THE EXAMINATION OF THE PHASE-TRANSFORMATION OF ARAGONITE INTO CALCITE BY MEANS OF DSC ANALYSIS

The X-ray diffraction method was used to determine the content of aragonite and calcite in samples of precipitated calcium carbonate synthesized in the laboratory. The transformation of the aragonite modification present into the stable calcite structure was traced by means of DSC analysis. All the samples were analysed by DSC under non-isothermal conditions in the temperature range 624-772 K at heating rates of 5, 10, 15, 20 and 25 K min-1. A mathematical analysis of experimental DSC curves defined the mechanism and the kinetics of the phase transformation of aragonite into calcite. The kinetic curves alpha = f(t) and alpha = f(T) are sigmoidal in shape, and their inflection points and the upsilon(m) point of the curves upsilon = f(t) and upsilon = f(T) are interrelated and are defined by the concept of a stationary point. The kinetic curves indicate that upsilon(m) corresponds to (dH/dt)m of the endothermal peak of transformation of aragonite into calcite, which is characteristic of a topochemical reaction.