Native Morphology of Hydrated Spheroidal Halloysite Observed by Environmental Transmission Electron Microscopy

Abstract

AbstractNatural mineral materials such as tabular and spheroidal halloysites have recently been suggested as candidates for intercalating metal ions or organic molecules. Their potential use as nanoadsorbents is related to their porous structure and water content. Although the two morphologies can coexist in natural deposits, spheroidal halloysites remain poorly characterized whereas much literature exists on tubular halloysites. The present study investigates the native morphology, internal porous structure, and behavior upon dehydration of spheroidal halloysite from Opotiki (New Zealand). This mineral was characterized in its natural hydrated state using a transmission electron microscope equipped with an environmental cell (EC-TEM). The sample was placed in a sealed block in which water vapor-saturated air circulated at a pressure of 30 Torr. The observed particles consisted of almost complete spheroids displaying polyhedral external surfaces. 1:1 layers stack concentrically as a pore-free, onion-like structure. The dynamic processes of dehydration created by slow depressurization of the cell resulted in a decrease in the layer-to-layer distance (d001) from ~10 Å to ~7 Å due to the loss of interlayer water molecules. Irreversible formation of spurious ‘internal pores’ was recorded during this process. These pores were not indigenous to the hydrated 10 Å halloysite and resulted from the collapse of the native layers. They cannot account for the physical chemical properties of spheroidal halloysite. Spheroidal halloysites would have a lower propensity for intercalating ions or molecules than tubular halloysites. Isolated facets were also observed in high-resolution-TEM and displayed a pseudo-hexagonal morphology. The three-dimensional microstructure of the spheroid appeared bent along the three pseudo equivalent yi directions of the kaolinite-like single layers. An analogy with polyhedral serpentine has allowed the proposal of a formation process of hydrated spheroidal halloysite triggered by enrichment in divalent ions in the growth system.