In the MnSiO3-MgSiO3 join a pyroxmangite-rhodonite peritectic exists on the liquidus at a composition of approximately (Mn0.67Mg0.33)SiO3 at 1350°C ± 5°C in air. Unit-cell dimensions are given for the continuous solid-solutions of pyroxmangite at compositional range (Mn0.60Mg0.40)SiO3—(Mn0.16Mg0.390)SiO2 and for rhodonite at compositional range (Mn0.60Mg0.40)SiO3—MgSiO3.

Single crystals of pyroxmangite, (Mn0.50Mg0.50)SiO3 and rhodonite, (Mn0.50Mg0.20)SiO3 were grown to a maximum size of 5mm × 3mm × 2mm by slow cooling of the melts. At 1200°C in air, the join MnSiO3—MgSiO3 consists of four phases, i.e., triclinic rhodonite, at MnSiO3—(Mn0.55Mg0.45)SiO3; triclinic pyroxmangite, at (Mn0.50Mg0.50(SiO3—(Mn0.25Mg0.75)SiO3; clinopyroxene, at (Mn0.20Mg0.80)SiO3—(Mn0.15Mg0.85)SiO3 and orthorhombic protoenstatite, at (Mn0.10Mg0.90)— MgSiO3. The phase transformation at fixed P and Tfor (Mn, Mg)SiO3 appears to be controlled by the mean cationic radii for the variable Mn and Mg contents. A narrow but distinct rhodonite-pyroxmangite coexistence region is found below the solidus around (Mn0.50Mg0.50)SiO3—(Mn0.55Mg0.45)SiO.

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