Influence of tetrahedral aluminium on the hydrothermal transformation of montmorillonite and beidellite to mixed‐layer illite‐montmorillonite and illite

Abstract In weathering crusts, sedimentary rocks and zones of hydrothermal transformation frequently mixed‐layer illite‐montmorillonite will occur. Based upon new experimental transformations of montmorillonite and beidellite under hydrothermal conditions in the presence of potassium chloride it is shown that mixed‐layer illite‐montmorillonite, with decreasing expandable proportion, occurs in the form of transitional stages of the montmorillonite → illite transformation. Besides the increase of reaction temperature or potassium concentration, it is first of all an increased charge of the (Si, Al)O4 tetrahedral layers of the dioctahedral smectites that contribute considerably to accelerating the formation of illite‐montmorillonite mixed‐layers, lowering their expandable proportion and converting these mixed layers to illite.