Illite

Illite is a group of micaceous, non-expanding phyllosilicate minerals that resemble muscovite but differ in composition and behavior. It is a 2:1 clay mineral, consisting of a silicate tetrahedral sheet linked to an octahedral sheet, with potassium ions occupying the interlayer space. The interlayer potassium binds the layers together, giving illite its characteristic lack of swelling in water.

Formation and occurrence: Illite forms during diagenesis of feldspar- and mica-rich rocks and in soils developed from these rocks. It is common in pelitic and clay-rich sediments and soils, particularly under low-grade metamorphic or diagenetic conditions. Illite frequently occurs with other clays such as kaolinite and smectite and may appear as illite–smectite mixed-layer minerals when interlayer expansion is present in varying proportions.

Chemistry and structure: An approximate formula for illite is (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10(OH)2. Illite is generally considered dioctahedral, with the octahedral sheet dominated by Al3+ and substitutions involving Mg2+ and Fe2+/3+. The interlayer K+ ions lock the layers together, giving a basal spacing near 9–10 Å in X-ray diffraction and rendering illite non-expanding, in contrast to smectite clays.

Properties and significance: In soils, illite contributes to cation exchange capacity and influences plasticity and mechanical behavior of clays. In geology, the presence and crystallinity of illite help interpret diagenetic maturity, burial history, and sediment provenance. Illite can be part of interstratified clays with swelling clays, reflecting complex alteration histories.