Ilmenite is widespread in igneous rocks as an accessory mineral, but is seldom concentrated or found in large crystals except in pegmatites and large bodies of plutonic rock. Its crystals are typically rhombohedral. It has no cleavage and a conchoidal fracture. It also occurs in metamorphic rocks.
Because of its resistance to weathering, ilmenite is commonly concentrated (along with magnetite) in heavy black sands where the host rock is deeply weathered. For many years ilmenite was an undesirable contaminant in iron ores, but today titanium is much more valuable. At high temperatures ilmenite and hematite dissolve together, but they separate as they cool, leading to occurrences where the two minerals are interlayered at a microscopic scale.
