The system Al2O3-SiO2.: Calorimetric and thermoanalytical studies of phases in the system.

Gels of synthetic kaolinites formed from mixtures of very pure metalorganic compounds of aluminium and silicon and with seven different Al/Si ratios, have been investigated by DTA/TG/DTG up to 1200 degreesC. The enthalpies associated with the exothermic reaction occuring at 980 degreesC, reaches a maximum at a composition corresponding to 2Al(2)O(3). 3SiO(2). This result has been discussed with reference to the wellknown work by Brindley and Nakahira from 1959 on the kaolinite-mullite reaction series, Good correlation between the experimental enthalpies and the enthalpies of crystallization of alumina and silica taken from the literature, have been obtained. Models for distribution of the Al-atoms between the octahedral and the tetrahedral positons in the mullite structure have been discussed, and models for the incongruent melting of both 3:2 mullite and 2:1 mullite have been evaluated.