Abstract

Vesuvianite grains in a calc-silicate layer from the Canigou Massif (Pyrenees, France) display oscillatory concentric birefringence-zoning and a locally developed, discordant, nearly isotropic marginal zone. The boundaries between the zones correspond to sharp changes in the abundance of Ti, Al, Mg and Fe, although only Ti displays a consistent positive correlation with birefringence. Systematic variation in cation abundances also occurs within individual zones of equal birefringence, indicating that compositional zoning is, to some extent, independent of birefringence. Several schemes of cation substitution appear to have been operative in the vesuvianite, and different substitutions seem to have dominated at different times. The dominant substitutions in the higher-birefringence zones seem to be Ti+Mg<-->2Al and Fe (super 3+) <-->Al, whereas Mg<-->Fe (super 2+) may occur in addition to Ti+Mg<-->2Al in the lower-birefringence zones.

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