The mechanism and kinetics of Al,Si ordering in Mg-cordierite

Under non-equilibrium conditions the hexagonal to orthorhombic ordering transformation in Mg-cordierite takes place via a continuous mechanism involving the development of a metastable short-range-ordered modulated structure. A Time-Temperature-Transformation diagram illustrates the kinetics ofthis transformation sequence. The activation energy for the formation of the modulated microstructure is found to be l20t l0 kcal/mole (500+40 kJ/mole) consistent with Al,Si ordering as the rate-limiting process. X-ray difractrometry, transmission electron microscopy and IR spectrometry have been used to trace the development of short-range and long-range order in cordierite. They show that ordering proceeds smoothly with time. Short-range ordering is well-developpd by the time the modulated microstructure forms, and continued annealing increases the long-range ordering to a point where it is sufficiently well-developed to be seen in X-ray diffraction studies.