Iceland spar

Short description: Transparent variety of calcite

Iceland spar, formerly called Iceland crystal (Icelandic: silfurberg is, lit. silver-rock) and also called optical calcite, is a transparent variety of calcite, a crystallized calcium carbonate, originally brought from Iceland and used in demonstrating the polarization of light.[1][2]

## Contents

* 1 Formation and composition
* 2 Characteristics and optical properties
* 3 Historical significance
* 4 Mining
* 4.1 Environmental issues
* 5 Uses
* 5.1 Modern applications
* 5.2 Geological significance
* 6 Conservation and protection
* 7 Cultural impact
* 8 See also
* 9 References

## Formation and composition

Calcite rhombohedral crystal structure

Iceland spar is a colourless, transparent variety of calcium carbonate (CaCO3).[3] It crystallizes in the trigonal system, typically forming rhombohedral crystals.[4] It has a Mohs hardness of 3 and exhibits double refraction, splitting a ray of light into two rays that travel at different speeds and directions.[3][5]

Iceland spar forms in sedimentary environments, mainly limestone and dolomite rocks, but also in hydrothermal veins and evaporite deposits.[6] It precipitates from solutions rich in calcium and carbonate ions, influenced by temperature, pressure, and impurities.[6][7]

The most common crystal structure of Iceland spar is rhombohedral, but other structures, such as scalenohedral or prismatic, can form depending on formation conditions.[8][9] Iceland spar is primarily found in Iceland but can occur in different parts of the world with suitable geological conditions.[3][10]

## Characteristics and optical properties

Calcite crystal birefringence

Iceland spar is characterized by its large, readily cleavable crystals, easily divided into parallelepipeds.[11][12] This feature makes it easily identifiable and workable.

Iceland spar possesses remarkable optical properties:

* It is highly transparent to visible light, which passes through with minimal absorption or scattering, ideal for optical applications requiring clarity.[13]

* It is birefringent, whereby its refractive index differs for light of different polarizations.[11][12] When a ray of unpolarized light passes through the crystal, it is divided into two rays of mutually perpendicular polarization directed at various angles. This double refraction causes objects seen through the crystal to appear doubled, and the crystal can produce vivid colours when viewed under polarized light.[14] This effect is known as the "Becke line" and can be used to determine a mineral's refractive index.[15][16]

* It is optically active, meaning it can rotate the plane of polarization of light passing through it, a property resulting from its asymmetric atomic arrangement.[17]

These optical properties contribute to the mineral's scientific use and aesthetic appeal.

## Historical significance

Iceland spar holds historical importance in optics and the study of light. That it exhibits double refraction was first described by the Danish scientist Erasmus Bartholin in 1669.[18][19]

The study of Iceland spar's double refraction by scientists including Christiaan Huygens,[20] Isaac Newton, and Sir George Stokes played a role in developing the wave theory of light.[21][22] Huygens, in particular, used double refraction to support his wave theory of light, in contrast to Newton's corpuscular theory.[23] Augustin-Jean Fresnel published a complete explanation of double refraction in light polarization in the 1820s.[24]

The understanding of double refraction in Iceland spar led to the development of polarized light microscopy, used to study the properties of materials.[25][26]

It is speculated Vikings used its light-polarizing property to tell the direction of the sun on cloudy days for navigational purposes.[27][28]

## Mining

Named after Iceland due to its abundance on the island,[29] Iceland spar occurs in locations worldwide including many mines producing related calcite and aragonite. Sources include China, the greater Sonoran Desert region of North America, Chihuahua, Mexico, and New Mexico, United States.[30][31][32] The clearest specimens, as well as the single largest, are from the Helgustaðir mine in Iceland.[33]

The mining process for Iceland spar varies based on the specific geological conditions of the deposit. Open-pit mining or quarrying is common for surface deposits.[34] Once extracted, the calcite is processed to remove impurities and prepared for applications including optical instruments and jewelry, and as a source of calcium carbonate for industrial use.[34][35]

### Environmental issues

Some potential environmental issues associated with Iceland spar mining include habitat destruction, water pollution, air pollution, soil degradation, and visual impact.[36][37] Mining activities can destroy natural habitats, mainly if the mining site is located in ecologically sensitive areas, leading to the loss of biodiversity and disrupting local ecosystems.[36] Water sources can be contaminated through the discharge of chemicals used in the extraction and processing of minerals, impacting aquatic life and water quality.[37] Mining activities can also lead to soil erosion and degradation, mainly if proper land reclamation measures are not implemented after mining ceases.[38] Open-pit mining operations can have a significant visual impact on the landscape, altering the natural scenery of an area.[36] These measures may include erosion control, environmentally friendly mining techniques, and the reclamation of mined areas to restore them to a natural state.[36]

