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

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-Al₂O₃ and SiO₂, before the "950°C" exothermic reaction in which metakaolinite is completely decomposed. The resulting spinel-type phase, which is associated with amorphous SiO₂ and some poorly crystalline "primary" mullite, is γ-Al₂0₃ (crystalline) rather than an Al-Si spinel. There is some evidence, however, that a fraction of the γ-Al₂O₃, may be an Al-Si spinel. At ≥1100°C secondary mullite therefore forms primarily from the γ-Al₂O₃/amorphous SiO₂ reaction and the recrystallization of primary mullite, whereas excess amorphous SiO₂ eventually crystallizes as cristobalite.