Clay mica is a three layer mineral with important properties for casting like swelling capacity (see Swelling), thixotropy and ion exchange capacity. The medium layer consists of (AlO₆) octahedrons and at each side, there is a layer of (SiO₄) tetrahedrons. The layers are not rigidly connected and can swell as a result of the storage of water. When illite is heated, it loses its crystal water at 100 - 200 °C and 500 - 600 °C. The crystal lattice further expands at 810 °C and is then destroyed under the formation of spinel. Besides illite, montmorillonite is the most important three layer mineral.

The uniform crystallographic structure of clay minerals (tetrahedron – octahedron) offers the possibility to combine various layer packages to regular or irregular crystallites. This type of structures is referred to as mixed layer minerals. The montmorillonite-illite mixed layer mineral can be frequently found.

Additional references:
Active bentonite
Betonite
Calcium bentonite
Sodium bentonite
Smectites