Phase Equilibria in System CaO-CO2-H2O and Related Systems, with Implications for Crystal Growth of Calcite and Apatite

Conditions for crystallization of calcite from melts in the presence of a vapor or gaseous phase are reviewed to provide possible guidelines for growth of calcite. Phase equilibrium data for the system CaO-CO₂ show that to precipitate calcite from molten CaCO₃ the temperature must be above 1230° C and excess CO₂ must be present at pressures greater than 39.5 bars. Phase equilibrium data for the system CaO-CO₂-H₂O show that calcite can be precipitated from melts at 650°C in the presence of an aqueous vapor phase at 10 bars or more. Crystallization can be induced either by introducing temperature variations, or by changing the composition of the coexisting aqueous vapor phase under isobaric, isothermal conditions. The system CaO-MgO-CO₂-H₂O is used to show that doped calcite crystals, Ca_xR₁-_xCO₃, could be precipitated, with precisely controlled compositions, from melts in the system CaO-RO-CO₂-H₂O. Apatite phase relations are also presented.