PETROGENETIC IMPLICATIONS OF THE FE3+ CONTENT OF MUSCOVITE IN PELITICSCHISTS

The Fe3+/Fe2+ ratio of Muscovite from typical metapelites is large. it reflects the opaque mineral assemblages in the rocks but appears to b e independent of metamorphic grade. After correction for Fe3+, the Mg/ (Mg + Fe2+) ratio is increased such that it affects any thermodynamic calculation. This study shows that a good approximation of the effects of Fe3+ on the Mg/(Mg + Fe2+) ratio Can be made by considering the op aque minerals present in a given sample. In other studies, the authors have shown the magnitude of similar approximations that can be made f or the biotite of common metapelites. Hence, corrections for the effec ts of Fe3+ on Mg/(Mg + Fe2+) ratios of these minerals can and should b e routinely made when dealing with typical metapelites.