Ilmenite
Ilmenite-155036.jpg
Ilmenite from Miass, Ilmen Mts, Chelyabinsk Oblast', Southern Urals, Urals Region, Russia. 4.5 x 4.3 x 1.5 cm
General
CategoryOxide mineral
Formula
(repeating unit)		iron titanium oxide, FeTiO
3
IMA symbolIlm[1]
Strunz classification4.CB.05
Dana classification04.03.05.01
Crystal systemTrigonal
Crystal classRhombohedral (3)
H-M symbol: (3)
Space groupR3 (no. 148)
Unit cella = 5.08854(7)
c = 14.0924(3) [Å]: Z = 6
Identification
ColorIron-black; gray with a brownish tint in reflected light
Crystal habitGranular to massive and lamellar exsolutions in hematite or magnetite
Twinning{0001} simple, {1011} lamellar
Cleavageabsent; parting on {0001} and {1011}
FractureConchoidal to subconchoidal
TenacityBrittle
Mohs scale hardness5–6
LusterMetallic to submetallic
StreakBlack
DiaphaneityOpaque
Specific gravity4.70–4.79
Optical propertiesUniaxial (–)
BirefringenceStrong; O = pinkish brown, E = dark brown (bireflectance)
Other characteristicsweakly magnetic
References[2][3][4]

Ilmenite is a titanium-iron oxide mineral with the idealized formula FeTiO
3. It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium[5] and the main source of titanium dioxide, which is used in paints, printing inks,[6] fabrics, plastics, paper, sunscreen, food and cosmetics.[7]

Structure and properties

Ilmenite is a heavy (specific gravity 4.7), moderately hard (Mohs hardness 5.6 to 6), opaque black mineral with a submetallic luster.[8] It is almost always massive, with thick tabular crystals being quite rare. It shows no discernible cleavage, breaking instead with a conchoidal to uneven fracture.[9]

Ilmenite crystallizes in the trigonal system with space group R3.[10][3] The ilmenite crystal structure consists of an ordered derivative of the corundum structure; in corundum all cations are identical but in ilmenite Fe2+ and Ti4+ ions occupy alternating layers perpendicular to the trigonal c axis.

Pure ilmenite is paramagnetic (showing only very weak attraction to a magnet), but ilmenite forms solid solutions with hematite that are weakly ferromagnetic and so are noticeably attracted to a magnet. Natural deposits of ilmenite usually contain intergrown or exsolved magnetite that also contribute to its ferromagnetism.[8]

Ilmenite is distinguished from hematite by its less intensely black color and duller appearance and its black streak, and from magnetite by its weaker magnetism.[9][8]


Discovery

In 1791 William Gregor discovered a deposit of black sand in a stream that runs through the valley just south of the village of Manaccan (Cornwall), and identified for the first time titanium as one of the constituents of the main mineral in the sand.[11][12][13] Gregor named this mineral manaccanite.[14] The same mineral was found in the Ilmensky Mountains, near Miass, Russia, and named ilmenite.[9]

Mineral chemistry

Pure ilmenite has the composition FeTiO3. However, ilmenite most often contains appreciable quantities of magnesium and manganese and up to 6 wt% of hematite, Fe2O3, substituting for FeTiO3 in the crystal structure. Thus the full chemical formula can be expressed as (Fe,Mg,Mn,Ti)O3.[8] Ilmenite forms a solid solution with geikielite (MgTiO
3) and pyrophanite (MnTiO
3) which are magnesian and manganiferous end-members of the solid solution series.[3]

Although ilmenite is typically close to the ideal FeTiO
3 composition, with minor mole percentages of Mn and Mg,[3] the ilmenites of kimberlites usually contain substantial amounts of geikielite molecules,[15] and in some highly differentiated felsic rocks ilmenites may contain significant amounts of pyrophanite molecules.[16]

At temperatures above 950 °C (1,740 °F), there is a complete solid solution between ilmenite and hematite. There is a miscibility gap at lower temperatures, resulting in a coexistence of these two minerals in rocks but no solid solution.[8] This coexistence may result in exsolution lamellae in cooled ilmenites with more iron in the system than can be homogeneously accommodated in the crystal lattice.[17] Ilmenite containing 6 to 13 percent Fe2O3 is sometimes described as ferrian ilmenite.[18][19]

Ilmenite alters or weathers to form the pseudo-mineral leucoxene, a fine-grained yellowish to grayish or brownish material[8][20] enriched to 70% or more of TiO2.[19] Leucoxene is an important source of titanium in heavy mineral sands ore deposits.[21]

