Compositions of pyroxenes in the assemblage orthoenstatite + diopside + anorthite + forsterite have been determined by electron‐microprobe analysis of experimental run products in the system CaO‐MgO‐Al2O3‐SiO2, from 925 to 1225°C on the l‐atm isobar. The experiments, using vanadate and plumbate solvents, produced reversals of both Ca:Mg ratios and Al contents of the pyroxenes. The results confirm thermodynamic inferences that, in this assemblage, the incorporation of aluminous components in both phases is strongly dependent on pressure but largely independent of temperature, whereas the exchange of Ca and Mg between orthopyroxene and clinopyroxene is strongly dependent on temperature but largely independent of pressure. On the 1‐atm isobar, the amount of Al incorporated in both pyroxenes is substantially less than present thermodynamic solution models predict.

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