The stability of CaCO3·6H2O (ikaite)

Experimental runs were made in cold-seal pressure vessels using synthetic CaCO₈·6H₂O, calcite and aragonite as starting materials. The reaction CaCO₃·6H₂O (ikaite) ? CaCO₃ (calcite I) + 6H₂O was reversed across its metastable extension into the aragonite stability field and the phase boundary is defined by brackets at 4.14kb, 14.3°C and 2.96 kb, ?3.0°C. An invariant point for CaCO₃·6H₂O, calcite I, aragonite and water thus occurs at about 3.02 kb and ?2.0°C. No other reaction could be reversed. Calculations based on the equilibrium phase boundary between calcite and ikaite and the available thermochemical data for calcite and water yield the stadard free energy of formation, standard enthalpy of formation and third law entropy of CaCO₃·6H₂O at 25°C and 1 bar total pressure; ?607.3 kcal/mole, ?705.8 kcal/mole, and 88.4 cal/deg mole, respectively.