Iron-Titanium Oxide Minerals and Synthetic Equivalents

Phase-equilibrium studies in the system FeO-Fe 2 O 3 —TiO 2 permit determination of the temperature and oxygen fugacity of formation of coexisting pairs of titaniferous magnetite and ilmenite in many rocks. Temperatures thus obtained are probably accurate to ±50°C. Temperatures indicated for most igneous and metamorphic rocks for which data are available are generally consistent with temperatures inferred by other methods. Temperatures for certain gabbroic rocks are too low for magmatic crystallization and probably reflect the migration of ilmenite from titaniferous magnetite to form separate granules upon cooling. The experimentally determined solubility of ilmenite in magnetite is much too small to account for most ilmeno-magnetites by simple exsolution. Subsolidus oxidation of magnetite-ulvöspinelg SS to yield ilmenite-magnetite intergrowths has been experimentally verified and probably takes place during cooling of many igneous and perhaps some metamorphic rocks. Oxidation at surface or hypabyssal conditions may produce metastable titanomaghemites. In order of increasing intensity of oxidation, the following Fe—Ti oxide pairs occur in plutonic rocks: ulvöspinel-rich magnetite SS +ilmenite SS , ulvöspinel-poor magnetite SS +ilmenite SS , ulvöspinel-poor magnetite SS +hematite SS , hematite SS +rutile.