Editing Rutile

{{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]]<sub>2</sub>
| 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) <br />[[H-M symbol]]: (4/m 2/m 2/m)
| symmetry = ''P''4<sub>2</sub>/mnm
| unit cell = ''a'' = 4.5937&nbsp;Å, ''c'' = 2.9587&nbsp;Å; ''Z''&nbsp;=&nbsp;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''<sub>ω</sub> = 2.613, ''n''<sub>ε</sub> = 2.909 (589&nbsp;nm)
| opticalprop = Uniaxial (+)
| birefringence = 0.296 (589&nbsp;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]] (TiO<sub>2</sub>), the most common natural form of TiO<sub>2</sub>. Rarer [[Polymorphism (materials science)|polymorphs]] of TiO<sub>2</sub> 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 (geology)|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 TiO<sub>2</sub> 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.&nbsp;A.&nbsp;H.
| last2=Assadi
| first2=M.&nbsp;H.&nbsp;N.
| last3=Li
| first3=S.
| last4=Yu
| first4=A.
| last5=Sorrell
| first5=C.&nbsp;C.
| title= Ab initio study of phase stability in doped TiO<sub>2</sub>
| 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 TiO<sub>2</sub> 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]]
The structure of rutile is so classic that it is discussed in textbooks as a reference motif, much like [[sodium chloride]] and [[nickel arsenide]].<ref>{{Greenwood&Earnshaw2nd}}</ref> The structure is adopted by not only TiO<sub>2</sub>, but also by [[germanium dioxide|GeO<sub>2</sub>]], [[ruthenium dioxide|RuO<sub>2</sub>]], [[tin dioxide|SnO<sub>2</sub>]], [[manganese dioxide|MnO<sub>2</sub>]], [[vanadium dioxide|VO<sub>2</sub>]], [[iridium dioxide|IrO<sub>2</sub>]], and [[chromium dioxide|CrO<sub>2</sub>]].<ref>{{cite journal |doi=10.1016/j.progsurf.2005.09.002 |title=The surface and materials science of tin oxide |date=2005 |last1=Batzill |first1=Matthias |last2=Diebold |first2=Ulrike |journal=Progress in Surface Science |volume=79 |issue=2–4 |pages=47–154 |bibcode=2005PrSS...79...47B }}</ref> Somewhat curiously, [[zirconium dioxide|ZrO<sub>2</sub>]] and [[hafnium dioxide|HfO<sub>2</sub>]] adopt another classical structural motif, the [[fluorite 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.<ref>[http://www.uwgb.edu/dutchs/Petrology/Rutile%20Structure.HTM "Rutile Structure"], Steven Dutch, Natural and Applied Sciences, University of Wisconsin&nbsp;– Green Bay.</ref> When formed under reducing conditions, oxygen vacancies can occur, coupled to Ti<sup>3+</sup> 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–H<sub>O</sub>–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 | url = https://resolver.caltech.edu/CaltechAUTHORS:20211015-191354505 }}</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>{{cite journal | journal=Journal of Crystal Growth| volume=359|pages= 83–91|year=2012|title=Abnormal grain growth of rutile TiO<sub>2</sub> induced by ZrSiO<sub>4</sub>|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 ''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&nbsp;–&nbsp;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 TiO<sub>2</sub> in powder and thin film form is frequently fabricated in laboratory conditions through solution based routes using inorganic precursors (typically [[Titanium tetrachloride|TiCl<sub>4</sub>]]) 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]]
[[Category:Minerals described in 1803]]