Experiments have been made with synthetic pyroxenes on part of the join FswErus-Wollastonite in the pyroxene quadrilateral. Results at 15 kbar define parts of a 'miscibility gap between two clinopyroxenes and two fields of orthopyroxene plus clinopyroxene. The miscibility gap is symmetrical about a composition near the center of the join in the quadrilateral, and the solvus appears to have a critical temperature in the interval 915-950±C. Molar volume considerations suggest that this temperature is not sensitive to pressure. A field of orthopyroxene plus clinopyroxene lies between Wo4 and Wo13 at 900-915±C and 15 kbar. Below 875±C, however, orthopyroxene coexists with Wo30 composition clinopyroxene, and consequently calcium-poor clinopyroxene (“pigeon-ite”) cannot form below 875±C at 15 kbar. Results at 15 kbar together with an experiment at 30 kbar indicate that the minimum temperature for stability of calcium-poor clinopyroxene (“pigeonite”) increases with increasing pressure. Experiments at lower pressures outline conditions at which pyroxenes on the join are not stable with respect to more calcic pyroxene, fayalitic olivine, and SiOa phase, and they outline part of the “forbidden zone” in the quadrilateral.

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