The reaction enstatitess+sillimanite ⇌ sapphiriness+quartz in the system MgO-Al2O3-SiO2 |
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| Authors: | N D Chatterjee W Schreyer |
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| Institution: | (1) Institut für Mineralogie, Ruhr-Universität, Bochum, West-Germany |
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| Abstract: | The univariant pressure temperature curve of the reaction aluminous enstatite solid solution+sillimanite sapphirine solid solution+quartz was determined experimentally in the pressure range between 12 and 20 kb. It is defined by four reversals at 15 kb, 1140°+10°C; at 1190°C, 15.9±1 kb; at 1300°C, 17.2±1 kb; and at 1400°C, 18±1 kb. Among the coexisting phases the Al-content of the enstatites increases strongly, with rising temperatures and pressures, up to values approaching that of pyrope composition, whereas the Al-content of the sapphirine solid solution appears to increase only slightly. Concomitantly, the sillimanites, most probably of invariant composition, exhibit growing Al/Si-disorder. These compositional and structural variations, in addition to the changing stoichiometry of the reaction equation, cause the progressively decreasing positive dP/dT slope of the equilibrium curve. — The assemblage sapphirine + quartz found in natural granulites is indicative of conditions of water pressure much lower than total pressure. |
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