Class : Silicates
    Subclass : Phyllosilicates
    Crystal system : Monoclinic
    Chemistry : Al₂Si₂O₅(OH)₄
    Rarity : Very common

Halloysite is a clay mineral from the kaolinite group ; it is a product of hydrothermal alteration of feldspars and other aluminum silicates, a hydrated form of kaolinite. It was named in honor of the Belgian geologist, Baron Omalius d'Halloy who was the first to study the mineral. Halloysite occurs in compact earthy collomorphic masses with conchoidal fracture or in small spherules, white, gray, brown, yellow, blue, green or red, catching on the tongue. Halloysite is frequently associated with kaolinite from which it sometimes seems to be derived by the action of sulfate solutions. It has a wide range of uses : in the ceramic and rubber industries as well as in the chemical industry for the manufacture of ion exchange materials, saponifiers and degreasers.

Main photo : Quartz with halloysite inclusions (Mango quartz) from Cabiche, Quípama, Boyacá, Colombia © David Ethan Ziga

No twin known for this mineral species. Halloysite can, on the other hand, completely replace (pseudomorph) other minerals, thus crystals of topaz replaced into halloysite are known at Varuträsk (Sweden).

No fakes recorded for this mineral species.