|ILMENITE AND RUTILE|
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide (FeTiO3). It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite.
Ilmenite is a common accessory mineral found in metamorphic and igneous rocks. It is found in large concentrations in ultramafic to mafic layered intrusions where it forms as part of a cumulate layer within the silicate stratigraphy of the intrusion. Ilmenite generally occurs within the pyroxenitic portion of such intrusions (the ‘pyroxene-in’ level).
Magnesian ilmenite is indicative of kimberlitic paragenesis and forms part of the MARID association of minerals (mica-amphibole-rutile-ilmenite-diopside) assemblage of glimmerite xenoliths. Managaniferous ilmenite is found in granitic rocks and also in carbonatite intrusions where it may also contain anomalous niobium. Many mafic igneous rocks contain grains of intergrown magnetite and ilmenite, formed by the oxidation of ulvospinel. Ilmenite also occurs as discrete grains, typically with some hematite in solid solution, and complete solid solution exists between the two minerals at temperatures above about 950°C.
The world’s largest open cast ilmenite mine is the Tellnes mine located in Sokndal, Norway and run by Titania AS (owned by Kronos Worldwide Inc.), a hard rock ilmenite mine, which produces most of Norway’s 380,000t of ilmenite production.
Rutile is a mineral composed primarily of titanium dioxide, TiO2. Rutile is the most common natural form of TiO2, with two rarer polymorphs anatase (sometimes known by the obsolete name ‘octahedrite’), a tetragonal mineral of pseudo-octahedral habit; and brookite, an orthorhombic mineral.
Rutile has among the highest refractive indices of any known mineral and also exhibits high dispersion. Natural rutile may contain up to 10% iron and significant amounts of niobium and tantalum.
Rutile is a common accessory mineral in high-temperature and high-pressure metamorphic rocks and in igneous rocks.
Rutile is the preferred polymorph of TiO2 in such environments because it has the lowest molecular volume of the three polymorphs; it is thus the primary titanium bearing phase in most high pressure metamorphic rocks, chiefly eclogites. Brookite and anatase are typical polymorphs of rutile formed by retrogression of metamorphic rutile.
Within the igneous environment, rutile is a common accessory mineral in plutonic igneous rocks, although it is also found occasionally in extrusive igneous rocks, particularly those which have deep mantle sources such as kimberlites and lamproites. Anatase and brookite are found in the igneous environment particularly as products of autogenic alteration during the cooling of plutonic rocks; anatase is also found formed within placer deposits sourced from primary rutile.
The occurrence of large specimen crystals is most common in pegmatites, skarns and particularly granite greisens. Rutile is found as an accessory mineral in some altered igneous rocks, and in certain gneisses and schists.