View source for Physics:Iceland spar

{{Short description|Transparent variety of calcite}}
[[File:Silfurberg.jpg|thumb|Iceland spar, possibly the Icelandic medieval sun stone used to locate the sun in the sky when obstructed from view<ref name="VikingPolarization" />]]

'''Iceland spar''', formerly called '''Iceland crystal''' ({{lang-is|silfurberg}} {{IPA-is|ˈsɪlvʏrˌpɛrk|}}, {{lit|silver-rock}}) and also called '''optical calcite''', is a transparent variety of [[Chemistry:Calcite|calcite]], or crystallized [[Chemistry:Calcium carbonate|calcium carbonate]], originally brought from Iceland, and used in demonstrating the [[Physics:Polarization (waves)|polarization]] of [[Physics:Light|light]].<ref name="1913Websters"/><ref name="OEDIcelandspar"/>

== Characteristics ==

Iceland spar occurs in large readily [[Physics:Cleavage (crystal)|cleavable]] crystals, which are easily divisible into [[Parallelepiped|parallelepiped]]s, and it is remarkable for its [[Physics:Birefringence|birefringence]].<ref name="Websters1828"/><ref name="Miers2010"/> This means that the [[Physics:Refractive index|refractive index]] of the crystal is different for light of different polarizations. A ray of unpolarized light passing through the crystal is divided into two rays of mutually [[Perpendicular|perpendicular]] polarization directed at different angles. This double refraction causes objects seen through the crystal to appear doubled.

Historically, the double-refraction property of this crystal was important to understanding the nature of light as a wave. This was studied at length by [[Biography:Christiaan Huygens|Christiaan Huygens]]<ref name="Huy1690"/> and [[Biography:Isaac Newton|Isaac Newton]].<ref name="1728Cyclopaedia" /> [[Biography:Sir George Stokes, 1st Baronet#Polarization|Sir George Stokes]] also studied the phenomenon.<ref name="Larmor2010"/> Its complete explanation in terms of light polarization was published by [[Biography:Augustin-Jean Fresnel|Augustin-Jean Fresnel]] in the 1820s.<ref name="Whittaker" />

== Occurrence ==

Mines producing Iceland spar include many mines producing related calcite and [[Chemistry:Aragonite|aragonite]]. As well as those famously in Iceland,<ref name="Russell2008" /> there are productive sources in the greater Sonoran Desert region; in Santa Eulalia, Chihuahua, Mexico;<ref name="GraniteGap"/> and in New Mexico, United States,<ref name="Kelly1940" /> as well as in {{wipe|China}}.<ref name="Guizhou"/> The clearest specimens, as well as the largest, have been from the [[Place:Helgustadir mine|Helgustaðir mine]] in Iceland.<ref name=":0">{{Cite web|title=Helgustaðanáma|url=https://ust.is/nattura/natturuverndarsvaedi/fridlyst-svaedi/austurland/helgustadanama/|access-date=2020-08-20|website=Umhverfisstofnun|language=is}}</ref>

== Uses ==

It has been speculated that the [[Physics:Sunstone (medieval)|sunstone]] ({{lang-non|sólarsteinn}}, a different mineral from the gem-quality [[Chemistry:Sunstone|sunstone]]) mentioned in medieval Icelandic texts such as Rauðúlfs þáttr was Iceland spar, and that Vikings used its light-polarizing property to tell the direction of the sun on cloudy days for [[Engineering:Longship#Navigation|navigational]] purposes.<ref name="VikingPolarization" /><ref name="Karlsen2003" /> The polarization of sunlight in the Arctic can be detected,<ref name="peasoup" /> and the direction of the sun identified to within a few degrees in both cloudy and twilight conditions using the sunstone and the naked eye.<ref name="Ropars11" /> The process involves moving the stone across the visual field to reveal a yellow [[Physics:Entoptic phenomenon|entoptic pattern]] on the [[Medicine:Fovea centralis|fovea]] of the eye, probably [[Physics:Haidinger's brush|Haidinger's brush]]. The recovery of an Iceland spar sunstone from a ship of the [[History:Elizabethan era|Elizabethan era]] that sank in 1592 off Alderney suggests that this navigational technology may have persisted after the invention of the magnetic [[Physics:Compass|compass]].<ref>{{Cite web |date=2017-07-02 |title=First Evidence of Viking-Like 'Sunstone' Found - Seeker |url=https://www.seeker.com/first-evidence-of-viking-like-sunstone-found-1767286293.html |access-date=2023-05-24 |archive-url=https://web.archive.org/web/20170702072159/https://www.seeker.com/first-evidence-of-viking-like-sunstone-found-1767286293.html |archive-date=2017-07-02 }}</ref><ref>Le Floch, A., Ropars, G., Lucas, J., Wright, S., Davenport, T., Corfield, M., & Harrisson, M. (2013). The sixteenth century Alderney crystal: a calcite as an efficient reference optical compass?. ''Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences'', 469(2153), 20120651.</ref>

[[Biography:William Nicol (geologist)|William Nicol]] (1770–1851) invented the  first polarizing prism, using Iceland spar to create his [[Nicol prism]].<ref>{{cite web | url=http://physics.kenyon.edu/EarlyApparatus/Polarized_Light/Nicol_Prism/Nicol_Prism.html | title=Nicol Prism | publisher=Kenyon College | access-date=23 January 2014 | author=Greenslade, Thomas B. Jr.}}</ref>

