rutile

[French, from German Rutil, from Latin rutilus, red. See rutilant.]

(click to enlarge)
Rutile on pyrophyllite from Mono County, Calif. (credit: B.M. Shaub)

Commercially important titanium mineral (titanium dioxide, TiO₂). It forms red to reddish brown, hard, brilliant metallic, slender crystals. Rutile has minor uses in porcelain and glass manufacture as a colouring agent and in making some steels and copper alloys. It is also used as a gem, but synthetic rutile is actually superior to natural crystals for gem use; it has fire (flashes of colour) and brilliance (light deflection) like those of diamond. Rutile is mined in Norway and is widespread in the Alps, the southern U.S., Mexico, and elsewhere.

For more information on rutile, visit Britannica.com.

The most frequent of the three polymorphs of titania, TiO₂; the two other polymorphs are brookite and anatase.

The mineral occurs as striated tetragonal prisms and needles, commonly repeatedly twinned. The color is deep blood red, reddish brown, to black, rarely violet or yellow. Specific gravity is 4.2, and hardness 6.5 on Mohs scale. Melting point is 1825°C (3317°F).

Rutile occurs as an accessory in many rock types, ranging from plutonic to metamorphic rocks, and even as detrital material in sediments and placers because of its resistance to weathering. Large crystals have been found in some granite pegmatites; in Brazil it often occurs as inclusions in clear quartz crystals (rutilated quartz). Rutile is commonly associated with apatite in high-temperature veins. In sufficient quantities, it is marketed as an ore of titanium. See also Titanium.

A common mineral, red-to-brown or black in color; contains 60% titanium; used in paints, as a coating on welding rods to stabilize the arc, and as an opacifier in ceramic glaze and in glass.

Environment

In plutonic and metamorphic rocks, often in seams in such rocks.

Crystals common. In high-temperature almost pegmatitic veins, in alpine pockets with hematite plates, in Brazilian quartz veins, and in rock crystal, it forms hairlike to reticular lattices like bridge girders. Embedded in metamorphic aluminous rocks, crystals are equidimensional and compact, often quite large. Often twinned into 6- or 8-sided forms known as sixlings or eightlings .

Black. In large crystals, golden to reddish brown in capillary needles or thin, flat crystals. Luster metallic adamantine; hardness 6-6Ɖ; specific gravity 4.2-4.3; streak light brown; fracture subconchoidal to uneven; cleavage basal and prismatic. Brittle; translucent to transparent and deep red-brown in thin pieces.

Titanium oxide (60.0% Ti, 40.0% O).

Infusible and insoluble in acid. Can be made soluble by fusing with borax powder, then tested for Ti by dissolving the mixture in hydrochloric acid, filtering and boiling the yellow solution with real tin to produce faint blue or violet color (see ilmenite, p. 149).

Difficult to confuse with other minerals, especially after a test for magnetism (negative) and for Ti. Crystals so common they are easy to identify. The waterworn crystals have an adamantine luster but often a bruised look which gives them a sort of light-colored "skin" that is easy to recognize. Black to reddish brown adamantine crystals may be recognized by their striated prisms or geniculated twins. Distinguished from cassiterite (specific gravity 6.8-7.1) by its lesser heft.

Common in embedded crystals in gneiss or schist, in pegmatites, and free-growing in veins of the alpine type. Since it is also a hard, heavy, and common accessory mineral of primary rocks, it occurs in alluvial concentrations of heavy sands.

Large black shiny crystals, more or less equidimensional, are found in a quartzite at Graves Mountain, Georgia, associated with kyanite and lazulite. Beautiful reticulated growths of slender crystals were found in open seams in North Carolina at Hiddenite. Fine oriented growths of flat reddish rutile needles on hematite plates are common among the Swiss "iron roses." Perfect eightlings and rutile replacements of brookite (TiO ₂ ) crystals (paramorphs) are common at Magnet Cove, Arkansas. Slender red-brown hairs of rutile penetrate quartz crystals--by replacement--to form rutilated quartz, also known as flèches d'amour or Venus hairstone. Brazil, Switzerland, and the U.S. (West Hartford, Vermont, and Alexander Co., North Carolina) have produced fine specimens of this growth of rutile and quartz.

