KAOLINITE MULLITE REACTION-SERIES - THE DEVELOPMENT AND SIGNIFICANCE OF A BINARY ALUMINOSILICATE PHASE

The semiquantitative estimations of 980-degrees-C exothermic reaction products of kaolinite by quantitative X-ray diffraction (QXRD) and chemical leaching techniques show the formation of a significant amount of amorphous aluminosilicate phase (approximately 30 to 40 wt%). The theoretically expected AlO4/AlO6 ratio in the 980-degrees-C reaction is in close agreement with the value measured by the X-ray fluorescence (XRF) technique and the experimental radial electron distribution (RED) profile agrees with the suggested 980-degrees-C formation of Si-Al spinel with mullite-like composition. Mullitization of kaolinite has been compared with a synthetic Al2O3-SiO2 mixture. In synthetic mixtures development of an intermediate amorphous aluminosilicate phase is an essential step prior to mullitization. Kaolinite forms mullite in two ways: (i) by polymorphic transformation of cubic mullite at 1150-degrees to 1250-degrees-C and (ii) by nucleation of mullite in the amorphous aluminosilicate phase and its subsequent growth above 1250-degrees-C. Thus chemical continuity is maintained throughout the reaction series and the intermediate spinel phase is silicon bearing and its subsequent transformation to mullite confirms the topotactic concept in the kaolinite transformation.