Stability of pyrope-quartz in the system MgO-Al2O3-SiO2 |
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| Authors: | B J Hensen E J Essene |
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| Institution: | (1) Department of Geophysics and Geochemistry, Australian National University, Australia;(2) Present address: Department of Geology, University of Michigan, Ann-Arbor, Michigan, USA |
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| Abstract: | Pyrope and quartz are stable with respect to aluminous enstatite and sillimanite at 1400 °C, 20 kb and at 1100 °C, 16 kb. The phase boundary limiting the coexistence of pyrope and quartz towards lower pressures is probably slightly curved. A slope of 15 bars/°C at 1400° and of 10 bars/°C at 1000 °C has been estimated from the experimental data. Between 1050 and 1100 °C the curve is intersected by the kyanite-sillimanite phase boundary. The calculated slope of the reaction aluminous enstatite + kyanite  pyrope + quartz is negative (ca. 18–25 bars/°C). The existence of a negative slope has been demonstrated experimentally. Experimental evidence indicates that the assemblage aluminous enstatite and sillimanite is metastable with respect to sapphirine + quartz at high temperature. The invariant point involving the phases pyrope-sapphirine-aluminous enstatite-sillimanite-quartz is estimated to occur at 1125°±25 °C and 16±1 kb. A model phase diagram for the silicasaturated part of the system MgO-Al2O3-SiO2 has been constructed. The position of three invariant points in this system has been estimated on the basis of presently available data. |
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