Pseudo-reversal phase transformation of hexacelsian to celsian in a purified kaolinite system

The solid-state reaction of purified kaolinite (Si/Al = 1.03 molar ratio) and barium carbonate (BaCO 3 ) to form hexacelsian, the metastable phase of BaAl 2 Si 2 O 8 , was conducted at various calcination conditions. As the calcination temperature increases, X-ray diffraction (XRD) patterns of calcined specimens for 48 h indicate the follows; 1) the exclusive formation of hexacelsian is achieved in the 1000–1100 °C range; 2) celsian, the stable phase of BaAl 2 Si 2 O 8 , becomes predominant in the 1100–1300 °C range; 3) celsian content decreases in the 1300–1500 °C range; and 4) celsian disappears, with only hexacelsian observed at 1600 °C. Scanning electron microscope images reveal that the particles size of hexacelsian in specimens calcined at 1600 °C is significantly larger than in those calcined at 1300 °C. Notably, hexacelsian remains predominant, but celsian reflections slightly appear in the XRD patterns of the specimen calcined at 1600 °C for 72 h. Therefore, the phase transformation of hexacelsin to celsian, which generally proceeds at around 1000 °C in many studies using Kaolin clays containing kaolinite or halloysite with by-products, is delayed when hexacelsian particles are relatively larger. Furthermore, the exclusive formation of hexacelsian is achieved at 1600 °C for 72 h when using another purified kaolinite (Si/Al = 1.01 in molar ratio) and finer BaCO 3 particles as raw materials. Consequently, the findings in this study demonstrate a “pseudo-reversal phase transformation” at higher temperatures and shorter calcination times. A “pseudo-phase transformation” was demonstrated in BaAl 2 Si 2 O 8 though the solid-state reaction in a purified kaolinite system. • The solid-state reaction of BaAl 2 Si 2 O 8 was conducted in a purified kaolinite system. • Celsian, the stable phase, disappeared as the temperature increased from 1600 to 1300 °C. • Hexacelsian, the metastable phase, was exclusively formed at 1600 °C. • At 1600 °C, celsian appeared when the calcination time was extended from 48 to 72 h. • A pseudo-reversal phase transformation was observed in BaAl 2 Si 2 O 8 polymorphs.