Modelling the structure of the metastable phases in the reaction sequence kaolinite-mullite by X-ray scattering experiments

The crystal structure of two kaolinites with a different degree of planar disorder and their high temperature (HT) decomposition products have been investigated by means of symmetrical transmission powder X-ray diffractometry and the subsequent calculation of the radial distribution function (RDF). An ordered and a disordered kaolinite (international standard Kga-1 and Kga-2) were selected along with their HT reaction products at 600 degrees C (metakaolinite) and at 900 degrees C (mullite precursor). The calcinations and quenches have been carried out ex-situ prior to the data collection. The procedure for collection and reduction of the data and their interpretation is outlined. The structural characterization of the HT phase transformations is obtained by qualitative analysis of the RDF curves and is related to the extent of planar disorder in the original material. A model for the structure of metakaolinite is proposed on the basis of the present experiments, and of a review of literature data. According to this model, the structure of metakaolinite displays an almost uncollapsed structure with a short-range order in which Al is mainly in four-fold coordination. An ab initio modelling of the RDF curves provides a quantitative evaluation of the proposed structural model. Differences in the HT products from different kaolinites are directly correlated with the density of stacking faults in the starting kaolinites. Moreover the structure of the precursor of mullite is described and discussed with respect to the literature data. The nature and role of gamma-alumina is discussed as well. Finally the DTA exothermic peak of kaolinite is interpreted in the light of the proposed general model.