halloysite

{{Infobox mineral | name = Halloysite | image = Halloysite variety Indianaite Hydrous Aluminum silicate Bedford, Lawrence County, Indiana 2770.jpg | imagesize = | alt = | caption = | category = Phyllosilicate minerals | group = Kaolinite-Serpentine group, kaolinite subgroup | formula = Al2Si2O5(OH)4 | molweight = | strunz = 9.ED.10 | system = Monoclinic | class = Domatic (m) (same H-M symbol) | symmetry = Cc | unit cell = a = 5.14, b = 8.9, c = 7.214 [Å]; β = 99.7°; Z = 1 | color = White; grey, green, blue, yellow, red from included impurities. | habit = Spherical clusters, massive | twinning = | cleavage = Probable on {001} | fracture = Conchoidal | tenacity = | mohs = 2–2.5 | luster = Pearly, waxy, or dull | streak = | diaphaneity = Semitransparent | gravity = 2–2.65 | density = | polish = | opticalprop = Biaxial | refractive = nα = 1.553–1.565nβ = 1.559–1.569nγ = 1.560–1.570 | birefringence = δ = 0.007 | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alterations = | references = }}

Halloysite is an aluminosilicate clay mineral with the empirical formula Al2Si2O5(OH)4. Its main constituents are oxygen (55.78%), silicon (21.76%), aluminium (20.90%), and hydrogen (1.56%). It is a member of the kaolinite group. 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 subsequently named after, the Belgian geologist Omalius d'Halloy.