group of common clay minerals that are hydrous aluminum silicates; they comprise the principal ingredients of kaolin (china clay). The group includes kaolinite and its rarer forms, dickite and nacrite, halloysite, and allophane, which are chemically similar to kaolinite but amorphous.

Kaolinite, nacrite, and dickite occur as minute, sometimes elongated, hexagonal plates in compact or granular masses and in micalike piles. They are natural alteration products of feldspars, feldspathoids, and other silicates. Anauxite, which was previously regarded as a kaolinite-group mineral possessing a higher than usual silica-alumina ratio, is now considered to be kaolinite and free silica (mainly noncrystalline). For chemical formula and detailed physical properties, see clay mineral (table).