High morphological stability and structural transition of halloysite (Hunan, China) in heat treatment

The raw mineral is a kind of tubular nanotube with a hollow interior channel, the specific surface area is 56.7 m2/g. Alunitization occurred in the impure mineral. Thermal treatment to the mineral induced shrinkage of the c-axis and yielded a 7 Å-type structure. The tube wall was transformed to an amorphous structure in the dehydration process at 450 °C. Phase segregation occurred to yield amorphous SiO2 and γ-Al2O3 at around 1000 °C. The tubular morphology can be maintained even after it is calcined at 1100 °C for 6 h, and the specific surface area is enlarged slightly to 65.7 m2/g. While after being calcined to 1300 °C, the mineral will generate mullite and silica, accompanied with collapse of the tubular walls. A schematic diagram is proposed to illustrate the structural transformation procedure. The results provide assurance to the use of halloysite in high temperature circumstances, such as three-way catalysts.