A natural kaolin material, which contains 45 wt % of alumina and less than 1 wt % of metal oxide impurities, has been submitted to thermal treatments in the temperature range 1 000-1 450 °C. The cristobalite development is studied by X-ray diffraction, using the broadening of line {101}. The apparent grain sizes of cristobalite, measured by the integral width of the line, change as a function of thermal treatment. Nevertheless, the apparent total content of the cristobalite remains constant. In the case of a chamotte material, which contains 42 wt % alumina and 3 to 4 wt % of metal oxide impurities, the apparent cristobalite grain sizes are of order of magnitude of the nanometer and the total apparent content is slightly lower. Finally, the thermal expansion coefficient, α (T), of the first material is higher, during the cristobalite transformation α → β, than that of the chamotte material.