Kaolin group minerals include kaolinite, nacrite, dickite, metahalloysite and hydrated halloysite. Except hydrated halloysite, these minerals have the same chemical compositions, represented by the chemical formula Al₂O₃. 2SiO₂.2H₂O or Al₂Si₅O₅ (OH)₄. Hydrated halloysite is represented as Al2Si2O5 (OH)5.2H2O, possessing the same structure as metahalloysite except that it contains an excess of water molecules.

A unit cell of kaolinite contains two layers of tetrahedral SiO₂ and  a octahedral Al (O₂OH)6. Nacrite and dickite structures show shifts from one sheet to another in different directions. Metahalloysite is characterized by an irregular arrangement of the sheets and hydrated halloysite contains a layer of water between the sheets.

In natural clay, these minerals may be formed singularly or together. Clay is formed by weathering of pegmatitic rock where it may remain or to be washed out of the original deposit and deposited in low-lying lands. The former deposit is referred to as residual clay or primary clay; the latter is called sedimentary or secondary clay.

Shale clay is secondary clay compacted and hardened over the course of a long time. This clay is usually associated with coal seams in the old strata formed during paleozoic and mesozoic period. Shale clay available in China is known as China flint clay. The shale clay deposit in Japan occurs in Iwate Prefecture and is referred to as Iwate flint clay. Otavi flint clay of South Africa is its another big deposit.  

The flint clay develops little plasticity even after grinding. Thus it, is used not as a crude clay but as an aggregate after calcination. Mineral constituents of this clay are kaolinite and alumina hydrate like gibbsite (Al₂O₃.3H₂O) or diaspore (Al₂O₃.H₂O). In general, alumina hydrate contained in flint clay of Chinese origin exists in the form of diaspore, where as Japanese flint clay contains gibbsite as its main alumina hydrate. The differences in the form of alumina cause different properties as a refractory material. In low alumina flint clay, anomalous expansion takes place between 1000 - 1300⁰C. This expansion is thought to be caused by illite which contains volatile matter which decomposes during heating. This clay is referred to as bloating clay. A well known example of this type of clay is used to produce bloaring bricks in USA.