Mullite is a non stoichiometric aluminum silicate, named for the island of Mull (Scotland), of approximate composition 3Al$_{2}$O$_{3}$ 2SiO$_{2}$. This material is unusual as a mineral (typically in pelitic xenoliths in basic igneous rocks, or in thermally metamorphosed shales), but frequent in silicate ceramics and refractories.
This paper show a microstructural XRD analysis of mullites, obtained by thermal transformation of two well characterized kaolins (kaolin GW and GP from Georgia, USA) at different temperatures -between 1050 and 1300$^{o}$C- and times of firing [1]. Peak profiles for the XRD line broadening analysis were obtained from 110, 120, 210, 001, 220, 111 and 121 reflections of mullite, and these profiles were fitted to pseudo-Voigt functions by using the FIT program. In order to obtain the size and strain parameters corresponding to the studied samples of mullite, the Warren and Averbach [2] and Voigt function [3] methods were applied. The obtained results show an isothermal growth of mullite crystallite for all the temperatures used in the synthesis of studied samples. The comparison of crystallite sizes obtained for different crystallographic directions show that the crystallites grow preferentially along the [001] direction. This result is in good agreement with the prismatic or needle-like crystalline morphologies frequently observed by electron microscopy techniques in mullites.
On the other hand, this study reveals that the crystallinity of kaolinite used as starting material affects the average crystallite size of mullites obtained by thermal transformation. It has been observed an inverse relation: more the crystallinity of kaolinite used, less the crystallinity of mullite obtained.
1. Serano F.J., Sanz A., Esteve V., Bastida J. &
Amig¢ J.M.: Proc. of the XVI Conf. on Applied
Crystallography. pp. 57-58. August 22-26, 1994 (Cieszyn,
Poland).
2. Warren B.E. & Averbach B.L.: J. Appl. Phys. 21,(1950),
595-599.
3. Keijser TH.H. DE, Langford J.I., Mittemeijer E.J.\&
Vogels, A.B.P.: J. Appl. Cryst.15,( 1982), 308-14.