Abstract
Phengites in the Piemonte calcschists from the western Alps, Italy were examined using an electron micro probe analyzer (EMP). The EMP analyses revealed that the phengites in five samples show variable Si/Al and Na/K ratios within a single thin section and even within individual crystals. Their interlayer cation content is less than 1.0 pfu (K + Na + Ca + Ba = 0.82-0.95 for O = 11), although most paragonites in these samples have values of 1.0. Their chemical compositions plot in the region above an ideal Tschermak substitution line on an Si-Al diagram even though paragonite compositional data invariably lie close to this line. The cation balance indicates that the interlayer cation vacancies are due not only due to K1Al1 − [ ]−1Si−1 substitution (pyrophyllite component) but also to a deficiency in alkaline elements. The charge balance in the crystals should be buffered by ions such as H3O+, NH4+ or Fe3+ that can not be detected by EMP. Large flakes of phengite (ca. 0.3 mm × 0.6 mm) are observed in one of five samples. A back-scattered electron image indicates that brittle deformation of these large crystals took place and was associated with formation of secondary low-Si and high-Na phengites, which overgrow the rim and fill crack of high-Si phengite. This strongly suggests that the vacancies developed during the Alpine HP metamorphism and were preserved during the retrograde metamorphism accompanying the ductile and brittle deformation.
