Subsolidus Phase Relations in the System MgO–SiO2–Cr–O and Thermodynamic Properties of Related Cr2+–Containing End–members

Abstract

Abstract Subsolidus phase relations have been determined in the systems SiO2–Cr–O and MgO–SiO2–Cr–O in equilibrium with metallic Cr, at 1100 to 1500°C and 0 to 2.88 GPa. The results show that there are no ternary phases in the SiO2–Cr–O system at these conditions, i.e., only the assemblage eskolaite–Cr–metal–quartz (or tridymite) is found. In the MgO–containing system, however, extensive substitution of Cr2+ for Mg is observed in (Mg, Cr2+)2SiO4 olivine, (Mg, Cr2+)2Si2O6 pyroxene, and (Mg, Cr2+)Cr2O4 spinel. Cr3+ levels in olivine and pyroxene are below detection limits. The pyroxene is orthohombic at XPxCr2+ <0.2, monoclinic at higher XPxCr2+. The structure of the spinels becomes tetragonally distorted at XPxCr2+ >0.2. The experimental data have been fitted to a thermodynamic model, and the authors obtained the mixing parameter (W) of Mg‐Cr2+ in olivine, pyroxene and spinel, and the relation between temperatures and free energies of formation for the end–members: Cr2+–olivine (Cr2SiO4), Cr2+–pyroxene (Cr2Si2O6) and Cr2+–spinel (Cr3O4).