Experimental runs were made in cold-seal pressure vessels using synthetic CaCO 8·6H 2O, calcite and aragonite as starting materials. The reaction CaCO 3·6H 2O (ikaite) ⇌ CaCO 3 (calcite I) + 6H 2O 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 CaCO 3·6H 2O, 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 CaCO 3·6H 2O at 25°C and 1 bar total pressure; -607.3 kcal/mole, -705.8 kcal/mole, and 88.4 cal/deg mole, respectively.