Abstract:
In the present paper, the sysem FeO-MgO-Al2O3-CaO-SiO2 has been studied in an attempt to determine the effects of aluminum and calcium on the stability of the silicate perovskite. A series of synthesized silicate glasses were converted to silicate-perovskites in a diamond cell at approximately 40 GPa and 1 500 ℃. Powder X-ray diffraction analyses of these quenched samples showed that at least 25 mol% Al2O3 was dissolved in perovskite forming (Mg,Al)(Si,Al)O3. The increase in volume of the perovskite phase was regarded as a function of increasing Al2O3 content. The addition of Al2O3 caused a significant increase in the molar volume of MgSiO3-Pv than FeSiO3. Because of the large volume diference between garnet and perovskite, there would be no stadility field for garnet in the lower mantle at depths greater than the 650 km seismic discontinuity. In th study of the compositions En80Wo18Co2 and En73Wo24Co3 at 40 GPa, the run products yielded X-ray diffraction patterns almost identical with those of calcium-free perovskites. The calcium would probably form a Ca-rich Al-bearing silicate perovskites at high pressure and high temperature, but it could not be quenched.
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