The most important property of smectite clay minerals is the behaviour in contact to water. The weak layer charge allows the interlayer cations to adsorb water molecules to form a hydration shell. This inner-crystalline swelling can be observed by an increase in volume of the clay minerals which can be many times the original volume. Monovalent cations present, such as Na-ions in the interlayer space of the minerals, can hydrate to the extent that individual platelets are separated from each other