Differences in thermal and mechanochemical polymorphism. Effects of impurity on the system aragonite-calcite

The effects of incorporated Pb²⁺ and Sr²⁺ on the thermal and mechanochemical transformations of aragonite into calcite have been studied. The rate of thermal transformation is remarkably retarded on doping. The rate of mechanochemical transformation, in contrast, increases slightly on doping with Pb²⁺, whereas it decreases slightly on doping with Sr²⁺. The difference between the two processes is interpreted by assuming that the reduction in the dislocation movement on doping retards the thermal transformation, whereas the retardation effect is overcome by the applied shear stress. In addition, the local decomposition of PbCO₃ during grinding in air might have enhanced the mechanochemical transformation of aragonite doped with Pb²⁺. After the mechanical treatment, the apparent enthalpy of the transformation was reduced, presumably because of the partial cancellation of the endothermic heat of transformation by the liberation of excess enthalpy, as revealed by differential scanning calorimetry.