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American Mineralogist; January 2007; v. 92; no. 1; p. 229-232; DOI: 10.2138/am.2007.2358
© 2007 Mineralogical Society of America
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Letter

Thermochemistry and high-pressure equilibria of the post-perovskite phase transition in CaIrO3

Hiroshi Kojitani*, Asuka Furukawa and Masaki Akaogi

Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan

Correspondence: * E-mail: hiroshi.kojitani{at}gakushuin.ac.jp

To study the ambient analog of the deep mantle MgSiO3 perovskite to post-perovskite phase transition, high-temperature drop calorimetry experiments of perovskite and post-perovskite phases of CaIrO3 system as well as high-pressure phase equilibrium experiments in the CaIrO3 system were made. The enthalpies for dissociation of CaIrO3 (298 K) to CaO + Ir + O2 (1573 K) were 486.7 ± 9.2 kJ/mol for post-perovskite and 454.5 ± 12.5 kJ/mol for perovskite. From the difference between them, the phase transition enthalpy from perovskite to post-perovskite at 298 K is –32.2 ± 15.5 kJ/mol. This gives 2.7 ± 15.6 kJ/mol as formation enthalpy of CaIrO3 perovskite from CaO + IrO2 at 298 K. Using the phase transition enthalpy and volume change of –0.48 ± 0.02 cm3/mol determined in this study, the phase equilibrium boundary is calculated to be P (GPa) = 0.040 T (K) – 67.1. The strongly positive slope agrees with that obtained in high-pressure experiments. This is consistent with a large positive Clapeyron slope of post-perovskite phase transition in MgSiO3 recently reported from experimental and theoretical studies.

Key Words: CaIrO3 • post-perovskite • perovskite • calorimetry • thermodynamics • high pressure




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