Melting experiments at one atmosphere show a liquidus field of orthoenstatite in the system forsterite–diopside–silica. The orthoenstatite liquidus field is intermediate in CaO content between the protoenstatite and pigeonite fields and intermediate in SiO2 content between the forsterite and tridymite fields. Previously unrecognized peritectic points are Fo + Pr + OEn + L (1445±5°C), Fo + OEn + Pi + L (1410±5°C), Trid + Pr + OEn + L (1419±5°C), and Trid + OEn + Pi + L (1387±5°C). Orthoenstatite (6–9 wt. percent diopside) is intermediate in composition between protoenstatite (<3 wt. percent diopside) and pigeonite (12–25 wt. percent) in the melting interval. In the subsolidus region there are two distinct fields of orthoenstatite separated by a 275 °C interval. With addition of Al to the system or with an increase of 2 kbar pressure the orthoenstatite fields apparently expand and become continuous.

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