This study aimed to investigate the characteristics of three types of natural materials (KLC, KPT, and SSB) obtained from regular deposits in Vietnam and their mullitization at calcinated temperatures of 1000 °C, 1200 °C, and 1400 °C. The samples were characterized by X-ray diffraction (XRD), X-ray fluorescence analysis (XRF), thermal analysis (the thermogravimetry/differential scanning calorimetry (TG/DSC)), and scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-EDS). The results indicated that kaolinite, halloysite, and sericite are the dominant minerals in the KLC, KPT, and SSB samples, with particle sizes under 2 µm. These minerals' morphology was typical, with pseudo-hexagonal and stratified kaolinite, tubular halloysite, and thin platy sericite. The samples' chemical compositions are mainly SiO₂ and Al₂O₃, followed by (K₂O+Na₂O), TiO₂, and total iron content. Thermal analysis shows that the metakaolin phase is formed at different temperatures for the halloysite and kaolinite samples, as indicated by endothermic peaks at 411 °C for KPT and 436 °C for KLC. The quartz phase transition is also detected at different temperatures for the samples, with low-to-high temperature endothermic peaks at 559 °C (KPT), 569 °C (KLC), and 575 °C (SSB). The mullitization process is observed to start at temperatures above 1000 °C and is critical at around 1400 °C, with well-crystal mullites appearing. The mineral composition and morphology of the starting materials influence the size of the mullite crystals. SSB has larger mullite crystals than KLC and KPT at a sintering temperature of 1400 °C. It suggests that mineral composition, mineral morphology, and particle size of starting materials can also be essential factors influencing to mullitization process and ceramic products

Keywords: Pseudo-hexagonal kaolinite, Tabular halloysite, Platy sericite, Mullite, Thermal analysis, Ceramics