Geochemistry of the Trioctahedral Micas from Degana Granite

The composition of micas from Degana granite show significant chemical variation and trioctahedral nature. High Al₂O₃, and FeO content and a very distinctive depletion in MgO characterize the biotite whereas zinnwaldite has comparatively lower FeO and MgO. The biotites plot in the field of lithian-siderophyllite. The trioctahedral micas show high positive correlation between Si and Li and between Fe and Al^iv. and negative correlation between Si and Al^iv and Si and (Fe²⁺), suggesting that the coupled substitution Li^vi+Si^iv=(Fe²⁺"+Al^iv was operative during their evolution. The analysis of the micas show chemical continuity, suggcsting their continuous evolution from biotite to zinnwaldite composition. Late to post magmatic process for formation of zinnwaldite is favoured.