojitani

Abstract Drop-solution enthalpies of Mg2SiO4 wadsleyite and SiO2 stishovite were determined to be 141.38 ± 1.13 and 4.05 ± 0.38 kJ·mol-1, respectively, by performing drop-solution calorimetry with lead borate solvent at 978 K. Isobaric heat capacity of Mg2SiO4 wadsleyite was also measured using differential scanning calorimetry in the temperature range of 300−820 K. In addition, their self-consistent thermoelastic parameters of thermal expansivity (α), isothermal bulk modulus at the standard state (KT0) and its temperature derivatives [(∂KT0/∂T)P] were reassessed by combining the least squares fitting of a third-order Birch−Murnaghan equation of state with Grüneisen relation equation α = γth,0CV/(KT0V), together with those for Mg2SiO4 forsterite, Mg2SiO4 ringwoodite and SiO2 coesite, where isochoric heat capacity (CV) was calculated using the Kieffer model and thermal Grüneisen parameter at the standard state (γth,0) was constrained from measured α data. Obtained thermodynamic parameters were used to calculate forsterite−wadsleyite and wadsleyite−ringwoodite phase boundaries in Mg2SiO4 and coesite−stishovite phase boundary in SiO2. Finally, the present self-consistent thermodynamic datasets were applied to thermodynamic calculations of phase boundaries in the MgSiO3 system among [ringwoodite + stishovite], [wadsleyite + stishovite] and akimotoite by varying the standard enthalpies of formation for ringwoodite and stishovite within their uncertainties. The calculation results suggest that the stability field of the MgSiO3 akimotoite phase spreads to lower pressure region by 2−4 GPa than what have been accepted so far.

Abstract High‐temperature calorimetry (HTC) originated in the 20th century as a niche method to enable measurements not easily accomplished with acid solution calorimetry, combustion calorimetry, vapor pressure, or EMF methods. Over time, HTC has evolved into a versatile approach to accurately quantify formation, phase transition, surface and interfacial enthalpies of a wide range of materials including minerals and refractory inorganic compounds. This evolution has been the result of numerous adjustments to experimental setups and procedures, followed by rigorous testing. The commercial availability and the scientific success of this technique have led to an increase in the number of laboratories applying HTC. However, the knowledge acquired by researchers over the past 70 years is scattered throughout the literature or only available as laboratory internal documentation and personal experience. This publication is a collaborative effort among several leading HTC laboratories to summarize and unify current state‐of‐the‐art HTC techniques and procedures. The text starts by summarizing various HT techniques that are commonly used for readers with an interest in HTC in general. It is then directed toward HTC users and includes a brief section on data evaluation procedures as well as a comprehensive compilation of reference data utilizing molten sodium molybdate and lead borate solvents. Finally, for experienced HTC users, an in‐depth discussion of some common difficulties and a discussion of uncertainties are presented.