View source for Kyanite

{{short description|Aluminosilicate mineral}}
{{Infobox mineral
| name        = Kyanite
| category    = [[Nesosilicate]]
| image       = Kyanite_crystals.jpg
| imagesize   = 260px
| caption     =
| formula     = {{chem2|Al2SiO5}}
| IMAsymbol   = Ky<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>
| strunz      = 9.AF.15
| system      = [[Triclinic]]
| class       = Pinacoidal ({{overline|1}}) <br/><small>(same [[H-M symbol]])</small>
| symmetry    = ''P''{{overline|1}}
| unit cell   = a = 7.1262(12)&nbsp;[[Angstrom|Å]] <br/>b = 7.852(10)&nbsp;Å <br/>c = 5.5724(10)&nbsp;Å <br/>α = 89.99(2)°, β = 101.11(2)° <br/>γ = 106.03(1)°; Z&nbsp;=&nbsp;4
| color       = Blue, white, rarely green, light gray to gray, rarely yellow, pink, orange, and black, can be zoned
| habit       = Columnar; fibrous; bladed
| twinning    = Lamellar on {100}
| cleavage    = [100] perfect [010] imperfect with 79° angle between
| fracture    = Splintery
| tenacity    = Brittle
| mohs        = 4.5–5 parallel to one axis <br> 6.5–7 perpendicular to that axis
| luster      = Vitreous to white
| refractive  = n<sub>α</sub> = 1.712 – 1.718 n<sub>β</sub> = 1.720 – 1.725 n<sub>γ</sub> = 1.727 – 1.734
| birefringence = δ = 0.012 – 0.016
| opticalprop = Biaxial (-); high [[Optical relief|relief]]
| pleochroism = Trichroic, colorless to pale blue to blue
| 2V          = 78°–83°
| streak      = White
| gravity     = 3.53–3.65 measured; 3.67 calculated
| melt        =
| fusibility  =
| diagnostic  =
| solubility  =
| diaphaneity = Transparent to translucent
| other       =
| references  = <ref name=Handbook>{{cite web| url= http://www.handbookofmineralogy.com/pdfs/kyanite.pdf| title= Kyanite| year= 2001| work= Handbook of Mineralogy| access-date= 2018-01-01| archive-date= 2019-05-08| archive-url= https://web.archive.org/web/20190508113856/http://www.handbookofmineralogy.com/pdfs/kyanite.pdf| url-status= dead}}</ref><ref name=Mindat>{{cite web|url=http://www.mindat.org/min-2303.html |title=Kyanite |publisher=MinDat |access-date=2013-06-14}}</ref><ref name=Webmin>{{cite web|url=http://webmineral.com/data/Kyanite.shtml |title=Kyanite Mineral Data |publisher=Webmineral.com  |access-date=2013-06-14}}</ref>
}}
'''Kyanite''' is a typically blue [[aluminosilicate]] [[mineral]], found in [[aluminium]]-rich metamorphic [[pegmatite]]s and [[sedimentary rock]]. It is the high pressure [[Polymorphism (materials science)|polymorph]] of [[andalusite]] and [[sillimanite]], and the presence of kyanite in [[metamorphic rocks]] generally indicates metamorphism deep in the [[Earth's crust]]. Kyanite is also known as '''disthene''' or '''cyanite'''.<ref>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth}}</ref>
Kyanite is strongly [[anisotropy|anisotropic]], in that its [[Mohs scale of mineral hardness|hardness]] varies depending on its crystallographic direction. In kyanite, this anisotropism can be considered an identifying characteristic, along with its characteristic blue color. Its name comes from the same origin as that of the color [[cyan]], being derived from the Ancient Greek word κύανος. This is typically rendered into English as ''kyanos'' or ''kuanos'' and means "dark blue."
Kyanite is used as a raw material in the manufacture of ceramics and abrasives, and it is an important [[index mineral]] used by geologists to trace [[metamorphic zone]]s.
