The phase composition of mineral aragonite and synthetic vaterite samples was determined qualititavely by using IR-spectrophotometric and X-ray diffraction analyses. The transformation of aragonite, A, and vaterite, V, into the stable modification calcite, C, was followed using differential scanning calorimetry analysis. In order to determine the kinetics and mechanisms of these phase transformations, a number of experimental 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) and 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.