Rutile is used as an ore of titanium and of purified titanium oxide. It has been synthesized in commercial-size crystals by the Verneuil process (see corundum, p. 145). Color of pure, wholly oxidized material is a pale yellow, almost white; the less oxidized is darker, blue to black. Free titanium may be a partial cause of the dark color of most rutile, though an invariable Fe impurity no doubt shares the responsibility.

Oriented growths of rutile within the crystal network of other minerals are common. Oriented three-dimensional sets of rutile hairs cause the streaks of the stars seen in star quartz and star corundums (star sapphires, star rubies).

Rutile
Acicular crystals of rutile protruding from a quartz crystal
General
Category	Mineral
Chemical formula	Titanium dioxide; TiO2
Identification
Color	Metallic grey to earthy red, brown, violet or black
Crystal habit	Acicular to prismatic
Crystal system	Tetragonal (+)
Cleavage	{110} good, 100 moderate, parting on {092} and {011}
Fracture	Uneven to sub-conchoidal
Mohs scale hardness	5.5 - 6.5
Luster	Adamantine
Streak	Bright red to dark red
Specific gravity	4.23 - 5.5
Pleochroism	Weak to distinct brownish red-green-yellow
Fusibility	Fusible in alkali carbonates
Solubility	Insoluble in acids
Common impurities	Fe, Nb, Ta
Major varieties

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.

Contents 1 Occurrence 2 Crystal structure 3 Uses and economic importance 4 Synthetic rutile 5 References 6 See also 7 Gallery

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, 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 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).^[1]

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.^[2] It therefore has a density of 4240 kg/m³. 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. ^[3]Rutile also shows a screw axis when its octahedron are viewed sequentially. ^[4]

Uses and economic importance

Epitaxial growth of acicular rutile on hematite.

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.

References

1. ^ "Sierra Rutile Mine". Titanium Resources Group. http://www.titaniumresources.com/site/en-GB/Page_26.aspx. Retrieved 2009-05-06. 
2. ^ Diebold, Ulrike (January 2003). "The surface science of titanium dioxide" (PDF). Surface Science Reports (Elsevier) 48 (5-8): 53-229. doi:10.1016/S0167-5729(02)00100-0. http://www.surface.tulane.edu/pdf/SurfSciRep.pdf. 
3. ^ "Rutile: Interactive 3D Structure", Nick Greeves and Sze-Kie Ho, Higher Education Academy Physical Sciences centre and EPSRC Complex Materials Discovery Portfolio Partnership. © 2008-2009 The University of Liverpool
4. ^ "Rutile Stucture", Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay

• http://webmineral.com/data/Rutile.shtml
• http://mineral.galleries.com/minerals/oxides/rutile/rutile.htm
• http://www.mindat.org/min-3486.html

See also

Wikimedia Commons has media related to: Rutile

• List of minerals
• Heavy mineral sands ore deposits
• Anatase
• Brookite
• Metamorphism
• Eclogite
• Ilmenite

Gallery

Rutile in Quartz

v • d • e Titanium minerals Oxides Simple Anatase · Brookite · Rutile Mixed Aeschynite-(Y) · Armalcolite · Betafite · Euxenite · Haggertyite · Hibonite · Ilmenite · Keilhauite · Perovskite · Polycrase · Zirconolite · Zirkelite Silicates Aenigmatite · Keilhauite · Lorenzenite · Melanite · Nenadkevichite · Titanite · Zircophyllite Other Heavy mineral sands (various) · Sabinaite (carbonate) · Warwickite (borate) · Zimbabweite (arsenite)

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