==Properties==
[[Image:Kyanite.JPG|thumb|left|Deep blue kyanite]]
[[File:Kyanite within quartz, as collected by Dr John Hunter, Hunterian Museum, Glasgow.jpg|thumb|left|Kyanite within quartz, [[Hunterian Museum]], Glasgow]]
Kyanite is an [[aluminum silicate]] mineral, with the chemical formula {{chem2|Al2SiO5}}. It is typically patchy blue in color, though it can range from pale to deep blue<ref  name=Sinkankas/> and can also be gray or white or, infrequently, light green.<ref name=Nesse2000>{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=9780195106916 |page=319}}</ref> It typically forms sprays of bladed crystals, but is less commonly found as distinct [[euhedral]] (well-shaped) crystals, which are particularly prized by collectors.<ref  name=Sinkankas/> It has a perfect {100} [[Cleavage (mineralogy)|cleavage]] plane, parallel to the long axis of the crystal, and a second good cleavage plane {010} that is at an angle of 79 degrees to the {100} cleavage plane. Kyanite also shows a parting on {001} at an angle of about 85 degrees to the long axis of the crystal.<ref name=Nesse2000/> Cleavage surfaces typically display a pearly luster. The crystals are slightly flexible.<ref  name=Sinkankas>{{cite book |last1=Sinkankas |first1=John |title=Mineralogy for amateurs. |date=1964 |publisher=Van Nostrand |location=Princeton, N.J. |isbn=0442276249 |pages=528–529}}</ref>
Kyanite's elongated, columnar crystals are usually a good first indication of the mineral, as well as its color (when the specimen is blue). Associated minerals are useful as well, especially the presence of the [[Polymorphism (materials science)|polymorphs]] of [[staurolite]], which occurs frequently with kyanite. However, the most useful characteristic in identifying kyanite is its anisotropism. If one suspects a specimen to be kyanite, verifying that it has two distinctly different [[Mohs hardness scale|hardness]] values on perpendicular axes is a key to identification; it has a hardness of 5.5 parallel to {001} and 7 parallel to {100}.<ref name=Handbook/><ref name=Mindat/> Thus, a steel needle will easily scratch a kyanite crystal parallel to its long axis, but the crystal is impervious to being scratched by a steel needle perpendicular to the long axis.<ref  name=Sinkankas/>
===Structure===
The kyanite structure can be visualized as a distorted [[face centered cubic]] lattice of oxygen ions, with aluminium ions occupying 40% of the octahedral sites and silicon occupying 10% of the tetrahedral sites. The aluminium octahedra form chains along the length of the crystal, half of which are straight and half of which are zigzag, with silica tetrahedra linking the chains together. There is no direct linkage between the silica tetrahedra, making kyanite a member of the nesosilicate class of [[silicate minerals]].<ref>{{cite journal |last1=Winter |first1=J.K. |last2=Ghose |first2=S. |title=Thermal expansion and high-temperature crystal chemistry of the Al 2 SiO 5 polymorphs |journal=American Mineralogist |date=1979 |volume=64 |issue=5–6 |pages=573–586 |url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/64/5-6/573/41023/Thermal-expansion-and-high-temperture-crystal |access-date=28 August 2021}}</ref>{{sfn|Nesse|2000|p=315}}
==Occurrence==
{{al2sio5 phase diagram}}
Kyanite occurs in [[biotite]] [[gneiss]], mica [[schist]], and [[hornfels]], which are [[metamorphic rock]]s formed at high pressure during regional [[metamorphism]] of a [[protolith]] which is rich in [[aluminium]] (a ''[[Pelite|pelitic]]'' protolith). Kyanite is also occasionally found in [[granite]] and [[pegmatite|pegmatites]]{{sfn|Nesse|2000|p=315}}<ref name=GeologyPage>{{cite web| title=Geology Page - Kyanite| url= http://www.geologypage.com/2014/05/kyanite.html |work=Geology Page | date=2014-05-16 | access-date=2020-02-20}}</ref> and associated [[quartz]] [[Vein (geology)|veins]],<ref>{{cite book |last1=Sinkankas |first1=John |title=Mineralogy for amateurs. |date=1964 |publisher=Van Nostrand |location=Princeton, N.J. |isbn=0442276249 |page=529}}</ref> and is infrequently found in [[eclogite]]s. It occurs as detrital grains in sedimentary rocks, although it tends to weather rapidly.<ref name=Nesse2000/><ref name=GeologyPage/> It is associated with [[staurolite]], [[andalusite]], [[sillimanite]], [[talc]], [[hornblende]], [[gedrite]], [[mullite]] and [[corundum]].<ref name=Handbook/>