Paragenesis

Ilmenite is a common accessory mineral found in metamorphic and igneous rocks.[3] It is found in large concentrations in layered intrusions where it forms as part of a cumulate layer within the intrusion. Ilmenite generally occurs in these cumulates together with orthopyroxene[22] or in combination with plagioclase and apatite (nelsonite).[23]

Magnesian ilmenite is formed in kimberlites as part of the MARID association of minerals (mica-amphibole-rutile-ilmenite-diopside) assemblage of glimmerite xenoliths.[24] Manganiferous ilmenite is found in granitic rocks[16] and also in carbonatite intrusions where it may also contain anomalously high amounts of niobium.[25]

Many mafic igneous rocks contain grains of intergrown magnetite and ilmenite, formed by the oxidation of ulvospinel.[26]

Processing and consumption

Tellnes opencast ilmenite mine, Sokndal, Norway
Tellnes opencast ilmenite mine, Sokndal, Norway

Most ilmenite is mined for titanium dioxide production.[27] Ilmenite and titanium dioxide are used in the production of titanium metal.[28][29]

Titanium dioxide is most used as a white pigment and the major consuming industries for TiO2 pigments are paints and surface coatings, plastics, and paper and paperboard. Per capita consumption of TiO2 in China is about 1.1 kilograms per year, compared with 2.7 kilograms for Western Europe and the United States.[30]

Ilmenite can be converted into pigment grade titanium dioxide via either the sulfate process or the chloride process.[31] Ilmenite can also be improved and purified to titanium dioxide in the form of rutile using the Becher process.[32]

Ilmenite ores can also be converted to liquid iron and a titanium-rich slag using a smelting process.[33]

Ilmenite ore is used as a flux by steelmakers to line blast furnace hearth refractory.[34]

Ilmenite can be used to produce ferrotitanium via an aluminothermic reduction.[35]

Feedstock production

Various ilmenite feedstock grades.[36]
Feedstock TiO
2 Content		 Process
(%)
Ore <55 Sulfate
Ore >55 Chloride
Ore <50 Smelting (slag)
Synthetic rutile 88-95 Chloride
Chloride slag 85-95 Chloride
Sulfate slag 80 Sulfate
Estimated contained TiO
2.
production[37][38]
(Metric tpa x 1,000,
ilmenite & rutile)
Year 2011 2012-13
Country USGS Projected
Australia 1,300 247
South Africa 1,161 190
Mozambique 516 250
Canada 700
India 574
China 500
Vietnam 490
Ukraine 357
Senegal - 330
Norway 300
United States 300
Madagascar 288
Kenya - 246
Sri Lanka 62
Sierra Leone 60
Brazil 48
Other countries 37
Total world ~6,700 ~1,250

Most ilmenite is recovered from heavy mineral sands ore deposits, where the mineral is concentrated as a placer deposit and weathering reduces its iron content, increasing the percentage of titanium. However, ilmenite can also be recovered from "hard rock" titanium ore sources, such as ultramafic to mafic layered intrusions or anorthosite massifs. The ilmenite in layered intrusions is sometimes abundant, but it contains considerable intergrowths of magnetite that reduce its ore grade. Ilmenite from anorthosite massifs often contain large amounts of calcium or magnesium that render it unsuitable for the chloride process.[39]

The proven reserves of ilmenite and rutile ore are estimated at between 423 and 600 million tonnes titanium dioxide. The largest ilmenite deposits are in South Africa, India, the United States, Canada, Norway, Australia, Ukraine, Russia and Kazakhstan. Additional deposits are found in Bangladesh, Chile, Mexico and New Zealand.[40]

Australia was the world's largest ilmenite ore producer in 2011, with about 1.3 million tonnes of production, followed by South Africa, Canada, Mozambique, India, China, Vietnam, Ukraine, Norway, Madagascar and United States.

The top four ilmenite and rutile feedstock producers in 2010 were Rio Tinto Group, Iluka Resources, Exxaro and Kenmare Resources, which collectively accounted for more than 60% of world's supplies.[41]

The world's two largest open cast ilmenite mines are:

Major mineral sands based ilmenite mining operations include:

Attractive major potential ilmenite deposits include:

Lunar ilmenite

Ilmenite has been found in Moon rocks,[46] and is typically highly enriched in magnesium similar to the kimberlitic association. In 2005[47] NASA used the Hubble Space Telescope to locate potentially ilmenite-rich locations. This mineral could be essential to an eventual Moon base, as ilmenite would provide a source of iron and titanium for the building of structures and essential oxygen extraction.

References

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