Crystal chemistry of dioctahedral micas from peraluminous granites: The pedrobernardo pluton (Central Spain).

The aim of this work is an introductory study on the crystal chemistry of igneous muscovite, with particular focus on samples from the Pedrobernardo pluton (central Spain), whose crystal structure and chemistry is compared with data from literature. This pluton is a layered sheet-like granite body with distinct horizontal zoning and it can be subdivided into three main zones (lower, middle, and higher zone) characterized by different crystallization conditions. When chemical composition of muscovite from Pedrobernardo pluton is compared to literature data, different chemical trends can be pointed out. Samples from middle and lower zones follow a same trend, whereas samples from higher zone follow another. In samples from middle and lower zones Ti4+ increases without affecting Fe content (Fe2+ and/or Fe3+, thereafter referred to as Fe). Fe and Mg2+ can enter into the layer in similar amount. In samples from higher zone Ti4+ content is lower than in lower and middle zones, whereas Fe is higher. Fe content is related to Ti4+ content and Fe seems to substitute for octahedral Al3+ more easily than Mg2+. Samples from the middle and lower zones are characterized by substitutions involving both tetrahedral and octahedral sites, whereas samples from higher zone are characterized by a preference for octahedral substitutions. Single crystal structure refinements suggest a partial occupancy of the octahedral trans-site (M1) related to F- for OH- substitution and probably to a different O-H vector orientation. Lateral cell parameters are found to be affected by the octahedral cis-site mean bond distance (M2-O), which, for a great number of samples under consideration is linked to Fe2+ content. M1-O distance as well as tetrahedral thickness was found to be affected by the c parameter, following the structural mechanisms induced by chemical substitutions previously described.These parameters seem thus to be affected more by electrostatic interactions involving the whole layer rather than by site occupancy and local chemical composition.