Distribution Coefficient (KDFe2+-Mg) of Orthopyroxene-Biotite and Garnet-Biotite Mineral Phases in Relanon to Equilibrium

Petrographic study of the granitic rocks of Chipurupalle-Razam area in Srikakulam district of Andhra Pradesh showed that biotite was formed either from orthopyroxene or garnet in reaction with potassium feldspar, under textural disequilibrium conditions. To test whether chemical equilibrium was attained during the formation of biotite, iron distribution coefficient (K_D^Fe2+-Mg) in orthopyroxene-biotite and garnet-biotite mineral phases was considered. K_D of orthopyroxene-biotite pair varies from 1.1 to 2.0, while that of garnet-biotite pair varies from 2.6 to 6.5. After considering the effects of temperature difference and of other elements on the distribution coefficient, the large variation in K_D values in the area was mainly related to the chemical disequilibrium conditions which operated during the biotite formation. An examination of K_D values from different areas showed that in a majority of orthopyroxene-biotite phases and in all garnet-biotite phases, biotite is Mg-rich while the coexisting orthopyroxene or garnet is Fe-rich; and this was explained by the principles of crystal chemistry.