Vermiculitization of smectite interfaces and illite layer growth as a possible dual model for illite-smectite illitization in diagenetic environments: a synthesis

A structural model is proposed for illite-smectite (I-S) from diagenetic en vironments which accounts for the presence of three different layer types w hich are defined as follows: montmorillonite (low-charge, octahedrally subs tituted, fully expandable), vermiculite (high-charge, octa- and tetrahedral ly substituted, only partly expandable) and illite (K0.9Si3.3Al0.7R1.83+R0. 22+O10(OH)(2)). All three layers may be found within the MacEwan crystallit es, whereas external edges of the crystallites are only vermiculitic during the illitization process. In the proposed model, a layer is defined symmet rically on each side of the interlayer space, leading to the existence of p olar 2:1 units. It is proposed that the I-S growth is a three step mechanis m: (1) formation, from sediments of variable composition, of montmorillonit e crystallites; (2) vermiculitization of the montmorillonite crystallite in terfaces and of inner montmorillonite layers; and (3) precipitation of illi te of fixed chemical composition. The I-S crystal grows by addition of illi te layers linked by K+ or NH4+ ions saturating the vermiculitic interfaces.