Stoichiometric consequences of metamorphic mineral-forming reactions in pelitic rocks

Metamorphic mineral-forming reactions are used to predict the volume proportions (percentages) of minerals in metapelitic rocks. Only minerals whose composition can be expressed in an idealised pelitic system (SiO₂Al₂O₂FeOK₂OH₂O) are considered in postulating the chemical reactions. The volume proportions predicted from the products of the chemical reactions are compared with the volume proportions of the minerals actually present in selected samples of metapelitic rocks from the Michigamme Formation (Michigan, U.S.A.).Evidence is presented to show that the reaction: 4 $\text{muscovite}\text{+3(}\text{Fe, Mg}\text{)}{}^{\mathrm{2+}}\text{? 3}\text{staurolite}\text{+6}\text{quartz}\text{+4}\text{K}{}^{\mathrm{+}}\text{+2}\text{H}{}^{\mathrm{+}}$ produces staurolite poikiloblasts in which the estimated quartz to staurolite ratio is 1:2.28.