Original paper

Crystal chemistry of dioctahedral micas from peraluminous granites: the Pedrobernardo pluton (Central Spain)

Benincasa, Emanuele; Brigatti, Maria Franca; Poppi, Luciano; Barredo, Fernando Bea

Abstract

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 the literature. This pluton is a layered sheet-like granite body with distinct horizontal zoning, which can be subdivided into three main zones (lower, middle, and upper) characterized by different crystallization conditions. Different chemical trends can be identified when the chemical compositions of muscovite from the Pedrobernardo pluton are compared with literature data. Samples from the middle and lower zones follow a similar trend, whereas samples from the upper zone follow a different trend. In samples from the middle and lower zones, Ti4+ increases without affecting Fe content (Fe2+ and/or Fe3+, referred to here as Fe). Fe and Mg2+ can enter into the layer in similar amounts. In samples from the upper zone, the Ti4+ content is lower than in the lower and middle zones, whereas Fe is higher. Fe content is related to Ti4+ content and 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 the upper 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 substitution of Ffor OH-, and probably also 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/iso-ents/isotech/rang.gif___rang), which is linked to Fe2+ content for a large number of the samples under consideration. The 〈M1-O/iso-ents/isotech/rang.gif___rang distance as well as tetrahedral thickness are found to be affected by the c parameter, following the structural mechanisms induced by chemical substitutions described in previous studies. These parameters thus appear to be affected more by electrostatic interactions involving the whole layer rather than by site occupancy and local chemical composition.

Keywords

muscovite-2 m 1crystal structurechemistryperaluminous granites.