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{{Short description|Oxide mineral composed of titanium dioxide}}
{{Infobox mineral
| name = Rutile
| category = [[Oxide minerals]]
| boxwidth =
| boxbgcolor =#5e5048
| boxtextcolor = #fff
| image = Rutile-ww7a.jpg
| imagesize = 260px
| caption =
| formula = [[Titanium|Ti]][[Oxygen|O]]2
| IMAsymbol = Rt<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3 |pages=291–320|doi=10.1180/mgm.2021.43 |bibcode=2021MinM...85..291W |s2cid=235729616 |doi-access=free}}</ref>
| molweight =
| strunz = 4.DB.05
| system = [[Tetragonal crystal system|Tetragonal]]
| class = Ditetragonal dipyramidal (4/mmm) [[H-M symbol]]: (4/m 2/m 2/m)
| symmetry = ''P''42/mnm
| unit cell = ''a'' = 4.5937 Å, ''c'' = 2.9587 Å; ''Z'' = 2
| 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
| habit = Acicular to [[prism (geometry)|Prismatic]] 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 = [[Fracture (mineralogy)#Uneven fracture|Uneven]] to sub-[[Conchoidal fracture|conchoidal]]
| mohs = 6.0–6.5
| luster = [[Lustre (mineralogy)#Adamantine lustre|Adamantine]] to metallic
| refractive = ''n''ω = 2.613, ''n''ε = 2.909 (589 nm)
| opticalprop = Uniaxial (+)
| birefringence = 0.296 (589 nm)
| pleochroism = Weak to distinct brownish red-green-yellow
| dispersion = Strong
| other = Strongly anisotropic
| streak = Bright red to dark red
| gravity = 4.23 increasing with Nb–Ta content
| density =
| melt =
| fusibility = Fusible in alkali carbonates
| diagnostic =
| solubility = Insoluble in [[acid]]s
| diaphaneity = Opaque, transparent in thin fragments
| impurities = Fe, Nb, Ta
| references = <ref name=Handbook>[http://rruff.geo.arizona.edu/doclib/hom/rutile.pdf Handbook of Mineralogy].</ref><ref name=Webmin>[http://webmineral.com/data/Rutile.shtml Webmineral data].</ref><ref name=Mindat>[http://www.mindat.org/min-3486.html Mindat.org].</ref><ref name=Klein>Klein, Cornelis and Cornelius S. Hurlbut, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, pp. 304–05, {{ISBN|0-471-80580-7}}.</ref>
}}

'''Rutile''' is an [[oxide mineral]] composed of [[titanium dioxide]] (TiO2), the most common natural form of TiO2. Rarer [[Polymorphism (materials science)|polymorphs]] of TiO2 are known, including [[anatase]], [[akaogiite]], and [[brookite]].

Rutile has one of the highest [[refractive index|refractive indices]] at [[visible wavelength]]s of any known crystal and also exhibits a particularly large [[birefringence]] and high [[dispersion (optics)|dispersion]]. Owing to these properties, it is useful for the manufacture of certain optical elements, especially [[Polarization (waves)|polarization]] optics, for longer [[light|visible]] and [[infrared|infrared wavelengths]] up to about 4.5 micrometres. Natural rutile may contain up to 10% [[iron]] and significant amounts of [[niobium]] and [[tantalum]].

Rutile derives its name from the Latin {{wikt-lang|la|rutilus}} ('red'), in reference to the deep red color observed in some specimens when viewed by transmitted light. Rutile was first described in 1803 by [[Abraham Gottlob Werner]] using specimens obtained in Horcajuelo de la Sierra, Madrid (Spain),<ref>{{Cite book |last=Calvo |first=Miguel |title=Minerales y Minas de España. Vol. IV. Óxidos e hidróxidos |publisher=Escuela Técnica Superior de Ingenieros de Minas de Madrid. Fundación Gómez Pardo |year=2009 |location=Madrid, Spain |pages=237 |language=es}}</ref> which is consequently the type locality.

==Occurrence==
[[Image:2005rutile.PNG|thumb|left|Rutile output in 2005]]
Rutile is a common accessory mineral in high-temperature and high-pressure [[metamorphic rock]]s and in [[igneous rock]]s.

