Allophane - Encyclopedia

    Class : Silicates
    Subclass : Phyllosilicates
    Crystal System : Amorphous
    Chemistry : Al₂SiO₅ 3H₂O
    Rarity : Fairly common

Allophane is a rather mysterious, probably amorphous, hydrated alumina silicate whose precise chemical composition, in particular its water content, remains to be established. It takes its name from the Greek allos (other) and phainein (to appear), because it has the particularity of changing its appearance when subjected to heat. It is a fairly widespread mineral as a product of the alteration of volcanic tuffs, or as a mineral of hydrothermal alteration in igneous rocks in which it derives from feldspars, in hydrothermal copper deposits or various sedimentary rocks. The allophane forms earthy masses, porcelain encrustations, sometimes white stalactites or variously colored blue, yellow to pale green, or dark green to brown. These colorations frequently reveal the presence of impurities, mainly iron oxides. It is a mineral that has some success with collectors when its color is blue.

No twin known for this mineral species.

No fakes reported for this mineral species, but can easily be confused with chrysocolla, gibbsite, hyalite opal, etc...