The micas in the Abas granite (northern Portugal) have been examined by means of electron microprobe, scanning and transmission electron microscopy, and partial chemical analyses on mica separates. Four different types of postmagmatic muscovites are distinguished in thin sections, and their chemical variations are described in terms of celadonite, paragonite, and pyrophyllite substitutions. The paragonite substitution indicates that the four muscovite types formed at successively lower temperatures. Generally, the biotites exhibit texturally magmatic features, whereas they chemically have a deficient octahedral occupancy, suggesting considerable alteration to muscovite or zinnwaldite. Our observations suggest that the chemical variations in the biotite composition can more likely be explained by solid solution rather than by fine-scale intergrowths of the end-member minerals. Deficient interlayer occupancy of the biotites is caused at least in part by fine-scale interlayering of retrograde chlorite.

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