Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations.

Clay minerals include the following groups:

Kaolin group which includes the minerals kaolinite, dickite, halloysite and nacrite.
> Some sources include the serpentine group due to structural similarities (Bailey 1980).
> Smectite group which includes dioctahedral smectites such as montmorillonite and nontronite and trioctahedral smectites for example saponite.
> Illite group which includes the clay-micas. Illite is the only common mineral.
> Chlorite group includes a wide variety of similar minerals with considerable chemical variation.
> Other 2:1 clay types exist such as sepiolite or attapulgite, clays with long water channels internal to their structure