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.