Polytypism: in micas

Abbott RN Jr, Burnham CW (1988) Polytypism in micas A polyhedral approach to energy calculations. Am Mineral 73 105-118... 

Ferraris G Ivaldi G (1994a) Some new hypothesis on the structural control of polytypes in micas. EMPG-V Abstr suppl No 1 to TERRA nova 6 52... 

Although the causes of the complexity of the phenomenon of polytypism in micas are multifaceted, they can be simplified to magic words , local (partial) symmetry, and a magic number , 3. As shown hereafter, each atomic plane in mica has an ideal symmetry of at least trigonal, which is preserved in each of the two kinds of sheets (tetrahedral and octahedral), but it is reduced to monoclinic when considering the layer as... 

Since the pioneering study of Mauguin (1928), polytypism of micas has been extensively investigated, both experimentally and theoretically. Polytypism in micas... 

Baroimet A (1980) Polytypism in micas A survey with emphasis on the ciystal growth aspects. In Current Topics in Materials Science Vol. 5, E Kaldis (ed) North-Holland Publishing Company, p 447-548... 

Polytypism in layered silicates has been well characterized. There are as many as 19 polytypes known among micas. The repeat unit in muscovite mica, [KAl2(OH)2(Si3Al)0 o], for instance, consists of a sheet of octahedrally coordinated aluminium ions sandwiched between two identical sheets of (Si, AOO tetrahedra, the large ions being located in interlayer positions. Surface oxygens of the tetrahedra in... 

Most of the trioctahedral tme-mica stmctures are M polytypes and a few are 2Mi, 2M2, and 3T polytypes. In dioctahedral micas, the 2Mi sequence dominates, although 3T and M structures have been found. Brittle mica crystal-structure refinements indicate that the IM polytype is generally trioctahedral whereas the 2Mi polytype is dioctahedral. The 10 structure has been found for the trioctahedral brittle mica, anandite (Giuseppetti and Tadini 1972 Filut et al. 1985) and recently was reported for a phlogopite from Kola Peninsula (Ferraris et al. 2000). The greatest number of the reported structures were refined from single-crystal X-ray diffraction data, with only a few obtained from electron and neutron diffraction experiments. 

Bailey (1984c) recommended a notation system for structural sites in micas. However, there is no agreement to the labeling of these sites e.g., either an italic or roman font is used with or without parentheses to separate the alphanumeric parts. Following recent papers (Nespolo et al. 1999c Nespolo 2001) and in agreement with the chapter of Nespolo and Durovic (this volume), the nomenclature of the OD theory of polytypes (Domberger-Schiff et al. 1982 Durovic 1994) is here adopted to label sites. 

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. 

Table 2. Comparative classification of mica polytypes in the homo-octahedral approximation. 

The relations of homomorphy in mica structures are summarized in Table 7. Full symbols are given for homo-and meso-octahedral polytypes, shortened symbols (the line of orientational characters) - for hetero-octahedral polytypes. The reason for the somewhat unusual layout of this table is related to the fact that two out of the six homo-octahedral MDO polytypes, IMand 20, have the same projection normal to [010] (YZ projection). Thus, for the framework of the non-octahedral atoms in the homo-octahedral MDO polytypes (and also for the corresponding homo-octahedral approximations), there exist jive different YZ projections labeled by Roman numbers I to V in the first column of Table 7. The significance of the YZ projections will be explained below in the section Identification of MDO polytypes . 

From the examples above it is evident that 1) the homo-octahedral approximation corresponds to applying to a polytype the relation of homomorphy 2) in micas, the classical Ramsdell notation rigorously applies to homo-octahedral polytypes only. 

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. 

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