The phase relations in the Na2CO3-(Fe0.87Mn0.06Mg0.07)CO3 system have been studied in Kawai-type multi-anvil experiments using graphite capsules at 6.0 GPa and 900–1400 °C. Subsolidus assemblages comprise the stability fields of Na2CO3 + Na2Fe(CO3)2 and Na2Fe(CO3)2 + siderite with the transition boundary at X(Na2CO3) = 50 mol%. Intermediate Na2Fe(CO3)2 compound has rhombohedral R3̄ eitelite structure with cell parameters a = 4.9712(16), c = 16.569(4) Å, V = 354.61(22). The Na2CO3-Na2Fe(CO3)2 eutectic is established at 1000 °C and 66 mol% Na2CO3. Na2Fe(CO3)2 disappears between 1000 and 1100 °C via incongruent melting to siderite and a liquid containing about 55 mol% Na2CO3. Siderite remains a subliquidus phase at 1400 °C at X(Na2CO3) ≤ 30 mol%.
The ternary Na2CO3-FeCO3-MgCO3 system can be built up from the corresponding binary systems: two systems with intermediate Na2(Mg,Fe)(CO3)2 phase, which melts congruently at the Mg-rich side and incongruently at the Fe-rich side, and the (Mg,Fe)CO3 system with complete solid solution. The phase relations suggest that the maximum contribution of FeCO3 component into the lowering solidus temperatures of Na-bearing carbonated mantle domains could not exceed several tens of degrees Celsius.