Ilmenite is an iron-oxide mineral of the oxide and hydroxide group with structural formula (Fe,Ti)2O3. The structure is similar to that of hematite, where the two Fe3+ in hematite are replaced by one Fe2+ and one Ti4+ in ilmenite. It consists essentially of a dense arrangement of Fe2+ and Ti4+ ions in octahedral coordination with oxygens in hexagonal closest-packing. The structure may also be seen as the stacking of alternating sheets of octahedrally coordinated Fe2+ and Ti4+ in hexagonal closest packing with oxygen. Since Fe2+ and Ti4+ account for a total valency of +6, only two out of three available oxygen octahedral positions are occupied. This arrangement makes the structure neutral with no charge excess or deficit.

Ilmenite may contain significant isomorphic substitutions of Mg2+ and Mn2+ for both Fe2+, and probably forms solid solutions with both geikielite (Mg,Ti)2O3 and pyrophanite (Mn,Ti)2O3 end-members. Ilmenite commonly shows intergrowth with hematite and magnetite. The crystals are totally opaque and appear as thick tabular crystals or anhedral masses embedded in igneous and metamorphic rocks, or as opaque shapeless sand-size grains. Ilmenite is a weakly magnetic mineral. The color is commonly iron-black, brownish-black, with metallic to sub-metallic luster. The name “ilmenite” is from the Ilmen Mountains in Russia.

Distribution

Ilmenite is an opaque accessory mineral in a wide range of igneous and metamorphic rocks, especially gabbroic varieties, where it is associated with hematite and magnetite as lattice-type intergrowths, and other minerals such as plagioclase and hornblende. Masses of ilmenite were found in several emery mines in the USA, Norway, Canada and Russia. Its principal use is as an ore of titanium.

Importance in soil environments:

Ilmenite is a heavy, opaque mineral formed at high temperatures. It occurs in igneous rock as an accessory mineral and in sediments due to its resistance to weathering. It is mostly found in the sand and silt fractions. It can be isolated from soils or sediments with a magnet. Alteration of ilmenite yields fine-grained products, such as leucoxene. Ilmenite may be found as a constituent of plinthites along with other iron oxides.

Wechsler, B.A., and C.T. Prewitt. 1984. Crystal structure of ilmenite (FeTiO3) at high temperature and high pressure. American Mineralogist. 69:176-185.