| Rutile | |
|---|---|
 | |
| General | |
| Category | Oxide minerals |
| Formula | TiO2 |
| IMA symbol | Rt[1] |
| Strunz classification | 4.DB.05 |
| Crystal system | Tetragonal |
| Crystal class | Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m) |
| Space group | P42/mnm |
| Unit cell | a = 4.5937 Å,c = 2.9587 Å;Z = 2 |
| Identification | |
| Color | Brown, reddish brown, blood red, red, brownish yellow, pale yellow, yellow, pale blue, violet, rarely grass-green, grayish black; black if high in Nb–Ta |
| Crystal habit | Acicular toPrismatic crystals, elongated and striated parallel to [001] |
| Twinning | Common 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 metallic |
| 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 = 2.613,nε = 2.909 (589 nm) |
| Birefringence | 0.296 (589 nm) |
| Pleochroism | Weak to distinct brownish red-green-yellow |
| Dispersion | Strong |
| Fusibility | Fusible in alkali carbonates |
| Solubility | Insoluble inacids |
| Common impurities | Fe, Nb, Ta |
| Other characteristics | Strongly anisotropic |
| References | [2][3][4][5] |
Rutile is anoxide mineral composed oftitanium dioxide (TiO2), the most common natural form of TiO2. Rarerpolymorphs of TiO2 are known, includinganatase,akaogiite, andbrookite.
Rutile has one of the highestrefractive indices atvisible wavelengths of any known crystal and also exhibits a particularly largebirefringence and highdispersion. Owing to these properties, it is useful for the manufacture of certain optical elements, especiallypolarization optics, for longervisible andinfrared wavelengths up to about 4.5 micrometres. Natural rutile may contain up to 10%iron and significant amounts ofniobium andtantalum.
Rutile derives its name from the Latinrutilus ('red'), in reference to the deep red color observed in some specimens when viewed by transmitted light. Rutile was first described in 1803 byAbraham Gottlob Werner using specimens obtained in Horcajuelo de la Sierra, Madrid (Spain),[6] which is consequently thetype locality.
Rutile is a common accessory mineral in high-temperature and high-pressuremetamorphic rocks and inigneous rocks.
Thermodynamically, rutile is the most stable polymorph of TiO2 at all temperatures, exhibiting lower totalfree energy thanmetastable phases of anatase or brookite.[7] Consequently, the transformation of the metastable TiO2 polymorphs to rutile is irreversible. As it has the lowestmolecular volume of the three main polymorphs, it is generally the primary titanium-bearing phase in most high-pressure metamorphic rocks, chieflyeclogites.
Within the igneous environment, rutile is a common accessory mineral inplutonic igneous rocks, though it is also found occasionally inextrusive igneous rocks, particularly those such askimberlites andlamproites that have deep mantle sources. Anatase and brookite are found in the igneous environment, particularly as products ofautogenic alteration during the cooling of plutonic rocks; anatase is also found inplacer deposits sourced from primary rutile.
The occurrence of large specimen crystals is most common inpegmatites,skarns, andgranitegreisens. Rutile is found as an accessory mineral in somealtered igneous rocks, and in certaingneisses andschists. In groups of acicularcrystals it is frequently seen penetratingquartz as in thefléches d'amour fromGraubünden,Switzerland. In 2005 the Republic ofSierra Leone inWest Africa had a production capacity of 23% of the world's annual rutile supply, which rose to approximately 30% in 2008.
The structure of rutile is so classic that it is discussed in textbooks as a reference motif, much likesodium chloride andnickel arsenide.[8] The structure is adopted by not only TiO2, but also byGeO2,RuO2,SnO2,MnO2,VO2,IrO2, andCrO2.[9]ZrO2 andHfO2 adopt another classical structural motif, thefluorite structure.
In the rutile motif, the metal "cations" have a coordination number of 6, meaning they are surrounded by an octahedron of 6 oxygen atoms. The oxygen anions have a coordination number of 3, in a trigonal planar coordination. Rutile also shows a screw axis when its octahedra are viewed sequentially.[10] When formed under reducing conditions, oxygen vacancies can occur, coupled to Ti3+ centers.[11] Hydrogen can enter these gaps, existing as an individual vacancy occupant (pairing as a hydrogen ion) or creating ahydroxide group with an adjacent oxygen.[11]
Rutile crystals are most commonly observed to exhibit a prismatic or aciculargrowth habit with preferential orientation along theirc axis, the [001]direction. This growth habit is favored as the {110} facets of rutile exhibit the lowestsurface free energy and are therefore thermodynamically most stable.[12] Thec-axis oriented growth of rutile appears clearly innanorods,nanowires andabnormal grain growth phenomena of this phase.
In large enough quantities in beach sands, rutile forms an important constituent ofheavy minerals andore deposits. Miners extract and separate the valuable minerals – e.g., rutile,zircon, andilmenite. The main uses for rutile are the manufacture ofrefractory ceramic, as apigment, and for the production oftitanium metal.
Finely powdered rutile is a brilliant white pigment and is used inpaints,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 tovisible light but are highly effective in theabsorption ofultraviolet radiation (sunscreen). 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 insunscreens to protect against UV-induced skin damage.
Small rutile needles present ingems are responsible for anoptical phenomenon known asasterism. Asteriated gems are known as "star" gems. Starsapphires, starrubies, and other star gems are highly sought after and are generally more valuable than their normal counterparts.
Rutile is widely used as awelding electrode covering. It is also used as a part of theZTR index, which classifies highly weathered sediments.
Rutile, as a large band-gapsemiconductor, has in recent decades been the subject of significant research towards applications as a functional oxide for applications inphotocatalysis anddilute magnetism.[13] Research efforts typically utilize small quantities of synthetic rutile rather than mineral-deposit derived materials.
Synthetic rutile was first produced in 1948 and is sold under a variety of names. It can be produced from the titanium oreilmenite through theBecher process. Very pure synthetic rutile istransparent and almost colorless, being slightly yellow, in large pieces. Synthetic rutile can be made in a variety of colors by doping. The highrefractive index gives anadamantineluster and strong refraction that leads to adiamond-like appearance. The near-colorlessdiamond substitute is sold as "Titania", which is the old-fashioned chemical name for this oxide. However, rutile is seldom used injewellery because it is not veryhard (scratch-resistant), measuring only about 6 on theMohs hardness scale.
As the result of growing research interest in thephotocatalytic activity of titanium dioxide, in both anatase and rutile phases (as well as biphasic mixtures of the two phases), rutile TiO2 in powder and thin film form is frequently fabricated in laboratory conditions through solution based routes using inorganic precursors (typicallyTiCl4) or organometallic precursors (typically alkoxides such astitanium isopropoxide, also known as TTIP). Depending on synthesis conditions, the first phase to crystallize may be the metastableanatase phase, which can then be converted to the equilibrium rutile phase through thermal treatment. The physical properties of rutile are often modified usingdopants to impart improved photocatalytic activity through improved photo-generated charge carrier separation, altered electronic band structures and improved surface reactivity.
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