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 derives its name from the Latin rutilus, red, in reference to the deep red color observed in some specimens when viewed by transmitted light.
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 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. In groups of acicular crystals it is frequently seen penetrating quartz as in the "fléches d'amour" from Grisons, Switzerland.
Uses and economic importance
Rutile, when present in large enough quantities in beach sands, forms an important constituent of heavy mineral sands ore deposits. It is primarily extracted for use in refractory manufacture or use as a base for paints. Rarely is it extracted as an ore of titanium.
Finely powdered rutile is a brilliant white pigment and is used in paints, plastics, papers, foods, and other applications that call for a bright white color. Titanium dioxide pigment is the single greatest use of titanium worldwide. Nanoscale particles of rutile are transparent to optical light but remain highly reflective to UV light. Hence, they are used in sunscreens.
Small rutile needles present in gems are responsible for an optical phenomenon known as asterism. Asterated gems are known as "star" gems. Star sapphires, star rubies, and other "star" gems are highly sought after and often more valuable than their normal equivalents.
Synthetic rutile was first produced in 1948 and is sold under a variety of names. Very pure synthetic rutile is transparent and almost colourless (slightly yellow) in large pieces. Synthetic rutile can be made in a variety of colors by doping, although the purest material is almost colourless. The high refractive index gives an adamantine lustre and strong refraction that leads to a diamond-like appearance. The near-colorless diamond substitute is sold under the name Titania, which is the old-fashioned chemical name for this oxide. However, rutile is seldom used in jewellery because it is not very hard (scratch-resistant), measuring only about 6 on the Mohs hardness scale.