Thermochemical investigations of calcium carbonate phase transitions - I. Thermal activated vaterite-calcite transition

The vaterite-calcite transition above 630 K has been studied by isothermal and non-isothermal differential scanning calorimetry. Vaterite samples prepared under different conditions were investigated. The transition temperature is strongly dependent on the sample preparation. The observed transition enthalpy Delta H-tr is nearly equal for different samples and experimental conditions. From 28 measurements a value of Delta H-tr= -(3.12+/-0.11) kJ mol(-1) was obtained. The activation energy for the polymorphic transition was calculated from the Arrhenius plot and by use of isoconversional methods, as a function of the degree of conversion. The influence of the kinetic model distortion and experimental uncertainties on the obtained data was discussed. The actual value of the activation energy was assessed at E-a=(250+/-10) kJ mol(-1) for nearly all examined samples. Functions, corresponded to the model mechanism of nuclei formation and growth, provide the unambiguous consideration of the transition kinetic for the investigated vaterite samples. Differences in the dynamic behaviour of several samples at the transition are established.