The spinel species magnetite (Fe3O4) is found in a huge range of different rock types. The equilibrium between quartz, olivine, and magnetite is used as a reference to specify a commonly-occuring oxygen pressure typical of rocks formed near the Earth's surface. In addition, magnetites in many igneous and metamorphic rocks are titaniferous (containing ulvospinel, Fe2TiO4, in solid solution). Titaniferous magnetites coexisting with ilmenite solid solution are sensitive to both temperature and redox conditions (oxygen fugacity), and in favorable conditions the compositions of both phases can indicate the temperature at which they formed.
from: Mineralogy and Optical Mineralogy (2008) Dyar, M. and Gunther, M.; Mineralogical Society of America.
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