When kaolinite is heated for differential thermal analysis (DTA) the second exothermic peak (∼1275°C) is due to growth of mullite crystals accelerated by formation of a liquid phase. The third exothermic peak (∼1460°C) is due to crystallization of cristobalite. Addition or presence of impurity oxides causes the second peak to shift in temperature and accelerates the formation of cristobalite so that the third peak may appear as low as the second exothermic peak temperature.
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Johnson et al. (1982) studied this question.
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