Kinetics and mechanism of the solid-state high-temperature transformation of andalusite (Al2SiO5) into 32-mullite (3Al2O3.2SiO2) and silica (SiO2)

The kinetics of the high-temperature transformation of andalusite (<40 μm) were investigated by means of X-ray techniques. The transformation interval was determined to lie between about 1250 and 1500°C. The best fitting of the kinetic data was achieved with an exponential law of the type: 1 − α = exp (−kt + ct/(t + 1)). The kinetic curves are associated with a topotactic transformation mode with preservation of the aluminium-oxygen octahedral chains and diffusion of aluminium, silicon and oxygen atoms. Below 1380°C, mullite (+SiO₂) formation is essentially restricted to an initial rapid period of the transformation, because nucleation and product growth occur only at energetically favored lattice sites (e.g. grain boundaries). Above 1380°C, mullite and SiO₂ nucleation and growth may take place throughout the volume of the low-temperature phase (andalusite) over the whole range investigated.