X‐ray diffraction patterns were obtained from an English china clay and a Georgia sedimentary kaolin, both raw and fired to various temperatures. The chief crystalline constituent of the raw clays was found to be kaolinite. Upon dehydration the kaolinite lattice was destroyed and the clays gave no diffraction pattern. Mullite was formed in both clays at 950°C and the amount increased with increase of firing temperature. In addition to the mullite, free alumina was present in the Georgia clay from 950 to 1100°C and cristobalite at temperatures above 1200°C.
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McVay et al. (1928) studied this question.