Phase relations in Mg₃Cr₂Si₃O₁₂ and formation of majoritic knorringite garnet at high pressure and high temperature

Abstract

High-pressure and high-temperature phase relations for Mg₃Cr₂Si₃O₁₂ have been studied at pressures from 8 to 16 GPa and temperatures of 1200-1800 °C using a Kawai-type multianvil apparatus. The low-pressure phase assemblage of MgSiO₃ pyroxene + Cr₂O₃ eskolaite was found to transform into a high-pressure phase assemblage of majoritic knorringite + eskolaite with a negative phase boundary of dP/dT = -0.010 GPa/°C. No pure Mg₃Cr₂Si₃O₁₂ knorringite garnet was observed over the entire range of the P-T conditions in the present study, suggesting that pure knorringite garnet may not be a stable phase under these pressure and temperature conditions. This result is inconsistent with earlier studies, where knorringite was reported to be stable at pressures higher than ∼ 10 GPa. On the other hand, the present result agrees well with recent results for the synthesis of majoritic knorringite at pressures of 11 and 14 GPa and at temperatures of 1500-1600 °C. Majoritic knorringite becomes more Cr-deficient with increasing pressure, which is similar to the Al-deficient nature of majoritic garnet in the MgSiO₃-Al₂O₃ system.