Editing Labradorite (section)

==Labradorescence==
{{See also|Adularescence}}
[[Image:Labradorescence.jpg|thumb|left|Labradorescence in labradorite]]
[[File:Labradorescence in labradorite.webm|thumb|left|upright|Video of labradorescence in labradorite, visible as the angle of view changes]]

Labradorite can display an [[iridescence|iridescent]] optical effect (or ''[[Lustre (mineralogy)#Schiller|schiller]]'') known as labradorescence. The term ''labradorescence'' was coined by [[Ove Balthasar Bøggild]], who defined it (labradorization) as follows:<ref name="bog" />

{{Quote|Labradorization is the peculiar reflection of the light from submicroscopical planes orientated in one direction (rarely in two directions); these planes have never such a position that they can be expressed by simple indices, and they are not directly visible under the microscope.}}

Contributions to the understanding of the origin and cause of the effect were made by [[Robert Strutt, 4th Baron Rayleigh]] (1923), and by Bøggild (1924).<ref name="bog">{{citation |url=http://www.sdu.dk/media/bibpdf/Bind%201-9%5CBind%5Cmfm-6-3.pdf |archive-url=https://web.archive.org/web/20150402111409/http://www.sdu.dk/media/bibpdf/Bind%201-9%5CBind%5Cmfm-6-3.pdf |url-status=dead |archive-date=April 2, 2015 |title=On the Labradorization of the Feldspars |first=Ove Balthasar |last=Bøggild |year=1924 |journal=Kongelige Danske Videnskabernes Selskab, Mathematisk-fysiske Meddelelelser |volume=6 |number=3 |pages=1–79 }}</ref><ref>{{Cite journal |doi=10.1007/BF03172469 |journal=Proceedings of the Indian Academy of Sciences, Section A |date=July 1950 |volume=32 |issue=1 |pages=1–16 |title=The structure of labradorite and the origin of its iridescence |first1=Chandrasekhara Venkata |last1=Raman |author-link=Chandrasekhara Venkata Raman |first2=Aiyasami |last2=Jayaraman |s2cid=128235557 |url=https://www.researchgate.net/publication/226907416 }}</ref><ref>{{citation |title=Studies of Iridescent Colour and the Structure Producing it. III. The Colours of Labrador Felspar |author=Lord Rayleigh |author-link=Robert Strutt, 4th Baron Rayleigh |journal=Proceedings of the Royal Society of London. Series A |volume=103 |number=720 |date=3 April 1923 |pages=34–45 |publisher=The Royal Society |jstor=94093 |doi=10.1098/rspa.1923.0037 |bibcode=1923RSPSA.103...34R |doi-access=free }}</ref>

The cause of this optical phenomenon is phase [[Solid solution#Exsolution|exsolution]] lamellar structure,<ref name="cn1"/> occurring in the Bøggild [[miscibility gap]].<ref name="cn2">{{citation |url=http://en.cnki.com.cn/Article_en/CJFDTOTAL-DZKQ200602005.htm |title=Relation Between Labradorescence and Internal Structure of Labradorite |first1=Xie |last1=Hao |first2=Pei |last2=Jing-cheng |first3=Li |last3=Li-ping |journal=Geological Science and Technology Information |date=February 2006 |access-date=2015-03-01 |archive-date=2021-11-06 |archive-url=https://web.archive.org/web/20211106051919/http://en.cnki.com.cn/Article_en/CJFDTOTAL-DZKQ200602005.htm |url-status=dead }}</ref> The effect is visible when the lamellar separation is between {{convert|128|and|252|nm|abbr=on}}; the lamellae are not necessarily parallel;<ref name="cn2" /> and the lamellar structure is found to lack long range order.<ref>{{Cite journal |doi=10.1002/pssb.19660180123 |title=On the origin of the colour of labradorite |journal=Physica Status Solidi B |volume=18 |pages=221–230 |year=1966 |last1=Bolton |first1=Herbert Cairns |last2=Bursill |first2=Leslie Arthur |last3=McLaren |first3=Alexander Clark |last4=Turner |first4=Robin G. |issue=1 |bibcode=1966PSSBR..18..221B |s2cid=95485108 }}</ref>

The lamellar separation only occurs in plagioclases of a certain composition; those of calcic labradorite (50–70% anorthite) and [[bytownite]] (formula: {{No break|(Ca<sub>0.7-0.9</sub>,Na<sub>0.3-0.1</sub>)[Al(Al,Si)Si<sub>2</sub>O<sub>8</sub>]}}, ''i.e.'', with an [[anorthite]] content of ~70 to 90%) particularly exemplify this.<ref name="cn1">{{citation |url=http://en.cnki.com.cn/Article_en/CJFDTOTAL-YSKW200805015.htm |title=Exsolution lamellar structure causes of iridescence in labradorite: evidence from TEM |journal=Acta Petrologica et Mineralogica |first1=Peng |last1=Yan-ju |first2=He |last2=Xue-mei |first3=Fang |last3=Qin-fang |date=May 2008 |access-date=2015-03-01 |archive-date=2021-11-06 |archive-url=https://web.archive.org/web/20211106051921/http://en.cnki.com.cn/Article_en/CJFDTOTAL-YSKW200805015.htm |url-status=dead }}</ref><ref>{{citation |work =The Feldspars: Proceedings of a NATO Advanced Study Institute, Manchester, 11–21 July 1972 |volume=2 |editor-first=William Scott |editor-last=MacKenzie |editor-first2=Jack |editor-last2=Zussman |publisher=Manchester University Press |year=1974 |title=23. Electron-optical study of a schiller labradorite |pages=478–490 }}</ref> Another requirement for the lamellar separation is a very slow cooling of the rock containing the plagioclase. Slow cooling is required to allow the Ca, Na, Si, and Al ions to diffuse through the plagioclase and produce the lamellar separation. Therefore, not all labradorites exhibit labradorescence (they might not have the correct composition, cooled too quickly, or both), and not all plagioclases that exhibit labradorescence are labradorites (they may be bytownite).

Some [[gemstone]] varieties of labradorite exhibiting a high degree of labradorescence are called [[spectrolite]].