illitesmectite

Illite-smectite (I-S) is a mixed-layer clay mineral in which illite-like 2:1 layers alternate with smectite-like 2:1 layers within a single crystal. The layers stack in sequences that range from mostly illite to mostly smectite, forming a continuum. The proportion of illite is commonly expressed as the illite fraction. I-S can be randomly interstratified or more ordered.

Each 2:1 layer comprises tetrahedral sheets bonded to octahedral sheets. Illite layers host interlayer potassium, which stabilizes the structure and limits swelling. Smectite layers can take up water in the interlayer space and swell. In illite-smectite, the interlayer region contains varying amounts ofK and water, producing a range of basal spacings and swelling behavior. X-ray diffraction shows the mixed-layer character as a broadened or split 001 reflection, reflecting the alternation of illite- and smectite-like parts.

Formation and occurrence: I-S forms during diagenesis of sediments or weathering of clays, and is common in soils, mudstones, shales, and hydrocarbon reservoirs. Burial, heat, and chemical conditions promote illitization of smectite, increasing the illite fraction with depth.

Significance and use: The illite content of I-S affects porosity and permeability in clastic rocks and swelling in soils. In geology and petroleum exploration, I-S content helps interpret diagenetic history and reservoir quality. Analytical methods include X-ray diffraction and, when needed, electron microscopy or thermodynamic modeling.