## Uses

Iceland spar, possibly the Icelandic medieval sun stone used to locate the sun's direction in overcast or foggy skies[27]

Iceland spar's unique optical properties made it historically useful in applications including telecommunications, polarizing microscopes, optical rangefinders, and gunsights.[39][40] It has been used in navigation as a polarizing filter to determine the sun's direction on overcast and foggy days.[41] It is speculated that the sunstone (Old Norse: sólarsteinn, a different mineral from the gem-quality 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 navigational purposes.[27][28] The polarization of sunlight in the Arctic can be detected,[41] 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.[42] 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.[43][44]

William Nicol (1770–1851) used Iceland spar to invent the first polarizing prism, the Nicol prism.[45]

### Modern applications

Iceland spar holds an essential place in modern applications including polarizing microscopes, lenses, and filters.[46] Its birefringence in geological and biological microscopy reveals material structure, and in education and research it is a practical tool to demonstrate optical principles.[47]

As a calcite, Iceland spar is used as a building material in cement and concrete. Its high purity and brightness make it an ideal filler in paints and coatings.[48] In metallurgy, calcite acts as a flux to lower the melting point of metals during smelting and refining.[49] It is used in agriculture as a soil conditioner and neutralizer to adjust soil pH levels and improve crop yields.[50] Calcite contributes to environmental remediation efforts, treating water and soil by neutralizing acidity and removing heavy metals.[50]

### Geological significance

Due to Iceland spar typically forming in sedimentary environments, particularly limestone and dolomite rocks, its formation is closely tied to these carbonate rocks' deposition and diagenesis (compaction and cementation).[51] Studying Iceland spar distribution can provide information about past environmental conditions, such as the presence of ancient seas and marine life, as carbonate rocks like limestone often form in marine environments.[52] The presence of Iceland spar can indicate hydrothermal activity, as calcite can form in hydrothermal veins.[53]

## Conservation and protection

Conservation efforts related to Iceland spar primarily focus on preserving specimens and mining sites.[54] One challenge in preserving specimens is the risk of damage during extraction, handling, and storage.[55] Mining sites that yield high-quality specimens are of interest for conservation and may be designated protected areas to prevent overexploitation.[54]

## Cultural impact

The Thomas Pynchon novel Against the Day uses the doubling effect of Iceland spar as a theme.[56]

