Micas polytypes

The structures in which there is trigonal prism coordination of the metal atom contain pairs of adjacent S layers which are directly superposed (and therefore not close-packed), but the multiple S-M-S layers are then packed in the same way as simple layers in normal c.p. sequences. The simplest structures of this kind are illustrated in Fig. 4.11 (p. 130), in which the nomenclature is similar to that used for mica polytypes. No example of the (2T) structure is yet known, but examples of three structures are ... 

Several authors used symbols and orientations that differ from convention to describe geometric arrangements of the layer and the stacking sequence of mica polytypes (e.g., Radoslovich 1961 Durovic 1994 Dornberger-Schiff et al. 1982). To make inter-structure comparisons of features easier, however, it is advantageous to define briefly the site nomenclature adopted and the parameters used to describe and characterize layer geometry. The direction defined by the stacking of 2 1 units defines the... 

Ohta T, Takeda H, Takeuchi Y (1982) Mica polytypism similarities in the crystal stmctures of coexisting IMand 2Mj oxybiotite. Am Mineral 67 298-310... 

Takeda H, Haga N, Sadanaga R (1971) Stractmal investigation of a polymorphic transition between 2M2-, lA/-lepidohte and 2Mi-muscovite. Mineral J 6 203-215 Takeda H, Ross M (1975) Mica polytypism Dissimilarities in the crystal structures of coexisting IM and 2M biotite. Am Mineral 60 1030-1040 Takeuchi Y (1965) Stractrrres of brittle micas. Clays Clay Minerals 13 1-25... 

The partial occupancy of the Ml site by divalent cations, and a greater Si content in the tetrahedral sheet, should decrease the compressibility. Therefore the presence of a phengitic component probably increases the mica stiffness, as shown by the increased bulk modulus of phengite-SZ in comparison with that of end-member muscovite (Pavese et al. 1999b). The difference found, however, could be partially ascribed to the different mica polytype. A statistical study on the occurrence of metamorphic phengites by Sassi et al. (1994) seems to show that the crystallization of the 3T polytype is mainly favored by high PIT ratio conditions. 

However, anandite-20 cannot be considered a real mica polytype (Ferraris et al. 2001c) for the following reasons its P cell is not compatible with the C-centered cell common to all mica polytypes its space group is not that expected (Ccmm) for the 20 mica polytype according to the OD theory S substituting OH is coordinated by the interlayer cation which thus has coordination number 13. 

Table 2. -symmetry S) and type of layer (Z) in the three families of mica polytypes within the Trigonal model. 5 indicates the electron density of the octahedral site (site occupancy). 

Probably features other than crystal chemistry concur to explain the wide distribution and persistence of variously sliced mica modules. The high symmetry of the mica modules could be a key feature, in the sense that it favors different stacking and connections with other modules both of the same kind and different nature. Mica polytypes and twins are clear examples of symmetry-assisted structures. The flexibility of... 

X-pp ellipes. The intensity distribution in the 1st ellipse (as well as other X-type ellipses) is typical of each mica polytype. Knowing the symmetry principle (subfamily A or B, or mixed-rotation, revealed by the 2nd ellipse) helps to obtain the stacking sequence from the intensity distribution in the 1st ellipse. Because the X rows are non-family rows in both the Pauling and trigonal model, the computation of the intensities in the X-type ellipses, to be compared with those experimentally measured, can be performed even in the simplest Pauling model. 

Ferraris G Gula A, Ivaldi G Nespolo M, Sokolova E, Uvarova Y, Khomyakov AP (2001c) First structure determination of an MDO-2G mica polytype associated with a IM polytype. Em J Mineral 13 ... 

Nespolo M (2001) Perturbative theory of mica polytypisrrr. Role of the M2 layer in the formation of inhomogeneotrs polytypes. Clays Clay Minerals 49 1-23 Nespolo M, Ferraris G (2001) Effects of the stacking faults on the calcrrlated electron density of mica polytypes -The Drrrovic effect Eur J Mineral 13 1035-1045... 

