Halloysite is an aluminosilicate clay mineral with the empirical formula Al2Si2O5(OH)4. Halloysite typically forms by hydrothermal alteration of alumino-silicate minerals.
It was first described in 1826 and named after the Belgian geologist Omalius d'Halloy.
Halloysite naturally occurs as small cylinders (nanotubes) that have a wall thickness of 10–15 atomic alumosilicate sheets, an outer diameter of 50–60 nm, an inner diameter of 12–15 nm, and a length of 0.5–10 μm. Their outer surface is mostly composed of SiO2 and the inner surface of Al2O3, and hence those surfaces are oppositely charged.
Two common forms are found, hydrated and dehydrated.
When hydrated, the clay exhibits a 1 nm spacing of the layers, and when dehydrated (meta-halloysite), the spacing is 0.7 nm, also the cation exchange capacity depends on the amount of hydration
Halloysite is an efficient adsorbent both for cations and anions, it has also been used as a petroleum cracking catalyst or can be intercalated with catalytic metal nanoparticles, thereby serving as a catalyst support.
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