Usually contains some water. Polymorphous; the three naturally occurring forms are andalusite, cyanite, sillimanite. Other aluminum silicate minerals are anauxite, dickite, kaolinite, kochite, mullite, newtonite, pyrophyllite, takizolite, termierite, ton. For prepn and properties see Gmelins, Aluminum (8th ed.) 35B, p 313-317 (1934).