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

Halloysite is one of the nanoscale tubular minerals in nature. This article reports the mineralogical character, dispersion treatment, structural, and morphological stabilities of a 10 Å-type halloysite mineral from Hunan, China. X-ray diffraction (XRD), transmission electron microscopy (TEM), particle-size distribution and N2 adsorption–desorption isotherms were adopted to characterize the raw and calcined minerals. Ammonium lauryl sulfate was used as the surfactant to obtain mono-dispersed halloysite nanotubes. 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.