Ceramics-Silikáty 27, (1) 1 - 18 (1983)

Polytypic crystal structures of pyrophyllite and talc with idealized symmetry of their tetrahedral sheets are OD structures consisting of two kinds of layers and belonging to category I. This explains the polytypism of these minerals from the point of view of their symmetry and facilitates the derivation of their standard (MDO) polytypes which are needed for establishing identification criteria. There are 30 non-congruent (22 non-equivalent) MDO polytypes within the pyrophyllite family and 12 (10) MDO polytypes within the talc family. The ideal ditrigonalization of tetrahedral sheets leaves the number of MDO polytypes unchanged, but reduces the stacking possibilities of individual OD layers and causes individual polytypes to appear as members of either of two crystallochemically independent subfamilies within the corresponding pyrophyllite or talc family. Individual polytypes are characterized by fully descriptive symbols which fit into a unified symbolism of polytypes of sheet silicates.