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)