{{Short description|Plagioclase feldspar with 50–70% anorthite}} {{Infobox mineral | name = Labradorite | category = Tectosilicate minerals | group = Feldspar group | series = Plagioclase feldspar series | boxwidth = | boxbgcolor = skyblue | image = Labradorite polie 3(Madagascar).jpg | imagesize = 260px | caption = Labradorite in a polished rock slab | formula = (Ca,Na)(Al,Si)<sub>4</sub>O<sub>8</sub> (Na:Ca = 30:70 to 50:50) | IMAstatus = Variety of anorthite | molweight = | system = Triclinic | class = Pinacoidal ({{overline|1}}) | symmetry = ''P''{{overline|1}} (no. 2) | unit cell = a = 8.155&nbsp;Å, b = 12.84&nbsp;Å <br/>c = 10.16&nbsp;Å; α = 93.5° <br/>β = 116.25°, γ = 89.133°; Z&nbsp;=&nbsp;6 | color = Gray, gray-white, or colorless with blue, pale green, or brown iridescence <br> Spectrolite: may display pink, orange, or purple iridescence | habit = Crystals typically thin and tabular, rhombic in cross section, striated; massive | twinning = Common by albite, pericline, Carlsbad, Baveno, or Manebach twin laws | cleavage = Perfect on {001}, less perfect on {010}, intersecting at near 90°; distinct on {110} | fracture = Uneven to conchoidal | mohs = 6–6.5 | luster = Vitreous to pearly on cleavages | polish = | refractive = n<sub>α</sub> = 1.554–1.563<br/>n<sub>β</sub> = 1.559–1.568<br/>n<sub>γ</sub> = 1.562–1.573 | opticalprop = Biaxial (+) | birefringence = δ = 0.008–0.010 | 2V = Measured: 85° | dispersion = None | pleochroism = | fluorescence = | absorption = | streak = White | gravity = 2.68 to 2.72 | density = | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Translucent to transparent | other = Labradorescence (iridescence, schiller optical effect) | references = <ref name="HBM">[http://rruff.geo.arizona.edu/doclib/hom/labradorite.pdf Handbook of Mineralogy]</ref><ref name="Mindat">[http://www.mindat.org/show.php?id=2308&ld=1#themap Mindat.org]</ref><ref name="Webmin">[http://webmineral.com/data/Labradorite.shtml Webmineral data]</ref> }} '''Labradorite''' ((Ca, Na)(Al, Si)<sub>4</sub>O<sub>8</sub>) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect (schiller).

Labradorite is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573 and twinning is common. As with all plagioclase members, the crystal system is triclinic, and three directions of cleavage are present, two of which are nearly at right angles and are more obvious, being of good to perfect quality (while the third direction is poor). It occurs as clear, white to gray, blocky to lath shaped grains in common mafic igneous rocks such as basalt and gabbro, as well as in anorthosites.

== Occurrence == The geological type area for labradorite is Paul's Island near the town of Nain in Labrador, Canada. It has also been reported in Poland, Norway, Finland and various other locations worldwide, with notable distribution in Madagascar, China, Australia, Slovakia and the United States.<ref name="Mindat" />

Labradorite occurs in mafic igneous rocks and is the feldspar variety most common in basalt and gabbro. The uncommon anorthosite bodies are composed almost entirely of labradorite.<ref name="Klein">Hurlbut, Cornelius S.; Klein, Cornelis; ''Manual of Mineralogy'', Wiley, 1985, 20th ed., p. 456, {{ISBN|0-471-80580-7}}</ref> It also is found in metamorphic amphibolites and as a detrital component of some sediments. Common mineral associates in igneous rocks include olivine, pyroxenes, amphiboles and magnetite.<ref name="HBM" />

==Labradorescence== {{See also|Adularescence}} Labradorite can display an iridescent optical effect (or ''schiller'') known as labradorescence. The term ''labradorescence'' was coined by {{ill|Ove Balthasar Bøggild|da}}, who defined it (labradorization) as follows:<ref name="bog" />

{{Blockquote|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 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).

==Spectrolite== 250px|right|thumb|Spectrolite with purple, pink, and orange iridescence '''Spectrolite''' is an uncommon variety of labradorite exhibiting a high degree of labradorescence. It exhibits a richer range of colors than other labradorites as for instance in Canada or Madagascar (which show mostly tones of blue-grey-green).<ref name="Gems">Michael O'Donoghue, ''Gems'', Butterworth-Heinemann, 6th ed., 2006, pp. 238-267, {{ISBN|0-7506-5856-8}}</ref><ref name="Schumann">Walter Schumann, ''Gemstones of the World,'' Sterling, 3rd ed., 2007, pp. 52 - 53, 182 {{ISBN|1-4027-4016-6}}</ref> Due to the unique colors mined in Finland, spectrolite has become a brand name for material mined only there. Sometimes spectrolite is incorrectly used to describe labradorite whenever a richer display of colors is present, regardless of locality: for example, labradorite with the ''spectrolite'' play of colors has sometimes described material from Madagascar.<ref name="Gems" />

Finnish geologist Aarne Laitakari (1890–1975) described spectrolite and sought its origin for years when his son Pekka discovered a deposit at Ylämaa in south-eastern Finland, while building the Salpa Line fortifications there in 1940.

The quarrying of spectrolite began after the Second World War and became a significant local industry. In 1973, the first workshop in Ylämaa began cutting and polishing spectrolite for jewels. After that, a gem center was established in Ylämaa with training for gem-cutting accompanied by an annual Gem and Mineral Show initiated by Esko Hämäläinen, mayor of Ylämaa municipality.

==Gallery== <gallery> File:Labradoryt, Madagaskar.JPG|Polished block from Madagascar File:Labradorite detail.jpg|Detail of labradorite File:LabradoriteAMD.jpg|Polished labradorite 18 × 20&nbsp;cm File:Labradorite (UCL Geology Collections).jpg|Polished labradorite from UCL Geology collections File:Labradorite with rare colours.JPG|Labradorite with rare colors (spectrolite) File:Ladrador iridescence.jpg|Polished labradorite File:Grey labradorite.jpg|Iridescence effect on grey labradorite File:Labradorescence.jpg|Labradorescence in labradorite File:LabradoriteOslo.jpg|Spectrolite from Ylämaa, Finland File:Labrador spektrolit - Ylamaa, Finlandia.jpg|Spectrolite – Ylämaa, Finland. File:Labradorescence in labradorite.webm|upright|Video of labradorescence in labradorite, visible as the angle of view changes File:Spectrolite1.jpg|Polished spectrolite showing the color play </gallery>

==See also== * {{Portal-inline|Minerals}} * Aventurescence * Lapis lazuli * Larvikite * Opal {{Clear}}

==References== Seppo Lahti I.1989 The origin of interference colours in spectrolite (iridescent labradorite).Geologi 41. {{Reflist}}

== External links == {{Commons category multi|Labradorite|Labradorescence}} * {{cite EB1911|wstitle=Labradorite}}

{{Jewelry}} {{Authority control}}

Category:Calcium minerals Category:Plagioclase feldspar series Category:Minerals in space group 2 Category:Provincial symbols of Newfoundland and Labrador Category:Sodium minerals Category:Triclinic minerals Category:Aluminosilicates