## See also

* Spar sunstone

## References

1. ↑ This article incorporates text from a publication now in the public domain:  Porter, Noah, ed. (1913) "Polarimetry" Webster's Dictionary Springfield, Massachusetts: C. & G. Merriam Co.
2. ↑ Iceland spar (3rd ed.), Oxford University Press, September 2005, http://oed.com/search?searchType=dictionary&q=Iceland+spar (Subscription or UK public library membership required.)
3. ↑ 3.0 3.1 3.2 Kristjansson, Leo (2002). "Iceland Spar: The Helgustadir Calcite Locality and its Influence on the Development of Science" (in en). Journal of Geoscience Education 50 (4): 419–427. doi:10.5408/1089-9995-50.4.419. ISSN 1089-9995. Bibcode: 2002JGeEd..50..419K. https://www.tandfonline.com/doi/full/10.5408/1089-9995-50.4.419.
4. ↑ Hughes, H. Herbert., Iceland spar and optical fluorite: U. S. Bureau of Mines, Information Circular 6468 (1931)
5. ↑ Wada, Shinobu; Suzuki, Hitomi (2003-01-06). "Calcite and fluorite as catalyst for the Knövenagel condensation of malononitrile and methyl cyanoacetate under solvent-free conditions". Tetrahedron Letters 44 (2): 399–401. doi:10.1016/S0040-4039(02)02431-0. ISSN 0040-4039. https://www.sciencedirect.com/science/article/pii/S0040403902024310.
6. ↑ 6.0 6.1 Rollion-Bard, Claire; Marin-Carbonne, Johanna (2011-06-01). "Determination of SIMS matrix effects on oxygen isotopic compositions in carbonates" (in en). Journal of Analytical Atomic Spectrometry 26 (6): 1285–1289. doi:10.1039/C0JA00213E. ISSN 1364-5544. https://pubs.rsc.org/en/content/articlelanding/2011/ja/c0ja00213e.
7. ↑ Morse, John W.; Arvidson, Rolf S.; Lüttge, Andreas (2007-02-01). "Calcium Carbonate Formation and Dissolution" (in en). Chemical Reviews 107 (2): 342–381. doi:10.1021/cr050358j. ISSN 0009-2665. PMID 17261071. https://pubs.acs.org/doi/10.1021/cr050358j.
8. ↑ Skomorovsky, Valery; Kushtal, Galina; Tokareva (Lopteva), Lyubov (2022-03-25). "Iceland spar and birefringent filter (BF) development" (in en). Solar-Terrestrial Physics 8 (1): 69–84. doi:10.12737/stp-81202209. ISSN 2500-0535. Bibcode: 2022STP.....8a..69S. http://naukaru.ru/en/nauka/article/45905/view.
9. ↑ Skropyshev, A. V. (1959). "Gaseous-Liquid Inclusions in Crystals of Iceland Spar" (in en). International Geology Review 1 (9): 1–11. doi:10.1080/00206815909473436. ISSN 0020-6814. Bibcode: 1959IGRv....1R...1S. https://www.tandfonline.com/doi/full/10.1080/00206815909473436.
10. ↑ Cook, Robert B. (2009). "Mineral Oddities: Theme of the 2009 Tucson Gem & Mineral Show" (in en). Rocks & Minerals 84 (1): 16–25. doi:10.3200/RMIN.84.1.16-25. ISSN 0035-7529. Bibcode: 2009RoMin..84...16C. https://www.tandfonline.com/doi/full/10.3200/RMIN.84.1.16-25.
11. ↑ 11.0 11.1 Template:WD1828
12. ↑ 12.0 12.1 Miers, Henry A., Mineralogy: an introduction to the scientific study of minerals. Nabu Press. ISBN 1-177-85127-X Chap. 6, p. 128.
13. ↑ Hughes, H. Herbert., Iceland spar and optical fluorite: U. S. Bureau of Mines, Information Circular 6468 (1931)
14. ↑ Skomorovsky, Valery; Kushtal, Galina; Tokareva (Lopteva), Lyubov (2022-03-25). "Iceland spar and birefringent filter (BF) development" (in en). Solar-Terrestrial Physics 8 (1): 69–84. doi:10.12737/stp-81202209. ISSN 2500-0535. Bibcode: 2022STP.....8a..69S. http://naukaru.ru/en/nauka/article/45905/view.
15. ↑ Kristjansson, Leo (2002). "Iceland Spar: The Helgustadir Calcite Locality and its Influence on the Development of Science" (in en). Journal of Geoscience Education 50 (4): 419–427. doi:10.5408/1089-9995-50.4.419. ISSN 1089-9995. Bibcode: 2002JGeEd..50..419K. https://www.tandfonline.com/doi/full/10.5408/1089-9995-50.4.419.
16. ↑ Choi, Ju H.; Eichele, Chad; Lin, Yuan C.; Shi, Frank G.; Carlson, Bob; Sciamanna, Steven (2008-03-01). "Determination of effective refractive index of molecular diamondoids by Becke line method". Scripta Materialia. Viewpoint set no. 43 "Friction stir processing" 58 (5): 413–416. doi:10.1016/j.scriptamat.2007.10.036. ISSN 1359-6462. https://www.sciencedirect.com/science/article/pii/S1359646207007543.
