Thermal transformation of different halloysite types probed by in situ X-ray absorption spectroscopy

Halloysite (HAL) is a natural dioctahedral 1:1 clay mineral ubiquitous in soils and weathered rocks, where it occurs in a variety of particle shapes from cylindrical to prismatic nanotubes and occasionally spheroidal particles. Much less attention has been paid to the thermal transformation of halloysite than to its planar polymorph, kaolinite. This study investigates the morphological and structural variability of HAL samples from different geological sources and their thermal transformation behaviors. In particular, in situ X-ray absorption near-edge structure spectroscopy at Al and Si K-edges demonstrates that thermal treatment induces a phase transformation from octahedral to tetrahedral coordination in Al sites, with minimal changes in Si coordination. Multivariate curve resolution alternating least squares analysis confirms a two-component transformation pathway, with transition onset temperatures dependent on morphology. Tubular halloysites undergo transformation at lower temperatures than kaolinite platelets, while spheroidal particles exhibit intermediate behavior. These findings highlight the influence of initial morphology and structure on thermal reactivity of halloysite.