illitisaatio

Illitisation, or illitisation (Finnish: illitisaatio), is a geological process in which certain clay minerals, most commonly smectite, transform into illite during diagenesis or low-grade metamorphism. The resulting illite is a non-expandable, micaceous clay mineral with a higher potassium content in the interlayers and a higher layer charge than its precursor clays.

The process occurs under burial conditions where temperatures rise from near-surface levels to modest diagenetic ranges, typically up to about 150–200°C, and in the presence of potassium-rich fluids. Through dewatering and chemical exchange, expandable smectite layers are replaced by the more stable illite structure as potassium is incorporated into the interlayers. The end product is a clay assemblage with reduced swelling, greater stability, and distinct textural and mineralogical characteristics. The rate and extent of illitisation depend on temperature, pressure, reaction duration, and potassium availability.

Significance in geology lies in its use as a marker of diagenetic maturation and burial history. Illite crystallinity and the illite–smectite interlayer ordering can be assessed by X-ray diffraction and used to estimate thermal maturity, burial depth, and tectonic evolution in sedimentary basins. Illitisation also influences porosity and permeability by diminishing clay swelling and altering clay fabric in rocks such as sandstones and shales.

In soils, illitisation refers to pedogenic transformation of clays toward illite as part of soil formation, yielding less expandable clays with typically stronger structure and altered cation exchange properties. This process contributes to the mineral assemblage of mature, temperate soils and reflects long-term chemical weathering and potassium availability.