Some petrogenetic aspects of Fe2O3: FeO ratio in igneous rocks

The "oxidation ratio" of igneous rocks (F = Fe₂O₃:FeO) depends on the partial pressure of oxygen during crystallization and therefore, in part, on depth of crystallization. For this reason F increases in the order: deep intrusives, dike rocks, extrusives, pyroclastics. Also, in general, F increases with increasing silica content, although there are exceptions. As it is not always easy to distinguish intrusive rocks from extrusive ones, or extrusives from pyroclastics, it is proposed that the oxidation ratio be used as a criterion of fades of igneous rocks. --F. E. Ingersoll.