Subsolidus equilibria between protopyroxene, orthopyroxene, pigeonite and diopside, in the MgSiO3-CaMgSi2O6 at 1 bar to 9.5 kbar, and 1012 to 1450 °C

Experiments on the join enstatite-diopside using gel and crystalline starting materials with 2-7 wt% water as flux, gave reversals of the equilibrium orthopyroxene = protopyroxene + diopside [PGT]. The path of the equilibrium is shown to be curved in PT space which has not been recognised in previous publications. The experiments also determine the invariant point, at which the four pyroxenes, protopyroxene, orthopyroxene, pigeonite, and diopside coexist, to be just below 1 kbar and just above 1275°C. The compositions of the pyroxenes determined in the experimental products were used to draw isopleths for protopyroxene, orthopyroxene and diopside. A few of the new experiments, and some from other studies, are relevant to the equilibrium orthopyroxene + diopside = pigeonite [PR] and suggest, but do not prove, that it exists to higher temperatures than shown in previous publications