Abstract polytypes

In micas (as well as in many other phyllosilicates) the Pauling model and also the homo-octahedral approximation are abstractions which are very useful, among others, for didactic purposes to gain first knowledge, but also for the calculation of identification diagrams of MDO polytypes, and for the calculation of PID functions, described in sections about experimental identification of mica polytypes below. A better approximation, but still an abstraction, is the Trigonal model, which is important for the explanation of subfamilies and for some features in the diffraction patterns. Also, when speaking of a specific polytype, a characteristic sequence of abstract mica layers is intended rather than deviations from stoichiometry, distribution of cations within octahedral sheets, distortion of coordination polyhedra, etc. 

---

Bloch AM, Zhukhlistov AP, Zvyagin BB (1990) Centrosymmetric and noncentrosymmetric one-layer polytypes of metasomatic sericites in the upper Devon of die Tuva Through. Abstracts of the 15th general Int l Mineral Assoc Meeting (Beijing China) 1 297... 

The geometrical equivalence must be fulfilled not necessarily by the real layers, but by their archetypes, i.e. the (partially) idealized layers to which the real layers can be reduced by neglecting some distortions occurring in the true structure. The notion of polytypism becomes thus unequivocal only when it is used in an abstract sense to indicate a structural type with specific geometrical properties. In micas, these archetypes are the layers described by the Trigonal model. Of the several kinds of layers presented in the previous section, the OD layers, and the OD packets, are the most suitable ones to both show and exploit the geometrical equivalence. 

Collected abstracts, 9th International Clay Cortference, Strasboirrg (France), p. 430 Weiss Z, Wiewiora A (1986) Polytypism of micas, in. X-ray Diffraction Identification. Clays Clay Minerals 34 53-68... 

---