The heating effect on Kaolin may be followed by using Electron Microscope. Upto 1100oC we can not observe the breakdown of the hexagonal lamellae. Above 1100oC the fine needles begin to separate and that is the first nuclei of Mullite. When we heat it further we can observe the well formed needles of Mullite scattered through out the fired body in disorderly manner.
Among the Aluminium Silicates only Mullite has been determined in fired clay ware with certainty. However many research workers claimed to have found Sillimanite. In addition to chemical analysis, x-ray method of examination has been used successfullly for the determintation of Mullite. Since the individual crystals are very small the method of Debye and Scherrer is mostly used.
The X-ray Spectogram of Mullite was found to be almost identical with that of Sillimanite. Mullite containing iron and Titanium yielded Spectra completely identical with those of Sillimanite. A high degree of similarity of atomic arrangement in the crystal lattice is, accordingly, peculiar to these two closely related Silicates.
… to be contd.
