A semiordered hydrous aluminosilicate, Al₂O₃(SiO₂)_1.3-2.0 2.5-3.0(H₂O), that is closely related to imogolite because both seem to be incipient 1:1 minerals structurally and chemically, although it has been suggested that allophane may be an incipient montmorillonite. The ratio of SiO₂ to Al₂O₃ is usually 1.3 to 2.0 for allophane, but has been reported as low as 0.83, a greater variation than in imogolite. Allophane shows curved walls with a more spherical morphology (30 – 50 Å spherical diameters) than imogolite, which is tube-like. Allophane is common in soilsderived from volcanic ash, but may be present in soils derived from basic igneous rocks in tropical climates or in podzol soils derived from more acidic rocks. Allophane may precipitate in hot springs rich in silicic acid and aluminum. Allophane is white or colorless when moist, but earthy when dried.
Syn., disordered allophane
Cf., imogolite