Thermal transformation of illite and smectite group clay minerals was studied using X-ray diffraction and transmission electron microscopy, and the results were compared with those of kaolinite. Differences were observed in the types and sizes of crystals formed at temperatures between 800 degrees and 1400 degrees C. Spinel-type phases were formed after 3 h at 1000 degrees C in all three clays, but the morphology and composition of the spinel crystals varied. Mullite crystals reached greater sizes in the illite (> 10 mu m needles) and smectite (> 1 mu m) materials than in the kaolinite (0.5 mu m). Iron-rich crystals formed in the illite and smectite materials, and feldspars present as impurities in these clays, along with interlayer cations, resulted in large quantities of liquid being formed above 1000 degrees C. Similar thermal breakdown and phase development schemes are proposed for all three materials, with the major differences observed ascribed to the presence or absence of interlayer cations, substitutional impurities, and accessory minerals. These results provide a basis for predicting phase development on firing a given clay mineral, if its characteristics and impurity levels are known.