| Rutile | |
|---|---|
Wine-red rutile crystals from Binn Valley, Switzerland (Size: 2.0 x 1.6 x 0.8 cm) |
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| General | |
| Category | Oxide minerals |
| Chemical formula | TiO2 |
| Strunz classification | 04.DB.05 |
| Crystal symmetry | Tetragonal 4/m 2/m 2/m; space group 136 |
| Unit cell | a = 4.5937 Å, c = 2.9587 Å; Z = 2 |
| Identification | |
| Color | Reddish brown, red, pale yellow, pale blue, violet, rarely grass-green; black if high in Nb–Ta |
| Crystal habit |
Acicular to Prismatic crystals, elongated and striated parallel to [001] |
| Crystal system | Tetragonal ditetragonal dipyramidal |
| Twinning | Comon on {011}, or {031}; as contact twins with two, six, or eight individuals, cyclic, polysynthetic |
| Cleavage | {110} good, 100 moderate, parting on {092} and {011} |
| Fracture | Uneven to sub-conchoidal |
| Mohs scale hardness | 6.0 - 6.5 |
| Luster | Adamantine to submetallic |
| Streak | Bright red to dark red |
| Diaphaneity | Opaque, transparent in thin fragments |
| Specific gravity | 4.23 increasing with Nb–Ta content |
| Optical properties | Uniaxial (+) |
| Refractive index | nω = 2.605–2.613 �nε = 2.899–2.901 |
| Birefringence | 0.2870-0.2940 |
| Pleochroism | Weak to distinct brownish red-green-yellow |
| Dispersion | strong |
| Fusibility | Fusible in alkali carbonates |
| Solubility | Insoluble in acids |
| Common impurities | Fe, Nb, Ta |
| References | [1][2][3][4] |
Rutile is a mineral composed primarily of titanium dioxide, TiO2.
Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:
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.
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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, though it is also found occasionally in extrusive igneous rocks, particularly those that 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. In groups of acicular crystals it is frequently seen penetrating quartz as in the "fléches d'amour" from Graubünden, Switzerland.
In 2005 the Republic of Sierra Leone in West Africa had a production capacity of 23% of the world's annual rutile supply, which rose to approximately 30% in 2008. The reserves, lasting for about 19 years, are estimated at 259,000,000 metric tons (285,000,000 short tons).[5]
Rutile has a primitive tetragonal unit cell, with unit cell parameters a=4.584Å, and c=2.953Å.[6] The titanium cations have a co-ordination number of 6 meaning they are surrounded by an octahedron of 6 oxygen atoms. The oxygen anions have a co-ordination number of 3 resulting in a trigonal planar co-ordination. Rutile also shows a screw axis when its octahedron are viewed sequentially.[7]
Rutile, when present in large enough quantities in beach sands, forms an important constituent of heavy mineral sands ore deposits. Miners extract and separate the valuable minerals (typically rutile, zircon, and ilmenite). The main uses for rutile are the manufacture of refractory ceramic, as a pigment, and for the production of titanium metal.
Finely powdered rutile is a brilliant white pigment and is used in paints, plastics, paper, 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 ultraviolet radiation. 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 nanoparticles. 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.
Rutile is widely used as a welding electrode covering.
Rutile is a part of the ZTR index to classify highly-weathered sediments.
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.
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