illitegroup

The illite group refers to a family of clay minerals within the mica group that are classified as non-expanding 2:1 phyllosilicates. Members of this group, most notably illite, are potassium-rich and form as diagenetic or low-temperature metamorphic products in soils and sedimentary rocks. They are important components of fine-grained clays in many geological settings.

Structurally, illite minerals consist of repeating 2:1 layered silicate sheets (two tetrahedral sheets around a central octahedral sheet) with interlayer potassium ions. The K+ ions link the sheets and prevent significant swelling, giving illite its characteristic non-expanding behavior. The octahedral sheets are typically composed of aluminum with substantial substitutions by magnesium or iron. This arrangement yields a relatively rigid crystal structure that contrasts with expandable clays such as smectites.

Key properties include limited swelling in water due to interlayer K+. Illite has a characteristic X-ray diffraction basal spacing near 10 Angstroms, which helps distinguish it from expandable clays. In soils, illite contributes to the mineral’s moderate to low cation exchange capacity compared with smectites, and its specific surface area is generally lower than that of highly expandable clays.

Occurrence and formation are tied to diagenetic transformation of aluminous and potassic minerals, weathering of feldspars and micas, and low-grade metamorphism. Illite and related minerals are widespread in marine and continental sediments, shales, and clay-rich soils. The illite group is often discussed alongside illite‑smectite mixed-layer minerals, which reflect varying degrees of illitization and interlayer swelling in transitional clays.