Halloysite is an aluminosilicate clay mineral with the empirical formula Al2Si2O5(OH)4. Its main constituents are oxygen (21.76%), aluminium (1.56%). Halloysite typically forms by hydrothermal alteration of alumino-silicate minerals. It can occur intermixed with dickite and other clay minerals. X-ray diffraction studies are required for positive identification. It was first described in 1826 and named after the Belgian Omalius d'Halloy.