Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XVIII, No.3


Phase Equllibria in the System MgSiO3-Al2O3-Fe2O3 at high Temperatures and Pressures, with Special Reference to the Solubility of Al2O3 and Fe2O3 in Enstatite

Arima, Makoto

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The phase equilibria in the system MgSiO3-Al2O3-Fe2O3 were determined experimentally in the temperature and pressure range 800 - 1300℃ and 8 - 16 Kb. The following phase assemblages were encountered, the single phase of enstatite solid solution(ss), enstatitess+hematite, enstatitess+sapphiriness+quartz, enstatitess+spinelss+quartz, enstatitess+spinelss+hematite+quartz. The solubility of Al2O3 and Fe2O3 in enstatite was determined experimentally. The solubility of MgFe3+AlSiO6 in enstatite by the substitution MgSi = Fe3+Al, increases with increasing temperature and decreasing pressure. It is confirmed that Al2O3 is incorporated in enstatite by the substitution MgSi = AlAl as well as MgSi = Fe3+Al. The solubility of Al2O3 in enstatite increases with increasing Fe2O3 content in it at constant temperature and pressure. The solubility of MgAlSiO6 by the substitution MgSi = AlAl, however, is constant at constant temperature and pressure, indicating the possibility of using the MgAl2SiO6 content of orthopyroxene as an indicator of temperature and/or pressure with taking mineral assemblage into account. Mössbauer spectra measurements were made on some synthetic enstatites. Fe3+ prefers to enter in the octahedral site (MI) rather than the tetrahedral site (T). Small amount of tetrahedral Fe3+ was confirmed indicating the substitution MgSi = Fe3+Fe3+ at high temperatures. Some geological significances of Al2O3 and Fe2O3 contents of orthopyroxenes are discussed. The present experimental results indicate that total Al2O3 contents of orthopyroxene can not be used as an indicat or of temperature and/or pressure, but Al2O3 content in MgAl2SiO6 should be used.