[[Thermodynamic]]ally, rutile is the most stable polymorph of TiO2 at all temperatures, exhibiting lower total [[Thermodynamic free energy|free energy]] than [[metastable]] phases of anatase or brookite.<ref>
{{cite journal
| last1=Hanaor
| first1=D. A. H.
| last2=Assadi
| first2=M. H. N.
| last3=Li
| first3=S.
| last4=Yu
| first4=A.
| last5=Sorrell
| first5=C. C.
| title= Ab initio study of phase stability in doped TiO2
| journal= Computational Mechanics
| year=2012
| volume=50
| issue=2
| pages=185–94
| doi=10.1007/s00466-012-0728-4| arxiv=1210.7555
| bibcode=2012CompM..50..185H
| s2cid=95958719
}}</ref> Consequently, the transformation of the metastable TiO2 polymorphs to rutile is irreversible. As it has the lowest [[Van der Waals surface|molecular volume]] of the three main polymorphs, it is generally the primary titanium-bearing phase in most high-pressure metamorphic rocks, chiefly [[eclogite]]s.

[[File:Quartz-159832.jpg|thumb|left|[[Rutilated quartz|Rutile in quartz]]]]
Within the igneous environment, rutile is a common accessory mineral in [[Intrusive rock|plutonic igneous rocks]], though it is also found occasionally in [[Extrusive rock|extrusive igneous rocks]], particularly those such as [[kimberlite]]s and [[lamproite]]s that have deep mantle sources. 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 in [[placer deposit]]s sourced from primary rutile.

[[File:Rutil.jpg|thumb|left|Milled rutile]]The occurrence of large specimen crystals is most common in [[pegmatite]]s, [[skarn]]s, and [[granite]] [[greisen]]s. Rutile is found as an accessory mineral in some [[metasomatism|altered igneous rocks]], and in certain [[gneiss]]es and [[schist]]s. In groups of acicular [[crystal]]s it is frequently seen penetrating [[quartz]] as in the {{lang|fr|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.

==Crystal structure==
[[Image:Rutile-unit-cell-3D-balls.png|thumb|left|The [[Crystal structure#Unit cell|unit cell]] of rutile. Ti atoms are gray; O atoms are red.]]
[[File:Rutile crystal structure.png|alt=A ball-and-stick chemical model of a rutile crystal|left|thumb|Extended crystal structure of rutile]]
Rutile has a [[Tetragonal crystal system|tetragonal]] [[Crystal structure#Unit cell|unit cell]], with unit cell parameters ''a'' = ''b'' = 4.584 Å, and ''c'' = 2.953 Å. <ref name=art>{{cite journal | journal=Journal of Crystal Growth| volume=359|pages= 83–91|year=2012|title=Abnormal grain growth of rutile TiO2 induced by ZrSiO4|doi=10.1016/j.jcrysgro.2012.08.015|arxiv=1303.2761| url=https://hal.science/hal-02315198 |bibcode=2012JCrGr.359...83H| last1=Hanaor| first1=Dorian A.H.| last2=Xu| first2=Wanqiang| last3=Ferry| first3=Michael| last4=Sorrell| first4=Charles C.| last5=Sorrell| first5=Charles C.| s2cid=94096447}}</ref> The titanium 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, resulting in a trigonal planar coordination. Rutile also shows a screw axis when its octahedra are viewed sequentially.<ref>[http://www.uwgb.edu/dutchs/Petrology/Rutile%20Structure.HTM "Rutile Structure"], Steven Dutch, Natural and Applied Sciences, University of Wisconsin – Green Bay.</ref> When formed under reducing conditions, oxygen vacancies can occur, coupled to Ti3+ centers.<ref name="palfey2021">{{cite journal | last1 = Palfey | first1 = W.R. | last2 = Rossman | first2 = G.R. | last3 = Goddard | first3 = W.A. III | title = Structure, Energetics, and Spectra for the Oxygen Vacancy in Rutile: Prominence of the Ti–HO–Ti Bond | date = 2021 | journal = The Journal of Physical Chemistry | volume = 12 | issue = 41 | pages = 10175–10181 | doi = 10.1021/acs.jpclett.1c02850| pmid = 34644100 | s2cid = 238860345 }}</ref> Hydrogen can enter these gaps, existing as an individual vacancy occupant (pairing as a hydrogen ion) or creating a [[hydroxide]] group with an adjacent oxygen.<ref name="palfey2021" />

Rutile crystals are most commonly observed to exhibit a prismatic or acicular [[Crystal habit|growth habit]] with preferential orientation along their ''c'' axis, the [001] [[Miller index|direction]]. This growth habit is favored as the {110} facets of rutile exhibit the lowest [[Surface energy|surface free energy]] and are therefore thermodynamically most stable.<ref name=art /> The ''c''-axis oriented growth of rutile appears clearly in [[nanorods]], [[nanowires]] and [[abnormal grain growth]] phenomena of this phase.

