THERMOCHEMICAL INVESTIGATIONS OF CALCIUM-CARBONATE PHASE-TRANSITIONS - I - THERMAL ACTIVATED VATERITE-CALCITE TRANSITION

The vaterite-calcite transition above 630 K has been studied by isothe rmal and non-isothermal differential scanning calorimetry. Vaterite sa mples prepared under different conditions were investigated. The trans ition 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 ene rgy for the polymorphic transition was calculated from the Arrhenius p lot and by use of isoconversional methods, as a function of the degree of conversion. The influence of the kinetic model distortion and expe rimental 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 mod el mechanism of nuclei formation and growth, provide the unambiguous c onsideration of the transition kinetic for the investigated vaterite s amples. Differences in the dynamic behaviour of several samples at the transition are established.