Studies on the Solid State Reactions of Aluminosilicates at High Temperatures

Abstract

This thesis describes a study of the high temperature reactions of the class of hydrous aluminosilicates typified by the kaolinite group of minerals. The work was carried out using X-ray diffractometry, infrared spectroscopy and electrical conductance measurements. The effects of impurity cations and anions and of the reaction atmosphere was also studied, using infrared and Mossbauer (γ-ray) spectroscopy. The high temperature reaction sequence of kaolinite can be represented by the following idealized equations:- The kinetics of mullite formation from metakaolinite and related clay minerals have been determined by X-ray and I.R methods, and the corresponding thermodynamic function compared with theoretical disorder entropy value calculated from idealized structural models. A comparison has also been made with activation energies for the conduction process. The slow step in the reaction has been identified as the formation of mullite from the spinel via a number of intermediate mullite-like phases differing from each other only in the arrangement of the octahedral aluminium atoms. This mechanism is supported by a normal co-ordinate analysis of the mullite "molecule" and the assignment of the infrared frequencies of mullite. The behaviour of cationic impurities can also be explained in terms of the influence of metal-oxygen interactions is confirmed by electric conductance measurements on cation-doped clays.