Reversed pyroxene phase equilibria in CaO-MgO-SiO₂ from 925° to 1,175° C at one atmosphere pressure

Abstract

Experiments using V₂O₅ as a high-temperature solvent have produced compositional reversals defining the miscibility gap between enstatite and diopside on the join Mg₂Si₂O₆-CaMgSi₂O₆ between 925° and 1,175° C at atmospheric pressure. These experiments locate an equilibrium near 1,000° C among diopside, protoenstatite, and orthoenstatite; they verify the stable coexistence of diopside and protoenstatite above 1,000° C and disprove the hypothesis that orthoenstatite has a stability field which is continuous from temperatures below 1,000° C to the solidus. The phase relations suggest that the orthorhombic low-Ca pyroxene on the solidus in this system (formerly identified as orthoenstatite) is a phase distinct from the orthoenstatite stable with diopside at low subsolidus temperatures. Data locating the orthoenstatite-diopside miscibility gap validate the use at low pressures of symmetric orthopyroxene and asymmetric clinopyroxene solution models in this system.