HIGH-PRESSURE PHASE-TRANSFORMATIONS IN THE SYSTEM CASIO3-AL2O3

Phase assemblages for five selected compositions in the system CaSiO3-Al2O3 have been investigated in the pressure range 100-300 kbar and at about 1000°C in a diamond-anvil press coupled with laser heating. At pressures below about 250 kbar, the assemblage of grossularite plus corundum is stable for compositions containing more than 25 mole% Al2O3. Above about 250 kbar, phase assemblages for the latter compositions are truncated by those in the join CaAl2O4-SiO2. Garnet solid solutions are stable between about 10 and 25 mole% Al2O3. Grossularite transforms to a new tetragonal form at pressures greater than about 250 kbar, but the stability field for the garnet solid solutions extends to pressures up to about 300 kbar. The perovskite modification appears to be stable at pressures above about 150 kbar, but is probably limited to nearly pure CaSiO3 composition. Phase behaviour for calcium-bearing silicates or aluminosilicates in the lower mantle are apparently more complicated than was suggested earlier. © 1979.