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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 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 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.
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.
Rutile has a tetragonal unit cell, with unit cell parameters a=4.584Å, and c=2.953Å. It therefore has a density of 4240 kg/m3.
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 visible light but are highly effective in the absorption of UV light. The UV absorption of nano-sized rutile particles is blue-shifted compared to bulk rutile, so that higher energy UV light is absorbed by the nano particles. Hence, they are used in sunscreens to protect against UV induced skin damage.
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 colorless (slightly yellow) in large pieces. Synthetic rutile can be made in a variety of colors by doping, although the purest material is almost colorless. 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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Rutile". A list of authors is available in Wikipedia.|