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.

Allophane from Mosset, France - © Philippe Font & MicroMinéral Market
Allophane from Valcroze, Gard, France - © Pascal Chollet
Allophane from Libiola Mine, Liguria, Italy - © Giuseppe Finello
Allophane from China - © Jordy Fabre

Allophane in the World

Allophane is a recognized weathering mineral at many sites around the world. The most beautiful specimens come from China (main photo © Jordy Fabre), notably from Qinglong Mine (Guizhou) and Laochang (Yunnan), where the blue guttular crusts can exceed 50 cm. Allophane is then associated with gibbsite and it is difficult to distinguish them from each other with the naked eye.

Allophane in France

In France, allophane is present in the aureole of alteration of the copper deposit of Chessy (Rhône), Decazeville (Aveyron), Mosset (Pyrénées-Orientales), Valcroze (Gard), Salsigne (Aude), Cap Garonne (Var), etc...


No twin known for this mineral species.

Fakes and treatments

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

Hardness : 3
Density : 2,75
Fracture : Conchoidal
Trace : White

TP : Translucent to transparent
RI : 1.468 to 1.512
Birefringence : None
Optical character : None
Pleochroism : None
Fluorescence : Sometime green

Solubility : Hydrochloric acid

Magnetism : None
Radioactivity : None


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