== Cultural impact ==

The Thomas Pynchon novel ''Against the Day'' uses the doubling effect of Iceland spar as a theme.<ref>{{Cite web |date=2007-09-30 |title=Pynchon's First Novel in 10 Years Has Sex, Explosives (Update1) |website={{wipe|Bloomberg News}} |url=https://www.bloomberg.com/apps/news?pid=20601088&sid=ab6WLEn2ciGU&refer=muse |access-date=2023-05-25 |archive-url=https://web.archive.org/web/20070930070316/https://www.bloomberg.com/apps/news?pid=20601088&sid=ab6WLEn2ciGU&refer=muse |archive-date=2007-09-30 }}</ref>

== References ==

{{refs|refs=
<ref name="1913Websters">{{WD1913|title=Polarimetry}}</ref>
<ref name="OEDIcelandspar">{{OED|Iceland spar}}</ref>
<ref name="Websters1828">{{WD1828|title=Birefringence}}</ref>
<ref name="Miers2010">Miers, Henry A., ''Mineralogy: an introduction to the scientific study of minerals''.  Nabu Press. {{ISBN|1-177-85127-X}} Chap.&nbsp;6, p.&nbsp;128.</ref>
<ref name="Huy1690">C. Huygens, ''Treatise on Light'' (Leiden: Van der Aa, 1690), translated by Silvanus P. Thompson, London: Macmillan, 1912, [https://archive.org/details/treatiseonlight031310mbp archive.org/details/treatiseonlight031310mbp]; Project Gutenberg edition, 2005, [http://www.gutenberg.org/ebooks/14725 gutenberg.org/ebooks/14725]; [http://www.grputland.com/2016/06/errata-in-various-editions-of-huygens-treatise-on-light.html Errata], 2016.</ref>
<ref name="1728Cyclopaedia">{{Cyclopaedia 1728|title=Isaac Newton}}</ref>
<ref name="Larmor2010">[[Biography:Joseph Larmor|Larmor, Joseph]] 2010. Retrieved January 2, 2011. ''[https://books.google.com/books?id=Bg9DAAAAIAAJ&pg=PA269&lpg=PA269 Memoir and scientific correspondence of the late Sir George Gabriel Stokes, bart., selected and arranged by Joseph Larmor]''.  Nabu Press.  {{ISBN|1-177-14275-9}} p.&nbsp;269.</ref>
<ref name="Russell2008">Russell, Daniel E . 17 February 2008. Retrieved December 31, 2010. "[http://www.mindat.org/article.php/190/Helgustadir+Iceland+Spar+Mine Helgustadir Iceland Spar Mine]" ''[[Mindat.org|mindat.org]]''</ref>
<ref name="GraniteGap">Retrieved January 2, 2011. "[http://granitegap.com/Minerals/Featured-Minerals/Calicite.aspx Calcite]"''Granite Gap'' "Several variety names exist for calcite. Iceland Spar is an ice-clear variety that demonstrates the effect of double refraction or birefringence ... Young mountain ranges in Mexico and South America also host fine localities for calcite. They include Chihuahua, Chihuahua; the Santa Eulalia Dist., Chihuahua; Mapimí, Durango; Guanajuato, Guanajuato; and Charcas, San Luis Potosí; all Mexico"</ref>
<ref name="Kelly1940">Kelley, Vincent C. 1940. Retrieved December 31, 2010. "[http://www.minsocam.org/msa/collectors_corner/arc/nmcalcite.htm Iceland Spar in New Mexico]". ''American Mineralogist'', Volume&nbsp;25, pp.&nbsp;357-367</ref>
<ref name="Guizhou">WANG Jing-teng, CHEN Hen-shui, YANG En-lin, WU Bo. 2009. Retrieved January 3, 2011. "[http://en.cnki.com.cn/Article_en/CJFDTOTAL-KJQB200933061.htm Geological Characteristics of Iceland Spar Mineral Deposit of Mashan District in Guizhou]". ''[[China National Knowledge Infrastructure]]'',  P619.2 [http://en.cnki.com.cn/Article_en/CJFDTOTAL-KJQB200933061.htm CNKI:SUN:KJQB.0.2009-33-061]</ref>
<ref name="VikingPolarization">[http://www.polarization.com/viking/viking.html ''The Viking Sunstone''], from Polarization.net.  Retrieved February 8, 2007.</ref>
<ref name="Karlsen2003">Karlsen, Leif K. 2003. ''Secrets of the Viking Navigators''.  One Earth Press.  {{ISBN|978-0-9721515-0-4}}, 220&nbsp;pp.</ref>
<ref name="peasoup">Hegedüs, Ramón, Åkesson, Susanne; Wehner, Rüdiger and Horváth, Gábor. 2007. "Could Vikings have navigated under foggy and cloudy conditions by skylight polarization? On the atmospheric optical prerequisites of [[Physics:Polarimetry|polarimetric]] Viking navigation under foggy and cloudy skies". ''[[Organization:Proceedings of the Royal Society|Proc. R. Soc.]] A''&nbsp;'''463'''&nbsp;(2080):&nbsp;1081–1095. {{doi|10.1098/rspa.2007.1811}}. {{issn|0962-8452}}.</ref>
<ref name="Ropars11">Ropars, G. et al., 2011. A depolarizer as a possible precise sunstone for Viking navigation by polarized skylight. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science. Available at: http://rspa.royalsocietypublishing.org/content/early/2011/10/28/rspa.2011.0369.abstract [Accessed December 5, 2011].</ref>
<ref name="Whittaker">[[Biography:E. T. Whittaker|Whittaker, E. T.]], ''A History of the Theories of Aether and Electricity''.  Dublin University Press, 1910.</ref>
}}

[[Category:Calcium minerals]]
[[Category:Carbonate minerals]]
[[Category:Optical materials]]
[[Category:Polarization (waves)]]
[[Category:Transparent materials]]
[[Category:Trigonal minerals]]

{{Sourceattribution|Iceland spar}}