17. ↑ Kristjánsson, L. (2012-05-16). "Iceland spar and its legacy in science" (in en). History of Geo- and Space Sciences 3 (1): 117–126. doi:10.5194/hgss-3-117-2012. ISSN 2190-5029. Bibcode: 2012HGSS....3..117K. https://hgss.copernicus.org/articles/3/117/2012/.
18. ↑ Kristjansson, Leo (2002). "Iceland Spar: The Helgustadir Calcite Locality and its Influence on the Development of Science" (in en). Journal of Geoscience Education 50 (4): 419–427. doi:10.5408/1089-9995-50.4.419. ISSN 1089-9995. Bibcode: 2002JGeEd..50..419K. https://www.tandfonline.com/doi/full/10.5408/1089-9995-50.4.419.
19. ↑ Bartholin, Rasmus; Archibald, Thomas; Buchwald, Jed Z.; Møller Pedersen, Kurt (1991). Experiments on Birefringent icelandic crystal: with a facsimile of the original publication 1669. Acta historica scientiarum naturalium et medicinalium (Reprod. en fac-sim. ed.). Copenhagen: the Danish national library of science and medicine. ISBN 978-87-7709-010-3.
20. ↑ 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.
21. ↑ 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 (first ed.). James and John Knapton, et al.
22. ↑ 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.
23. ↑ Huygens, Christiaan (1912). Treatise On Light. Osmania University, Digital Library Of India. Macmillan And Company., Limited. http://archive.org/details/treatiseonlight031310mbp.
24. ↑ Whittaker, E. T., A History of the Theories of Aether and Electricity. Dublin University Press, 1910.
25. ↑ Kristjansson, Leo (2002). "Iceland Spar: The Helgustadir Calcite Locality and its Influence on the Development of Science" (in en). Journal of Geoscience Education 50 (4): 419–427. doi:10.5408/1089-9995-50.4.419. ISSN 1089-9995. Bibcode: 2002JGeEd..50..419K. https://www.tandfonline.com/doi/full/10.5408/1089-9995-50.4.419.
26. ↑ Skomorovsky, Valery; Kushtal, Galina; Tokareva (Lopteva), Lyubov (2022-03-25). "Iceland spar and birefringent filter (BF) development" (in en). Solar-Terrestrial Physics 8 (1): 69–84. doi:10.12737/stp-81202209. ISSN 2500-0535. Bibcode: 2022STP.....8a..69S. http://naukaru.ru/en/nauka/article/45905/view.
27. ↑ 27.0 27.1 27.2 The Viking Sunstone, from Polarization.net. Retrieved February 8, 2007.
28. ↑ 28.0 28.1 Karlsen, Leif K. 2003. Secrets of the Viking Navigators. One Earth Press. ISBN 978-0-9721515-0-4, 220 pp.
29. ↑ Russell, Daniel E . 17 February 2008. Retrieved December 31, 2010. "Helgustadir Iceland Spar Mine" mindat.org
30. ↑ 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"
31. ↑ Kelley, Vincent C. 1940. Retrieved December 31, 2010. "Iceland Spar in New Mexico". American Mineralogist, Volume 25, pp. 357-367
32. ↑ 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
33. ↑ "Helgustaðanáma" (in is). https://ust.is/nattura/natturuverndarsvaedi/fridlyst-svaedi/austurland/helgustadanama/.
34. ↑ 34.0 34.1 Hughes, H. Herbert., Iceland spar and optical fluorite: U. S. Bureau of Mines, Information Circular 6468 (1931)
35. ↑ Kristjansson, Leo (2002). "Iceland Spar: The Helgustadir Calcite Locality and its Influence on the Development of Science" (in en). Journal of Geoscience Education 50 (4): 419–427. doi:10.5408/1089-9995-50.4.419. ISSN 1089-9995. Bibcode: 2002JGeEd..50..419K. https://www.tandfonline.com/doi/full/10.5408/1089-9995-50.4.419.
36. ↑ 36.0 36.1 36.2 36.3 Duffy, Lawrence (2015). "BCS 312: Land and Environments of the Circumpolar North II". https://www.uarctic.org/media/1060449/bcs-312-m2-feb-2015.pdf.
37. ↑ 37.0 37.1 Saliu, Muyideen Alade; Shehu, Shaib Abdulazeez (2012). "Effects of Calcite and Dolomite Mining on Water and Soil Qualities: A case study of Freedom Group of Companies, Ikpeshi, Edo State Nigeria". Journal of Emerging Trends in Engineering and Applied Sciences 3 (1): 19–24.
