Numerous investigations of the effect of heat on minerals of the kaolinite group have established that these minerals are characterized by two major thermal reactions below 1000° C. An intense and abrupt endothermal reaction between 500 and 600° C. has been attributed to the loss of chemically combined water from the kaolinite structure. At about 980° C. these minerals experience an exothermal effect. The intensity and rapidity of this reaction indicate a crystallization process. Certain investigators have ascribed this reaction to the crystallization of gamma alumina (Insley and Ewell, 1935). Comeforo, Fischer, and Bradley (1948), on the other hand, have concluded that it represents crystallization into mullite nuclei of articulated units derived from the original material.