Nespolo M, Takeda H, Kograe T, Ferraris G (1999c) Periodic interrsity distribntion (PID) of mica polytypes Symbohsm, stmctural model orierrtation and axial settings. Acta Crystallogr A55 659-676 Newrrham RE (1961) A refinement of the dickite stmcture and some remarks on polymorphism of the kaolin minerals. Minerd Mag 32 683-704... 

Nickel HN, Grice JD (1998) The IMA commission on new minerals and mineral names Procedtrres and guidelines on mineral nomenclature, 1998. Can Mineral 36 913-926 Ohta T, Takeda H, Take uchi Y (1982) Mica polytypism Similarities in the crystal stmctures of coexisting IM and 2Mi oxybiotite. Am Mineral 67 298-310... 

Smyth JR, Jacobsen SD, Swope RJ, Angel RJ, Arlt T, Domanik K, Holloway JR (2000) Crystal stractures and compressibility of synthetic 2Mi and 3T phengite micas. Eur J Mineral 12 955-963 Soboleva SV (1987) Mica polytypes Theoretical and applied aspects. Mineral J (Ukraine) 9 26-41 Takeda H, Burnham CW (1969) Fluor-polylithionite A lithium mica with nearly hexagonal (Si205) " ring. Mineral J (Japan) 6 102-109... 

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

The fundamental merit of Z symbols is that they can describe also meso-octahedral polytypes. Their shortcoming is that the symbols describing homo-octahedral mica polytypes are identical with those describing meso-octahedral polytypes consisting of Ml layers, and additional information must be given also. Moreover, in their present form, they cannot handle hetero-octahedral polytypes. 

C) OD symbols. The OD school, inspired by Z symbols, derived the most general symbols to describe mica polytypes (Durovic and Domberger-Schiff 1979 Dornberger-Schiff et al. 1982 Backhaus and Durovic 1984 Durovic et al. 1984 Weiss and Wiewiora 1986). These symbols consist of a sequence of characters referring to one period, placed between vertical bars two lines of characters are used the first line indicates the packet orientations, and the second line the packet-to-packet displacements. A dot separating the orientational characters for packets P2y and q2y+i indicates the position of Oc layer. The OD symbols are thus expressed ... 

Mica polytypes can belong to five symmetries A, T, O, M and A (Takeda 1971). In both the Pauling and the Trigonal models, the lattice of triclinic polytypes is metrically monoclinic, and the (001) projection of the c axis, labeled c , can take three values 0, a /3, 6 /3, on the basis of which mica polytypes are classified into orthogonal. Class a and Class b respectively. The number A of layers building a polytype can be expressed as ... 

LOCAL AND GLOBAL SYMMETRY OF MICA POLYTYPES FROM THEIR STACKING SYMBOLS... 

Table 5a. Conversion of characters appearing in the OD s nnbols of mica polytypes. The individual operations are characterized by their extended Hermann-Mauguin (H-M) symbols and by the corresponding operations indexed in orthogonal (ORT) C2 -setting axes. Cf. Table 1 and Backhaus and Durovic (1984).

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. 

IDEAL SPACE-GROUP TYPES OF MICA POLYTYPES AND DESYMMETRIZATION OF LAYERS IN POLYTYPES... 

Figure 13. Definition of the S, and axial settings of mica polytypes.

The geometrical characteristics of the reciprocal lattice rows parallel to c, each taken as a whole, are termed "row features". In the Trigonal model all mica polytypes have the same row features, described by the regular tessellation 3,6 (Takeda and Donnay 1965 see the section Tessellation of the hp lattice ), and the nine R, were classified into three types (Fig. 16) ... 

Family structure aud family reflectious of mica polytypes... 

Table 16. Orientation of the individuals building a twin in Class a mica polytype. Angles in parenthesis express the counter clockwise rotations of the whole twinned edifice. Shift stands for the shift of the origin along c. (010) means reflection of the twin lattice across the (010).plane, which is equivalent to inverting the direction of rotation of the individuals in the twin, i.e. to the symbol transformation 3IJ... P 3(6-I)(6-J)...(6-P).

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