Crystallography of mica polytypes
Published:January 01, 1997
Several approaches have been introduced to describe mica polytypes, using numeric representations (Ross et al., 1966; Takeda & Sadanaga, 1969; Zvyagin, 1962, 1967, 1974; Zvyagin et al., 1979; Zhukhlistov et al., 1990; Takeda & Ross, 1995), vector schemes (Smith & Yoder, 1956; Takéuchi & Haga, 1971) or both (Dekeyser & Amelinckx, 1953; Thompson, 1981). Among them, two are most suitable to represent the layer stacking in mica polytypes.
RTW symbols (Ross et al., 1966). Through an orientation-free, rotational description, they give an immediate representation of the stacking sequence. These symbols represent the simplest tool to derive all the possible polytypes with a given number of layers (Takeda, 1971; Mogami et al., 1978).
Zvyagin’s three-storied azimuthal orientation symbols, for shortness hereafter indicated as “Z symbols”. We make reference to the “second generation“ ones, described in Zvyagin et al., (1979) and in Zvyagin (1985), and not to the original, abandoned ones, introduced in Zvyagin (1962) and Zvyagin (1967). These symbols use an orientational description linked to a space-fixed reference, permit the localization of symmetry elements in the space, and are the ones to be used in calculating structure factor equations.
The OD interpretation of micas has been presented by Dornberger-Schiff et al. (1982a), Backhaus & Durovic (1984), Durovic et al. (1984) and Weiss & Wiewióra (1986).
Mica structures and polytypes exhibit local symmetry higher than those shown by their space group and unit cell translations (Sadanaga & Takeda, 1968). These features can be studied
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Modular Aspects of Minerals
Since the first beginning of the crystal chemical study of the inorganic compounds, a simple modular approach was developed, by looking at the crystal structures as built up through the assembling of simple polyhedral units. This approach was no only useful for a vivid and insightful description of the complex atomic arrangements of natural and synthetic compounds, but, through the use of simple and powerful rules for assembling polyhedral units, it displayed an extraordinary heuristic power, suggesting reliable models for many complex structural assemblages. The polyhedral approach also laid the basis for meaningful classifications which were applied to all the classes of inorganic compounds.