Thermodynamic aspects of the vaterite-calcite phase transition

Although vaterite is the least stable anhydrous calcium carbonate polymorph, it is formed as a metastable phase in some normal and pathological biomineralisation processes. In this work, thermodynamic aspects of the vaterite-calcite phase transition were comprehensively studied. Vaterite samples were prepared by different methods and characterised for the composition, crystal structure, specific surface and grain size. All products were identified to be pure vaterite by careful X-ray diffraction measurements. The enthalpy and Gibbs energy of transition were determined by precise calorimetric and potentiometric measurements. The reliability of the thermodynamic data for the vaterite-calcite phase transition derived from this work was shown by the use of different calorimetric methods to determine the enthalpy of transition and the independent measurements of heat capacity and entropy of vaterite. Our recommended values are Delta(trs)G*=-2.9-0.2 kJ mol(-1), Delta H-trs*=-3.4-0.2 kJ mol(-1) and Delta S-trs*=-1.7-0.9 J K-1 mol(-1), where the uncertainties are given as twice the standard deviations.