Kaolin M1 and M2 studied by X-ray diffraction focus on the mullite phase, which is the main phase present in both products. The Williamson–Hall and Warren–Averbach methods for determining the crystallite size and microstrains of integral breadth β are calculated by the FullProf program. The integral breadth ( β) is a mixture resulting from the microstrains and size effect, so this should be taken into account during the calculation. The Williamson–Hall chart determines whether the sample is affected by grain size or microstrain. It appears very clearly that the principal phase of the various sintered kaolins, mullite, is free from internal microstrains. It is the case of the mixtures fritted at low temperature (1200 °C) during 1 h and also the case of the mixtures of the type chamotte cooks with 1350 °C during very long times (several weeks). This result is very significant as it gives an element of explanation to a very significant quality of mullite: its mechanical resistance during uses at high temperature remains.

In this work, we have studied the Kaolin M1 and M2 by the X-ray diffraction method, and we have focused on mullite phase which is the main phase present in common in both products. An update of the program of the method of Stokes was carried out, it was necessary to the microstructural analysis. The completed version is less sensitive to the choice by excess of the number of coefficients of Fourier with regard to the effect of truncation and the office plurality of the errors. The determination of the symmetry of the cell of the principal phase (mullite) in the studied fritted Kaolins was carried out. In Kaolin M1, the size of crystallites of the dominant phase varies between 80 to 170 Å. In Kaolin M2, the size of the crystalline grains of mullite varies between 100 to 400 Å. The size of crystallites was confirmed by the joint method of Williamson-Hall. A distribution of sizes of crystallites was carried out. It shows a dominance of the size of approximately 140 Å for the principal phase of Kaolin M1 and a dominance of the size of approximately 230 Å for the same principal phase of Kaolin M2. By the study of the profiles of line by DRX, it appears very clearly that the principal phase of the various sintered Kaolins, mullite, is free from internal microstrains. It is the case of the mixtures fritted not only at low temperature (1100℃) during 1 hour but also the case of the mixtures of the type ‘chamotte’ cooks with 1350℃ during very long times.