Kinetics of phase formation in jarosite glass-ceramic

The crystallisation behaviour of an iron-rich glass was studied by means of density measurements, Differential Thermal Analysis and X-ray Diffraction. The results highlighted that there is no typical nucleation process in which the number of nuclei is influenced by the thermal treatment and influence the crystallisation process. The kinetics of phase formation was investigated in isothermal and non-isothermal conditions; the Avrami parameter value, 1.5, is indicative of a three dimensional diffusion controlled growth of the pyroxene crystals on a fixed number of nuclei. The activation energy of crystal growth, obtained by the isothermal and non isothermal methods, resulted in being 377kJ mol(-1) and 298 kJ mol(-1), respectively. Considering the different temperature range, these values are in fair agreement and consistent with values reported by other authors. Using TEM-EDS analysis the high immiscibility of the initial glass was highlighted; the two liquid phases, one richer in Fe and the other in Si and Ca, promote the formation of magnetite and pyroxene during the thermal treatment, respectively. (C) 1999 Elsevier Science Limited. All rights reserved.