kaolinite, clay mineral crystallizing in the monoclinic system and forming the chief constituent of china clay and kaolin. It is a hydrous aluminum silicate commonly formed by the weathering and decomposition of rocks containing aluminum silicate compounds; feldspar is a chief source. Kaolinite has the same chemical composition as dickite and nacrite (both of which are also clay or kaolin minerals) but differs from them in origin, in optical properties, in reaction to heat, and in certain other physical properties. Kaolinite is the basic raw material for ceramics, and large quantities are also used in the manufacture of coated paper.