Halloysite

halloysite [hə′lȯi‚sīt]

(mineralogy)

Al₂Si₂O₅(OH)₄·2H₂O Porcelainlike clay mineral whose composition is like that of kaolinite but contains more water and is structurally distinct; varieties are known as metahalloysites.

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Halloysite 

(named for the Belgian geologist J. B. Omalius d’Halloy, 1783-1875), a claylike mineral of the group of layered silicates having the chemical composition Al₄[Si₄O₁₀](OH)₈·4H₂O. It is close in composition to kaolinite, from which it is distinguished by a higher water content. Halloysite crystallizes in a monoclinic system. It is found in the form of claylike crypto-crystalline aggregates. The coloration is usually white but can sometimes be from yellowish to brown (from iron oxides), with a dull luster. Its hardness on the mineralogical scale is one to two; its density is 2,000 to 2,200 kg/m³. Halloysite is a typical product of the weathering of the aluminum silicates of igneous rocks, especially feldspars. It is common, but large accumulations are unusual.