A structural model is proposed for illite-smectite (I-S) from diagenetic environments which accounts for the presence of three different layer types which are defined as follows: montmorillonite (low-charge, octahedrally substituted, fully expandable), vermiculite (high-charge, octa- and tetrahedrally substituted, only partly expandable) and illite (K0.9Si3.3Al0.7R3+1.8R2+0.2O10(OH)2). All three layers may be found within the MacEwan crystallites, whereas external edges of the crystallites are only vermiculitic during the illitization process. In the proposed model, a layer is defined symmetrically on each side of the interlayer space, leading to the existence of polar 2:1 units. It is proposed that the I-S growth is a three step mechanism: (1) formation, from sediments of variable composition, of montmorillonite crystallites; (2) vermiculitization of the montmorillonite crystallite interfaces and of inner montmorillonite layers; and (3) precipitation of illite of fixed chemical composition. The I-S crystal grows by addition of illite layers linked by K+ or NH+4 ions saturating the vermiculitic interfaces.