Wollastonite polytypes in the CaO-SiO2 system. Part I. Crystallisation kinetics

This work deals with the identification and kinetics of crystallisation of the wollastonite polytypes which form in the system CaO-SiO2 in the tempera ture range 700-1000 degrees C. The kinetics of phase transformations in the system have been determined from analysis of in situ synchrotron X-ray pow der diffraction data. Two different systems with the same compositions were investigated: one is more reactive, having grains with a larger surface ar ea per unit volume, the other is less reactive, being composed of grains wi th a smaller surface area per unit volume. 1T-wollastonite forms first and progressively transforms in an intermediate 1Td-wollastonite disordered for m. Both phases in turn transform into 2M-wollastonite polytype. Differences in the polytypes forming and reaction kinetics were observed for the two s ystems. In the more reactive system, the conversion 1T to 1Td polytypes is the dominant process. The kinetic parameters calculated using the Avrami mo dels fully describe the reaction process of formation of both polytypes and transformation of one polytype into the other.