Subsolidus Phase Relations in the System MgO-SiO_2-Cr-O and Thermodynamic Properties of Related Cr~(2+)-Containing End-members

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℃ 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 XCrPx2+ < 0.2, monoclinic at higher XCrPx2+ . Thestructure of the spinels becomes tetragonally distorted at XCr2+Sp >0.2. The experimental datahave 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)

Subsolidus Phase Relations in the System MgO-SiO_2-Cr-O and Thermodynamic Properties of Related Cr~(2+)-Containing End-members

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 X^Px_Cr2+ <0.2, monoclinic at higher X^Px_Cr2+. The structure of the spinels becomes tetragonally distorted at X^Px_Cr2+ >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).