Oxidation of Iron-Titanium Oxides in Igneous Rocks

Abstract

Iron-titanium oxides in the compositional range magnetite-ulvospinel-ilmenite are likely to undergo oxidation during cooling in the presence of magmatic gases or air. The mineralogy and magnetic properties of igneous rocks depend on the initial composition of the oxide phases, the composition (particularly with regard to sulfur) and amount of gas phase, and the relative rates of cooling, unmixing, and oxidation. Tlfe oxides in an igneous rock may have a more complicated chemical and magnetic history than a deceptively simple mineralogy might lead one to suspect. Several mechanisms for self-reversal of the remanent magnetization are described. Approximate free energies of formation have been calculated for ulvospinel and maghemite. Partial pressures of oxygen have been calculated for some oxidation reactions involving ulvospinel. The equilibrium between members of the ilmenite-hematite and ulvospinel-magnetite series has been investigated.