This provides an opening for another ion of similar size such as potassium to be incorporated in the lattice. The octahedron has eight sides and six corners. It can be thought of as a double pyramid with a square base an apex both above and below the base. An octahedron results when the centers of six spheres such as oxygen atoms closely packed together are connected by lines. The space between the oxygen atoms of an octahedron is a little larger than that in a tetrahedron and is generally occupied by an aluminum atom forming the aluminum-hydroxyl octahedron.
Kaolinite clay is composed of one tetrahedral layer and one octrahedral layer.
The middle oxygen-hydroxyl layer is shared as part of both the tetrahedral and octrahedral layers. The characteristic C-axis spacing of the kaolinite clay, i.e., the vertical distance from one repeating unit to the next, is 7.2 A. Illite clay is composed of one tetrahedral layer on both the top and bottom of the clay with an octahedral layer sandwiched between. The two middle oxygen-hydroxyl layers are shared as part of both the tetrahedral and octrahedral layers both top and bottom. Note some substitution of aluminum for silicon in the tetrahedral layer. This type of clay may also be referred to as hydrous mica.