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 oC 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/oC. 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 oC. 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.