Spectroscopic investigations of the kaolinite-mullite reaction sequence

The changes of Al coordination in the kaolinite-mullite thermal sequence in the presence of mineralizers were investigated. Small amounts of mineralizers added to kaolinite promoted, selectively (900 degrees ), either (a) direct mullite formation or (b) spinel. X-ray fluorescence measurements of the Al/b K/ alpha shift showed that, in case (a), the Al(IV) content decreases from the metakaolinite level to values found in 3:2 mullite (without intermediary spinel formation); in case (b) Al(IV) content decreases to values corresponding to pure gamma -Al/sub 2/O/sub 3/ (lower than in mullite). The formation of a mixed Al-Si spinel as a major product is excluded. Results are discussed in the light of complementary ESCA measurements.