Iceland spar, formerly called Iceland crystal (Icelandic: silfurberg [ˈsɪlvʏrˌpɛrk], lit. 'silver-rock') and also called optical calcite, is a transparent variety of calcite, or crystallized calcium carbonate, originally brought from Iceland, and used in demonstrating the polarization of light (see polarimetry). It occurs in large readily cleavable crystals, is easily divisible into parallelepipeds, and is remarkable for its birefringence. This means that the refractive index of the crystal is different for light of different polarization. A ray of unpolarized light passing through the crystal is divided into two rays of 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 Christiaan Huygens and Isaac Newton. Sir George Stokes also studied the phenomenon. Its complete explanation in terms of light polarization was published by Augustin-Jean Fresnel in the 1820s.
Mines producing Iceland spar include many mines producing related calcite and aragonite as well as those famously in Iceland, productively in the greater Sonoran Desert region as in Santa Eulalia, Chihuahua, Mexico and New Mexico, United States, as well as in China. The clearest specimens, as well as the largest, have been from the Helgustadir mine in Iceland.
It has been speculated that the sunstone (Old Norse: sólarsteinn, a different mineral from the gem-quality sunstone) mentioned in medieval Icelandic texts[which?] was Iceland spar, and that Vikings used its light-polarizing property to tell the direction of the sun on cloudy days for navigational purposes. The polarization of sunlight in the Arctic can be detected, 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. The process involves moving the stone across the visual field to reveal a yellow entoptic pattern on the fovea of the eye, probably Haidinger's brush. The recovery of an Iceland spar sunstone from a ship of the Elizabethan era that sank in 1592 off Alderney suggests that this navigational technology may have persisted after the invention of the magnetic compass.
- The Viking Sunstone, from Polarization.net. Retrieved February 8, 2007.
- This article incorporates text from this source, which is in the public domain. Porter, Noah, ed. (1913). "Polarimetry". Webster's Dictionary. Springfield, Massachusetts: C. & G. Merriam Co.
- "Iceland spar". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
- This article incorporates text from this source, which is in the public domain. Webster, Noah (1828). "Birefringence". Webster's Dictionary. Springfield, Massachusetts: C. & G. Merriam Co.
- Miers, Henry A., Mineralogy: an introduction to the scientific study of minerals. Nabu Press. ISBN 1-177-85127-X Chap. 6, p. 128.
- C. Huygens, Treatise on Light (Leiden: Van der Aa, 1690), translated by Silvanus P. Thompson, London: Macmillan, 1912, archive.org/details/treatiseonlight031310mbp; Project Gutenberg edition, 2005, gutenberg.org/ebooks/14725; Errata, 2016.
- This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed. (1728). "Isaac Newton". Cyclopædia, or an Universal Dictionary of Arts and Sciences (1st ed.). James and John Knapton, et al.
- Larmor, Joseph 2010. Retrieved January 2, 2011. 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. 269.
- Whittaker, E. T., A History of the Theories of Aether and Electricity. Dublin University Press, 1910.
- Russell, Daniel E . 17 February 2008. Retrieved December 31, 2010. "Helgustadir Iceland Spar Mine" mindat.org
- Retrieved January 2, 2011. "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"
- Kelley, Vincent C. 1940. Retrieved December 31, 2010. "Iceland Spar in Mew Mexico". American Mineralogist, Volume 25, pp. 357-367
- WANG Jing-teng, CHEN Hen-shui, YANG En-lin, WU Bo. 2009. Retrieved January 3, 2011. "Geological Characteristics of Iceland Spar Mineral Deposit of Mashan District in Guizhou". China National Knowledge Infrastructure, P619.2 CNKI:SUN:KJQB.0.2009-33-061
- "Helgustaðanáma". Umhverfisstofnun (in Icelandic). Retrieved 2020-08-20.
- Karlsen, Leif K. 2003. Secrets of the Viking Navigators. One Earth Press. ISBN 978-0-9721515-0-4, 220 pp.
- 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 polarimetric Viking navigation under foggy and cloudy skies". Proc. R. Soc. A 463 (2080): 1081–1095. doi:10.1098/rspa.2007.1811. ISSN 0962-8452.
- 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].
- First evidence of Viking-like 'sunstone' found. Discovery.com. Accessed 8 March 2018.
- 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.
- Greenslade, Thomas B. Jr. "Nicol Prism". Kenyon College. Retrieved 23 January 2014.
- https://www.bloomberg.com/apps/news?pid=20601088&sid=ab6WLEn2ciGU&refer=muse Seligman, Craig, "Pynchon's First Novel in 10 Years Has Sex, Math, Explosives", review of Against the Day at web-site of Bloomberg News, article dated November 20, 2006, accessed November 26, 2006