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 biomin eralisation processes. In this work, thermodynamic aspects of the vaterite- calcite phase transition were comprehensively studied. Vaterite samples wer e prepared by different methods and characterised for the composition, crys tal structure, specific surface and grain size. All products were identifie d to be pure vaterite by careful X-ray diffraction measurements. The enthal py and Gibbs energy of transition were determined by precise calorimetric a nd potentiometric measurements. The reliability of the thermodynamic data f or the vaterite-calcite phase transition derived from this work was shown b y the use of different calorimetric methods to determine the enthalpy of tr ansition and the independent measurements of heat capacity and entropy of v aterite. 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.