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


Phase Relationship of Ca2Mg3Al2Si6Al2O22(OH)2 -Ca2Mg3Fe2^3+Si6Al2O22(OH)2 Join at High Temperature and High Pressure : The Stability of Tschermakite

Oba, Takanobu

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The join tschermakite(Ts) ― ferri-tschermakite(Fts) was studied at temperatures between 750 and 1000℃ under water vapor pressures of 5 to 24 kb. Tschermakite is stable up to 800℃ between approximately 7 and 17 kb. Run products at pressures higher than 17 kb include amphibole associated with clinopyroxene + garnet + quartz + vapor. With increasing temperature at vapor pressures more than about 18 kb, amphibole breaks down to the assemblage, garnet + clinopyroxene + orthopyroxene + quartz + vapor. At lower pressures, the equilibrium mineral assemblage consists of anorthite + orthopyroxene + forsterite + vapor. At 950℃ and 15 kb, no amphibole was found, and run products contain clinopyroxene + orthopyroxene + quartz + vapor. At 1000℃ and 12 kb, a liquid phase is found. No pure ferri-tschermakite could be synthezised at temperatures between 800 and 1000℃, and pressures of 7 to 20 kb. Amphibole, associated with clinopyroxene + orthopyroxene + magnetite + hematite + vapor, is stable up to 850℃ at pressures between 8 and 19 kbar. The solubility limit of Ca2Mg3Fe2^3+Si6Al2O22(OH)2 in tschermakite is 35 mole % at 850℃ and 12 kb.