Kaolin is the common name for the mineral products comprised totally or substantially of the aluminum silicate clay mineral kaolinite. Milled and air-classified grades of raw kaolin may contain small amounts of related sheet silicates (mica, illite, chlorite, smectite) and quartz. Most of the kaolin used by the coatings industry is water-washed to remove these mineral impurities. Kaolin has a platy structure, but unlike talc and mica, its value in coatings derives more from its contribution to optical properties than to physical properties.
The kaolinite structure can be depicted as a layer of silica rings joined to a layer of alumina octahedra through shared oxygens, as shown in Figure 1. A well-formed individual kaolinite particle has the shape of a hexagonal plate. In nature these plates occur in stacks or "books" that exhibit varying degrees of stacking regularity. Because an individual kaolinite particle has an oxygen surface on one side and a hydroxyl surface on the other, it is strongly hydrogen bonded to the plates above and below it.