Three-layer mineral

The basic components of clay minerals are SiO4 tetrahedrons and Al(OH)6 octahedrons (figure 1). These basic components are linked to form indefinitely level layers. Depending on the conditions of formation, so-called layer packages were formed during the overlapping of these networks. Accordingly, the clay minerals are classified in two-layer and three-layer minerals.

The three-layer mineral consists of a combination of one octahedron layer with two tetrahedron layers. The large variety of clay minerals in this group results from the fact that in these kinds of three-layer minerals it is possible to replace the Si4+ ion of the tetrahedron layer by Al3+ ions and/or the Al3+ ion of the octahedron layer by Mg2+, i.e. by lower-valence ions. This results in the generation of negative excess charge in the layer packages, which is balanced out by cations (e.g. Na+, K+).This ability is the basis for the technically relevant properties of this kind of clay minerals, such as swelling, thixotropy, and ion exchange capacities. The most important three-layer mineral is montmorillonite.

Additional references:
Active bentonite
Calcium bentonite
Sodium bentonite

  • Figure 1: Structure of the montmorillonite