Effects of CO2 on the phase behavior of the enstatite–forsterite system at high pressures and temperatures

The effect of CO2 on the various phase transitions in the enstatite (MgSiO3)–forsterite (Mg2SiO4) system observed at high pressures and high temperatures is investigated on the basis of the available experimental results and the zero-pressure density data for all the phases. Except for the coexistence with magnesite and stishovite, the high-pressure phase transition scheme for the MgSiO3–CO2 system remains very much the same as for pure enstatite when CO2 <30.48 wt.%. The transformation scheme for the Mg2SiO4–CO2 system, however, becomes rather complicated when minute amounts of CO2 are present. Not only the ordinary high-pressure modifications of Mg2SiO4 (e.g., β-phase and spinel modification) are present, but those of MgSiO3 and SiO2 plus magnesite are also present in the Mg2SiO4–CO2 system. It has also been found that all the high-pressure phase transitions known in MgSiO3 should not occur when CO2 ≥30.48 wt.%, and that all those known in Mg2SiO4 should not occur when CO2 ≥38.48 wt.%. With the presence of a small amount of CO2, the number of high-pressure phase transitions in the Mg2SiO4–CO2 system is about twice as that found in forsterite, and the increase in density associated with each transition in both forsterite and enstatite is reduced.