Al2Si2O5(OH4). A hydrated aluminium silicate mineral which is the principal constituent of kaolin clay. Kaolinite crystals have a lamellar or plate-like structure that gives the clay its slippery feel. It is a natural alteration product of aluminium silicate rocks, such as feldspar. Pure kaolinite is a primary clay. Most secondary clays contain only a small percentage of kaolinite (Fournier 1996). Museum of Fine Arts, Boston (2000); Fournier (1996). [J. Cassar, R. de Angelis]