Kyanite is one of the most common minerals, having the composition {{chem2|Al2SiO5}}. Minerals with identical compositions but a different, distinct crystal structure are called ''[[Polymorphism (materials science)|polymorphs]]''. There are two polymorphs of kyanite: [[andalusite]] and [[sillimanite]]. Kyanite is the most stable at high pressure, andalusite is the most stable at lower temperature and pressure, and sillimanite is the most stable at higher temperature and lower pressure.{{sfn|Nesse|2000|p=76}} They are all equally stable at the [[triple point]] near 4.2 [[kbar]] and {{convert|530|C||sp=us}}.<ref>{{cite journal |last1=Bohlen |first1=S.R. |last2=Montana |first2=A. |last3=Kerrick |first3=D.M. |year=1991 |title=Precise determinations of the equilibria kyanite⇌ sillimanite and kyanite⇌ andalusite and a revised triple point for Al2SiO5 polymorphs |journal=American Mineralogist |volume=76 |number=3–4 |pages=677–680|url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/76/3-4/677/42549/Precise-determinations-of-the-equilibria-kyanite |access-date=28 August 2021}}</ref>  This makes the presence of kyanite in a metamorphic rock an indication of metamorphism at high pressure.
Kyanite is often used as an [[index mineral]] to define and trace a [[metamorphic zone]] that was subject to a particular degree of metamorphism at great depth in the crust. For example, [[George Barrow (geologist)|G. M. Barrow]] defined kyanite zones and sillimanite zones in his pioneering work on the mineralogy of metamorphic rocks. Barrow was characterizing a region of Scotland that had experienced [[regional metamorphism]] at depth. By contrast, the metamorphic zones surrounding the Fanad pluton of Ireland, which formed by [[contact metamorphism]] at a shallower depth in the crust, include andalusite and sillimanite zones but no kyanite zone.<ref>{{cite book |last1=Yardley |first1=B. W. D. |title=An introduction to metamorphic petrology |date=1989 |publisher=Longman Scientific & Technical |location=Harlow, Essex, England |isbn=0582300967 |pages=8–10}}</ref>
Kyanite is potentially stable at low temperature and pressure. However, under these conditions, the reactions that produce kyanite, such as:
:{{chem2|[[muscovite]] + staurolite + quartz → biotite + kyanite + H2O}}
never take place, and hydrous aluminosilicate minerals such as muscovite, [[pyrophyllite]], or [[kaolinite]] are found instead of kyanite.{{sfn|Yardley|1989|p=68-69}}
Bladed crystals of kyanite are very common, but individual euhedral crystals are prized by collectors.<ref  name=Sinkankas/> Kyanite occurs in [[Manhattan#Geology|Manhattan schist]], formed under extreme pressure as a result of a [[continental collision]] during the assembly of the [[supercontinent]] of [[Pangaea]].<ref>{{cite web| title= How ancient collision shaped New York skyline | first=Helen |last=Quinn| url= https://www.bbc.co.uk/news/science-environment-22798563  |work=BBC Science | publisher= BBC.co.uk| date= 6 June 2013 | quote= Prof Stewart was keeping an eye out for a mineral known as kyanite, a beautiful blue specimen commonly seen in the Manhattan schist. 'Kyanite is a key mineral to identify, we know it only forms at very deep depths and under extensive pressure,' he said.  'It's like a fingerprint, revealing a wealth of information.'  The presence of this mineral reveals that the Manhattan schist was compressed under incredibly high pressure over 300 million years ago.  The schist formed as a result of two enormous landmasses coming together to form a supercontinent, known as Pangaea. | access-date=2013-06-13}}</ref> It is also found in pegmatites of the [[Appalachian Mountains]] and in [[Minas Gerais, Brazil]]. Splendid specimens are found at [[Pizzo Forno]] in Switzerland.<ref  name=Sinkankas/>
Kyanite can take on an orange color, which notably occurs in Loliondo, Tanzania.<ref name=":0" /> The orange color is due to inclusions of small amounts of [[manganese]] (Mn<sup>3+</sup>) in the structure.
