A poorly crystalline aluminosilicate mineral with an ideal composition SiO2 Al2O3 2. 5H2( + ). It appears as threads consisting of assemblies of a tube unit with inner and outer diameters of 1. 0 and 2. 0 nm, respectively. Imogolite is commonly found in association with allophane, and is similar to allophane in chemical properties. Imogolite is mostly found in soils derived from volcanic ash, and in weathered pumices and Spodosols.