ABSTRACT
Halloysite is a naturally occurring alumino silicate and its structural makeup is similar to kaolinite, that is, it features a laminar structure comprising repeating layers of one tetrahedral (silica) sheet and one octahedral (alumina) sheet. The two sheets are held together to form a single layer with a thickness of about 0.72 nm. Unlike kaolinite, halloysite is commonly seen in a tubular shape (Figure 14.1a, b), and for this reason, halloysite particles are commonly referred to as halloysite nanotubes (HNTs). Starting from a crystal lattice of a kaolinite sheet, it tends to an energetically stable structure by distorting (when hydrated) and deforming into a tube. The question on why it rolls instead of performing a tetrahedral rotation addresses the mechanics to correct for misfit of the octahedral (alumina) and tetrahedral (silica) sheets (Singh et al., 1996). Singh et al. (1996) presented an elegant model to explain that the rolling mechanism encounters lower resistance from the Si-Si repulsion (as compared to tetrahedral rotation) to correct the same amount of misfit.