==Application==
[[File:Rutile needles.jpg|thumb|Acicular crystals of rutile protruding from a [[quartz]] crystal]]
In large enough quantities in beach sands, rutile forms an important constituent of [[heavy mineral]]s and [[ore deposit]]s. Miners extract and separate the valuable minerals – e.g., rutile, [[zircon]], and [[ilmenite]]. The main uses for rutile are the manufacture of [[refractory|refractory ceramic]], as a [[pigment]], and for the production of [[Titanium metallurgy|titanium metal]].

Finely powdered rutile is a brilliant white pigment and is used in [[paint]]s, [[plastic]]s, [[paper]], foods, and other applications that call for a bright white color. [[Titanium dioxide]] pigment is the single greatest use of titanium worldwide. [[Nanoparticle|Nanoscale particles]] of rutile are transparent to [[Visible spectrum|visible light]] but are highly effective in the [[Absorption (electromagnetic radiation)|absorption]] of [[ultraviolet]] 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 in [[sunscreen]]s to protect against UV-induced skin damage.

Small rutile needles present in [[gemstone|gems]] are responsible for an [[optical phenomenon]] known as [[asterism (gemmology)|asterism]]. Asteriated gems are known as "star" gems. Star [[sapphire]]s, star [[ruby|rubies]], and other star gems are highly sought after and are generally more valuable than their normal counterparts.

Rutile is widely used as a [[Shielded metal arc welding|welding electrode covering]]. It is also used as a part of the [[ZTR index]], which classifies highly weathered sediments.

=== Semiconductor ===
Rutile, as a large band-gap [[semiconductor]], has in recent decades been the subject of significant research towards applications as a functional oxide for applications in [[photocatalysis]] and [[Magnetic semiconductor|dilute magnetism]].<ref> [https://arxiv.org/abs/1304.1854 Magnetism in titanium dioxide polymorphs] J. Applied Physics </ref> Research efforts typically utilize small quantities of synthetic rutile rather than mineral-deposit derived materials.

==Synthetic rutile==
Synthetic rutile was first produced in 1948 and is sold under a variety of names. It can be produced from the titanium ore [[ilmenite]] through the [[Becher process]]. Very pure synthetic rutile is [[Transparency (optics)|transparent]] and almost colorless, being slightly yellow, in large pieces. Synthetic rutile can be made in a variety of colors by doping. The high [[refractive index]] gives an [[Lustre (mineralogy)#Adamantine lustre|adamantine]] [[lustre (mineralogy)|luster]] and strong refraction that leads to a [[diamond]]-like appearance. The near-colorless [[diamond simulant|diamond substitute]] is sold as "Titania", which is the old-fashioned chemical name for this oxide. However, rutile is seldom used in [[jewellery]] because it is not very [[Hardness|hard]] (scratch-resistant), measuring only about 6 on the [[Mohs hardness scale]].

As the result of growing research interest in the [[Photocatalysis|photocatalytic]] 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 (typically [[Titanium tetrachloride|TiCl4]]) or organometallic precursors (typically alkoxides such as [[titanium isopropoxide]], also known as TTIP). Depending on synthesis conditions, the first phase to crystallize may be the metastable [[anatase]] phase, which can then be converted to the equilibrium rutile phase through thermal treatment. The physical properties of rutile are often modified using [[dopants]] to impart improved photocatalytic activity through improved photo-generated charge carrier separation, altered electronic band structures and improved surface reactivity.

==See also==
* [[List of minerals]]

==References==
{{Reflist}}

==External links==
{{Commons category|Rutile}}
* {{Cite Americana|short=1|wstitle=Rutile}}

{{Titanium minerals}}

{{Authority control}}

[[Category:Titanium minerals]]
[[Category:Oxide minerals]]
[[Category:Rutile group]]
[[Category:Tetragonal minerals]]
[[Category:Minerals in space group 136]]

Rutile - Revision history

Ajay Kumar: Created a new article