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Abstract

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.

  1. 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).

  2. 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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