kaolinitegroup

The kaolinite group is a collection of dioctahedral 1:1 phyllosilicate minerals that share the fundamental composition Al2Si2O5(OH)4 and are commonly referred to as kaolinite group minerals. The principal members are kaolinite, dickite, nacrite, and halloysite. Halloysite is often described as a hydrated form, with a composition written as Al2Si2O5(OH)4·nH2O, and it typically forms as tubes or fibers, whereas the other members occur as platy, layered minerals.

Structurally, the kaolinite group minerals are isostructural, consisting of alternating silica tetrahedral sheets and alumina octahedral sheets (a 1:1 layer), with hydrogen bonds linking adjacent layers. The differences among the members arise from variations in stacking order, symmetry, and interlayer water content. Kaolinite represents the low-crystallinity form, while dickite and nacrite are more crystalline polymorphs with distinct crystallographic symmetries. Halloysite differs notably in interlayer hydration, which drives its tubular or filamentous morphology.

Formation and occurrence are widespread in soils and weathered rocks. They form by chemical weathering of aluminosilicate minerals such as feldspars and micas and are common constituents of clay-sized fractions in temperate and tropical environments. Halloysite often forms in near-surface hydrothermal or tropical weathering settings where water intercalation can produce nanotubular structures.

Physically and chemically, the kaolinite group minerals typically have low cation-exchange capacity and limited shrink–swell behavior, with relatively low surface areas (except halloysite nanotubes, which can offer high surface area). They are widely used in ceramics, paper production, and as adsorbents in environmental and industrial applications. The name derives from kaolin, a widely mined deposit in the Kaoling region of China.