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. 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

Metamorphosed pillow basalt from the actively spreading basin west of the Marianas Island Arc contains albite or oligoclase of intermediate to low structural state. A feldspathic vein occupying late fractures in one metabasalt was found to contain highly disordered plagioclase ranging in composition from An 3 to An 31 . From the absence of thermal metamorphic effects in the chlorite-montmorilloniteactinolitic amphibole-epidote-sodic plagioclase assemblage of the host rock we infer formation of the vein at low temperature. Individual feldspar grains consist of lamellae or irregular domains (compositionally distinct volumes, not the crystallographers' domains) of varied compositions. Electron microprobe analyses of these domains yield a bimodal distribution of compositions. The large sizes of the domains and the inferred low temperature of formation of the vein suggest to us that the domains may have formed as primary metastable growth features. The bimodality and other features of the compositional distribution suggest that the composition adjusted, but incompletely so, in accord with the peristerite solvus.