Smectites

Layered silicates with three-layer structure belonging to the group of clay minerals (s. Three-layer mineral)

This structure includes two tetrahedron layers which are electrostatically linked via the cations of an octahedron intermediate layer. The layers are not rigidly connected but can swell as a result of the storage of water and other substances. The most familiar representative of smectites is the montmorillonite.

Additional references:
Swelling