Interpretation of the Kaolinite‐Mullite Reaction Sequence from Infrared Absorption Spectra

Abstract

The phases in the kaolinite‐mullite reaction sequence were reexamined by ir absorption spectrophotometry. Particular attention was paid to the controversial intermediate Al‐containing phases. Amorphous materials were leached from fired kaolinite samples with NaOH to help identify crystalline phases. Metakaolinite partially decomposes, releasing amorphous γ‐Al 2 O 3 and SiO 2 , before the “950°C” exothermic reaction in which metakaolinite is completely decomposed. The resulting spinel‐type phase, which is associated with amorphous SiO 2 and some poorly crystalline “primary” mullite, is γ‐Al 2 0 3 (crystalline) rather than an Al‐Si spinel. There is some evidence, however, that a fraction of the γ‐Al 2 O 3 , may be an Al‐Si spinel. At ≥1100°C secondary mullite therefore forms primarily from the γ‐Al 2 O 3 /amorphous SiO 2 reaction and the recrystallization of primary mullite, whereas excess amorphous SiO 2 eventually crystallizes as cristobalite.