For vermiculite, isomorphous substitution mainly occur at

Vermiculite is a type of clay consisting of two tetrahedral silicate layers coupled with one octahedral magnesium hydroxide-like layer.
Vermiculite is a hydrous phyllosilicate mineral that undergoes significant expansion when heated. Vermiculite clays are weathered micas in which the potassium ions between the molecular sheets are replaced by magnesium and iron ions
. Isomorphic substitution is the process of replacing one structural cation for another of similar size. It only occurs between the cations of similar ionic radii.
In vermiculite, isomorphous substitution mainly occurs at the Tetrahedral sheet.