Subsolidus Phase Relations in the Systems CaCO₃-MgCO₃-CoCO₃and CaCO₃-MgCO₃-NiCO₃

Subsolidus phase relations in the ternary systems CaCO₃-MgCO₃-CoCO₃ and CaCO₃-MgCO₃-NiCO₃ have been investigated at 600°, 650°, 700°, and 750° C. in the system containing cobalt and at 600°, 700°, and 750° C. in the system with nickel. All the runs were made in squeezer-type equipment, for the most part at 15 kb. applied pressure. Equilibrium could not readily be attained at temperatures below 600° C. Decomposition was a constant source of difficulty, particularly in runs rich in NlCO₃, and higher pressures were required in some cases. The configuration of the system with CaCO₃ is similar to the system CaCO₃-MgCO₃-FeCO₃, although solid solubility of CoCO₃ in calcite and dolomite is somewhat less than FeCOe at equivalent temperatures. A three-phase area (cobaltoan-magnesian calcite, calcian-magnesian cobaltocalcite, and cobaltoan dolomite) is still present at 750° C, and at all temperatures there are also three two-phase areas and three single-phase areas (Co, Mg-calcites, complete MgCO₃-CoCO₃ solid solutions, and a thin field of cobaltoan dolomites). The system containing NiCO₃ is more complex; solid solubility is quite limited, and a solubility gap interrupts the MgCO₃-NiCO₃ binary system, producing two three-phase areas (both containing nickeloan dolomite, differing slightly in composition). In addition there are four small single-phase fields and five two-phase fields. It has been shown that the dolomite-type compound CaFe(CO₃)₂ is not stable. Similarly the compounds CaCo(CO₃)₂ and CaNi(CO₃)₂ do not exist. The solubility of CoCO₃ in dolomite at 750° C. is slightly more than 25 mole per cent and of NiCO₃ approximately 15 per cent.