Rutile

Rutile
Wine-red rutile crystals from Binn Valley, Switzerland (Size: 2.0 x 1.6 x 0.8 cm)
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:

• 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.

Occurrence

Rutile output in 2005

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.

Rutile in quartz

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]

Crystal structure

The unit cell of rutile. Ti atoms are grey; O atoms are red.

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.^{[citation needed]} Rutile also shows a screw axis when its octahedron are viewed sequentially.^[7]

Uses and economic importance

Acicular crystals of rutile protruding from a quartz crystal

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

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.

See also

• List of minerals

References

1. ^ Handbook of Mineralogy
2. ^ Webmineral data
3. ^ Mindat.org
4. ^ Klein, Cornelis and Cornelius S. Hurlbut, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, p. 304-305, ISBN 0-471-80580-7
5. ^ "Sierra Rutile Mine". Titanium Resources Group. http://www.titaniumresources.com/site/en-GB/Page_26.aspx. Retrieved 2009-05-06. ^{[dead link]}
6. ^ Diebold, Ulrike (2003). "The surface science of titanium dioxide". Surface Science Reports 48 (5-8): 53–229. doi:10.1016/S0167-5729(02)00100-0. http://www.surface.tulane.edu/pdf/SurfSciRep.pdf. 
7. ^ "Rutile Structure", Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay