Phase Diagram and Thermal Expansion of orthopyroxene-, clinopyroxene- and ilmenite-structured MgGeO3

Simon Hunt, James R Santangeli, David P Dobson, Ian G Wood

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The MgGeO3 system is a low-pressure analogue for the Earth forming (Mg,Fe)SiO3 system and exhibits recoverable orthopyroxene, clinopyroxene and ilmenite structures below 6 GPa. The pressure-temperature conditions of the clinopyroxene to ilmenite phase transition are reasonably consistent between studies, having a positive Clapeyron slope and occurring between 4 and 7 GPa in the temperature range 900-1600 K. There are, though, significant discrepancies in the Clapeyron slope of the orthopyroxene to clinopyroxene phase transition in existing works which also disagree on the stable phase at ambient conditions. The most significant factor in these differences is the method used; high-pressure experiments and thermophysical property measurements yield apparently contradicting results. Here, we perform both high pressure and temperature experiments as well as thermal expansion measurements to reconcile the measurements. High pressure and temperature experiments, yield a Clapeyron slope of −1.0%&.' ().& MPa/K for the MgGeO3 orthopyroxene-clinopyroxene phase transition, consistent with previous high pressure and temperature experiments. The MgGeO3 orthopyroxene-clinopyroxene-ilmenite triple point is determined to be at 0.98 GPa and 752 K, with the ilmenite phase stable at ambient conditions. The high temperature (> 600 K) thermal expansion of the clinopyroxene phase is greater than that of the other phases. Debye-Grüneisen relationships fitted to the volume temperature data give Debye temperatures for the orthopyroxene, clinopyroxene and ilmenite phases of 602(7) K, 693(10) K and 758(13) K and V0 of 897.299(16) Å3, 433.192(10) Å3 and 289.156(6) Å3 respectively. The Clapeyron slopes calculated directly from the Debye-Grüneisen relationships are consistent with previous thermophysical property measurements. The presence of significant anharmonicity and/or formation of defects in the clinopyroxene phase at high temperatures, which is not apparent in the other phases, accounts for the previous contradictions between studies. The inferred increased heat capacity of the clinopyroxene corresponds to an increase in entropy and an expanded phase field at high temperatures.
Original languageEnglish
JournalAmerican Mineralogist
Publication statusAccepted/In press - 14 Dec 2020


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