PHASE-TRANSFORMATION OF CALCIUM-CARBONATE POLYMORPHS

The phase composition of mineral aragonite and synthetic vaterite samp les was determined qualititavely by using IR-spectrophotometric and X- ray diffraction analyses. The transformation of aragonite, A, and vate rite, V, into the stable modification calcite, C, was followed using d ifferential scanning calorimetry analysis. In order to determine the k inetics and mechanisms of these phase transformations, a number of exp erimental DSC curves were elaborated mathematically and the stationary point theory was applied. The activation energy, Ea, and the enthalpy , Delta H, were found to be, respectively, 234.5 +/- 5.6 kJ mol(-1) an d 122 Jg(-1) for the phase transformation A --> C, and 252.8 +/- 48.7 kJ mol(-1) and -21.2 Jg(-1) for the V --> C transformation.