Melting and subsolidus phase relations in the system K₂CO₃–MgCO₃ at 3 GPa

As part of an investigation of carbonate systems under mantle pressures and temperatures, phase relations in the K₂CO₃–MgCO₃ system have been studied at 3 GPa and 800–1300°C. Subsolidus assemblages comprise the stability fields of K₂CO₃ + K₂Mg(CO₃)₂ and K₂Mg(CO₃)₂ + MgCO₃ with the transition boundary near 50 mol% K₂CO₃ in the system. The K₂CO₃–K₂Mg(CO₃)₂ eutectic is located at 840°C and 52 mol% K₂CO₃. The K₂CO₃ content in the melt coexisting with potassium carbonate increases to 85 mol% as temperature increases to 1050°C. K₂CO₃ remains solid up to 1250 and melts at 1300°C. K₂Mg(CO₃)₂ melts incongruently at 890°C to produce magnesite and a liquid containing 51 mol% K₂CO₃. As temperature increases to 1300°C, the K₂CO₃ content in the liquid coexisting with magnesite decreases to 27 mol%.