Abstract
Analytical results of I.R., XRF and MAS-NMR techniques applied to characterize the intermediate phases namely metakaolinite and spinel phase have been reviewed. The exhaustive analyses showed that no positive evidence has been obtained so as to conclude that the intermediate 980°C spinel is γ-Al2O3. The major drawback in arriving to a definite conclusion is probably due to unaccountability of the co-existence of a substantial amount of amorphous alumino silicate phase at 980°C. The measured AlO4 content of 1000°C heated kaolinite by XRF analysis agrees closely with the theoretically expected global AlO4 value of the 980°C reaction where Si-Al spinel and amorphous alumino silicate phases forms with the liberation of amorphous silica. Lastly metakaolinite does not have any fixed structure. It transforms to mullite in two parallel paths as explained from I.R. studies of successive heated kaolinites. First path corresponds to nucleation of mullite in amorphous alumino-silicate matrix and subsequent growth thereof which is related to the observation that the ratio of AlO4 to AlO6 does not change too much between the temperature range 1000°C to 1400°C in some kaolinite where mullitization predominantly takes place following this path.
The second path corresponds to the polymorphic transformation of cubic to orthorhombic form where the global AlO4/AlO6 ratio of 1000°C heated kaolinite changes and subsequently attains to the value close to 3: 2 mullite.
