Allophane

Allophane is a mineraloid, meaning it is a naturally occurring solid substance that has an amorphous or poorly crystalline structure. It is a hydrated aluminum silicate with a general chemical formula of Al2O3·SiO2·nH2O. The exact composition can vary, leading to a range of properties. Allophane is typically found in the form of amorphous, gel-like masses, earthy coatings, or as small, irregular grains. Its color can range from colorless and white to yellow, brown, blue, or green, often due to impurities. It has a waxy to dull luster and a conchoidal fracture. The hardness of allophane is difficult to determine due to its amorphous nature but is generally considered to be soft, typically around 2 on the Mohs scale. It is an opaque to translucent mineraloid.

Allophane is commonly found in weathered argillic or allitic soils and in altered volcanic rocks. It forms as a secondary mineral through the weathering and alteration of other aluminum and silicon-bearing minerals, particularly feldspars and micas. It is often associated with other clay minerals like kaolinite, halloysite, and montmorillonite, as well as gibbsite. Allophane is known for its porous structure, which allows it to adsorb and retain water and other substances. This property makes it relevant in geological and environmental studies. It can also be found in hydrothermal alteration zones. While not a major ore mineral, its presence can be an indicator of specific geological processes and soil conditions.