Ilmenite

Ilmenite is a black or steel-gray mineral with the formula FeTiO3, a titanium-iron(II) oxide. It is weakly magnetic and serves as the primary ore of titanium, providing most of the titanium dioxide used as a white pigment in products like paints, inks, fabrics, plastics, paper, sunscreen, food, and cosmetics.

This mineral is heavy (specific gravity 4.7), moderately hard (Mohs 5.6–6), and opaque, with a submetallic luster. It typically occurs in massive form; thick tabular crystals are rare. It has no clear cleavage and breaks with a conchoidal to uneven fracture. Ilmenite crystallizes in the trigonal system (space group R3), with a structure derived from corundum but featuring alternating layers of Fe2+ and Ti4+ ions perpendicular to the trigonal c axis.

Pure ilmenite is paramagnetic, but it often forms solid solutions with hematite, making it weakly ferromagnetic. Natural deposits also commonly contain intergrown or exsolved magnetite, which adds to its magnetic attraction. It can be distinguished from hematite by its less intense black color, duller appearance, and black streak, and from magnetite by its weaker magnetism.

The mineral was first identified in 1791 by William Gregor, who found a black sand in a stream near Manaccan, Cornwall, and named it manaccanite after isolating titanium from it. The same mineral was later discovered in the Ilmensky Mountains near Miass, Russia, and renamed ilmenite.

Its ideal composition is FeTiO3, but natural ilmenite usually contains significant magnesium and manganese, along with up to 6% hematite (Fe2O3). The full formula is often written as (Fe,Mg,Mn,Ti)O3. It forms a solid solution series with geikielite (MgTiO3) and pyrophanite (MnTiO3). Kimberlite ilmenites are rich in geikielite, while those in highly differentiated felsic rocks may contain more pyrophanite. Above 950°C, ilmenite and hematite mix completely, but a miscibility gap at lower temperatures leads to exsolution lamellae in cooled samples. Ilmenite with 6–13% Fe2O3 is sometimes called ferrian ilmenite. Weathering alters ilmenite to leucoxene, a fine-grained material enriched to over 70% TiO2, which is an important titanium source in heavy mineral sands.

Ilmenite is a common accessory mineral in igneous and high-grade metamorphic rocks. It concentrates in layered intrusions as cumulate layers, often with orthopyroxene, or with plagioclase and apatite (nelsonite). Magnesian ilmenite occurs in kimberlites within the MARID assemblage (mica, amphibole, rutile, ilmenite, diopside). Manganiferous ilmenite appears in granitic rocks and carbonatites, sometimes with high niobium. Many mafic igneous rocks contain intergrown magnetite and ilmenite from the oxidation of ulvospinel.

Most ilmenite is mined for titanium dioxide production, which is also used to make titanium metal. The main consumers of TiO2 pigment are paints, plastics, and paper. Per capita consumption is about 1.1 kg per year in China, compared to 2.7 kg in Western Europe and the United States. Titanium is the ninth most abundant element in Earth's crust (about 0.6%). Ilmenite is processed into a titanium concentrate—called synthetic rutile if it exceeds 90% TiO2, or titaniferous slag if lower. Over 80% of global titanium concentrate comes from ilmenite, with 13% from titaniferous slags and 5% from rutile. It can be converted to pigment-grade TiO2 via the sulfate or chloride process, or upgraded to rutile using the Becher process. Smelting ilmenite yields liquid iron and titanium-rich slag. Steelmakers use ilmenite ore as a flux for blast furnace hearth linings, and it can also produce ferrotitanium through aluminothermic reduction.

Most ilmenite is recovered from heavy mineral sands, where weathering reduces iron content and increases titanium percentage. It can also be extracted from hard rock sources like ultramafic to mafic layered intrusions or anorthosites.