A PETROGENETIC GRID FOR THE SYSTEM MGO-AL2O3-SIO2

A comprehensive analysis of phase relations in the system MgO-Al2O3-Si O2 (MAS) was carried out using primarily the data from phase equilibri um experiments in MAS spinel and garnet peridotite systems. The result ing phase relations were extended to the whole MAS system by maintaini ng internal consistency and using other relevant experimental data. Eq uilibria include cordierite and sapphirine, which are usually omitted in the construction of comprehensive petrogenetic grids. A larger numb er of petrologically important equilibria involving these and the othe r MAS phases appear to be stable and have not been enumerated previous ly. The analysis produced an internally consistent set of thermodynami c parameters that closely agree with the measured thermochemical value s, although some outstanding discrepancies still remain. The result pr ovides a simple and practical tool for calculating the phase relations in the MAS system, and potentially also in more complex chemical syst ems. This is demonstrated with calculated temperature-pressure phase d iagrams.