Structure of tubular halloysite-(10 Å) and its transition to halloysite-(7 Å) by infrared spectroscopy and X-ray diffraction

Halloysite nanotubes (often abbreviated as HNTs) are technologically important owing to their unique structural and morphological features. Some of these features pre-exist in the naturally hydrated halloysite-(10 Å) parent clay mineral; others may develop during its dehydration towards halloysite-(7 Å). This is the first infrared spectroscopic study of the transition to halloysite-(7 Å), which, in combination with X-ray diffraction (XRD), aimed at advancing the structural description of the process. Three cylindrical and two polygonal halloysite-(10 Å) samples, in both their H- and D-forms, were measured by attenuated total reflectance (ATR), non-invasively and in situ, following step-wise equilibration from 70% relative humidity (RH) to