Halloysite

Halloysite is an aluminosilicate clay mineral with the empirical formula Al2Si2O5(OH)4. Its main constituents are aluminium (20.90%), silicon (21.76%) and hydrogen (1.56%). Halloysite typically forms by hydrothermal alteration of alumino-silicate minerals. It can occur intermixed with dickite, kaolinite, montmorillonite and other clay minerals. X-ray diffraction studies are required for positive identification. It was first described in 1826 and named after the Belgian geologist Omalius d'Halloy. 2Occurrence The formation of halloysite is due to hydrothermal alteration, and it is often found near carbonate rocks. For example, halloysite samples found in Wagon Wheel Gap, Colorado, United States are suspected to be the weathering product of rhyolite by downward moving waters. In general the formation of clay minerals is highly favoured in tropical and sub-tropical climates due to the immense amounts of water flow. Halloysite has also been found overlaying basaltic rock, showing no gradual changes from rock to mineral formatio