==Uses==
Kyanite is used primarily in [[refractory]] and ceramic products, including [[porcelain]] plumbing and [[dishware]]. It is also used in electronics, [[Insulator (electrical)|electrical insulators]] and [[abrasives]].{{sfn|Nesse|2000|p=316}}
At temperatures above 1100 °C, kyanite [[Chemical decomposition|decomposes]] into [[mullite]] and [[Fused quartz|vitreous silica]] via the following reaction:
:{{chem2|3(Al2O3*SiO2) → 3Al2O3*2SiO2 + SiO2}}
This transformation results in an expansion.<ref>{{cite book |title= Thermal Analysis of Materials |last=Speyer |first=Robert |year=1993 |publisher=[[CRC Press]] |isbn=0-8247-8963-6 |page=166 |url=https://books.google.com/books?id=5vTPIN_Y_FMC}}</ref> Mullitized kyanite is used to manufacture [[refractory material]]s.{{sfn|Nesse|2000|p=316}}
Kyanite has been used as a semiprecious [[gemstone]], which may display cat's eye [[chatoyancy]], though this effect is limited by its anisotropism and perfect cleavage. Color varieties include orange kyanite from Tanzania.<ref name=":0">{{Cite journal|last1=M. Chadwick|first1=Karen|last2=R. Rossman|first2=George|date=2009-01-01|title=Orange kyanite from Tanzania|url=https://www.researchgate.net/publication/30772542|journal=Gems and Gemology|volume=45}}</ref> The orange color is due to inclusions of small amounts of [[manganese]] (Mn<sup>3+</sup>) in the structure.<ref>{{cite journal |author=M. Gaft |author2=L. Nagli |author3=G. Panczer |author4=G. R. Rossman |author5=R. Reisfeld  |title=Laser-induced time-resolved luminescence of orange kyanite Al<sub>2</sub>SiO<sub>5</sub> |journal=Optical Materials |volume=33 |issue=10 |pages=1476–1480 |date=August 2011 |bibcode = 2011OptMa..33.1476G |doi = 10.1016/j.optmat.2011.03.052 }}</ref>
==References==
;Specific citations:
{{reflist}}
;General references:
{{refbegin}}
*[https://web.archive.org/web/20051003215717/http://mineral.galleries.com/minerals/silicate/kyanite/kyanite.htm Mineral Galleries]
*{{Cite journal |last1=Faye |first1=G. H. |last2=Nickel |first2=E. H. |year=1969 |title=On the origin of colour and pleochroism of kyanite |journal=The Canadian Mineralogist |volume=10 |pages=35–46 |url=http://rruff.info/rruff_1.0/uploads/CM10_35.pdf}}
{{refend}}
==External links==
{{commons category|Kyanite}}
* {{Cite EB1911|wstitle=Cyanite|short=x}}
{{Jewellery}}
[[Category:Gemstones]]
[[Category:Aluminium minerals]]
[[Category:Nesosilicates]]
[[Category:Triclinic minerals]]
[[Category:Industrial minerals]]
[[Category:Minerals in space group 2]]