38. ↑ Saliu, Muyideen Alade; Shehu, Shaib Abdulazeez (2012). "Effects of Calcite and Dolomite Mining on Water and Soil Qualities: A case study of Freedom Group of Companies, Ikpeshi, Edo State Nigeria". Journal of Emerging Trends in Engineering and Applied Sciences 3 (1): 19–24.
39. ↑ Kristjansson, Leo (2002). "Iceland Spar: The Helgustadir Calcite Locality and its Influence on the Development of Science" (in en). Journal of Geoscience Education 50 (4): 419–427. doi:10.5408/1089-9995-50.4.419. ISSN 1089-9995. Bibcode: 2002JGeEd..50..419K. https://www.tandfonline.com/doi/full/10.5408/1089-9995-50.4.419.
40. ↑ Orrell, Lew (1995). "Mining calcite crystals during World War II". Mining History Journal. https://www.mininghistoryassociation.org/Journal/MHJ-v2-1995-Orrell.pdf.
41. ↑ 41.0 41.1 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.
42. ↑ 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].
43. ↑ "First Evidence of Viking-Like 'Sunstone' Found - Seeker". 2017-07-02. https://www.seeker.com/first-evidence-of-viking-like-sunstone-found-1767286293.html.
44. ↑ 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.
45. ↑ Greenslade, Thomas B. Jr.. "Nicol Prism". Kenyon College. http://physics.kenyon.edu/EarlyApparatus/Polarized_Light/Nicol_Prism/Nicol_Prism.html.
46. ↑ Skomorovsky, Valery; Kushtal, Galina; Tokareva (Lopteva), Lyubov (2022-03-25). "Iceland spar and birefringent filter (BF) development" (in en). Solar-Terrestrial Physics 8 (1): 69–84. doi:10.12737/stp-81202209. ISSN 2500-0535. Bibcode: 2022STP.....8a..69S. http://naukaru.ru/en/nauka/article/45905/view.
47. ↑ Kristjansson, Leo (2002). "Iceland Spar: The Helgustadir Calcite Locality and its Influence on the Development of Science" (in en). Journal of Geoscience Education 50 (4): 419–427. doi:10.5408/1089-9995-50.4.419. ISSN 1089-9995. Bibcode: 2002JGeEd..50..419K. https://www.tandfonline.com/doi/full/10.5408/1089-9995-50.4.419.
48. ↑ Dhar, Priyanka; Thornhill, Maria; Kota, Hanumantha Rao (2020-09-23). "An Overview of Calcite Recovery by Flotation" (in en). Materials Circular Economy 2 (1): 9. doi:10.1007/s42824-020-00006-y. ISSN 2524-8154.
49. ↑ Solihin (February 2018). "The review of recent carbonate minerals processing technology". IOP Conference Series: Earth and Environmental Science 118 (1). doi:10.1088/1755-1315/118/1/012065. ISSN 1755-1307. Bibcode: 2018E&ES..118a2065S.
50. ↑ 50.0 50.1 Wojas, Natalia A.; Swerin, Agne; Wallqvist, Viveca; Järn, Mikael; Schoelkopf, Joachim; Gane, Patrick A. C.; Claesson, Per M. (2019-04-01). "Iceland spar calcite: Humidity and time effects on surface properties and their reversibility". Journal of Colloid and Interface Science 541: 42–55. doi:10.1016/j.jcis.2019.01.047. ISSN 0021-9797. PMID 30682592. Bibcode: 2019JCIS..541...42W.
51. ↑ Dhar, Priyanka; Thornhill, Maria; Kota, Hanumantha Rao (2020-09-23). "An Overview of Calcite Recovery by Flotation" (in en). Materials Circular Economy 2 (1): 9. doi:10.1007/s42824-020-00006-y. ISSN 2524-8154.
52. ↑ Chen, Zhe; Zhou, Chuanming; Xiao, Shuhai; Wang, Wei; Guan, Chengguo; Hua, Hong; Yuan, Xunlai (2014-02-25). "New Ediacara fossils preserved in marine limestone and their ecological implications" (in en). Scientific Reports 4 (1): 4180. doi:10.1038/srep04180. ISSN 2045-2322. PMID 24566959. Bibcode: 2014NatSR...4.4180C.
53. ↑ Rollion-Bard, Claire; Marin-Carbonne, Johanna (2011-06-01). "Determination of SIMS matrix effects on oxygen isotopic compositions in carbonates" (in en). Journal of Analytical Atomic Spectrometry 26 (6): 1285–1289. doi:10.1039/C0JA00213E. ISSN 1364-5544. https://pubs.rsc.org/en/content/articlelanding/2011/ja/c0ja00213e.
54. ↑ 54.0 54.1 "Helgustaðanáma - Stjórnunar- og verndararáætlun 20172026". 2017. https://www.ust.is/library/Skrar/Einstaklingar/Nattura/Verndaraaetlanir/i-vinnslu/Helgustaðanáma_stjogverndaráætlun 2017_drög til kynningar.pdf.
55. ↑ Hughes, H. Herbert., Iceland spar and optical fluorite: U. S. Bureau of Mines, Information Circular 6468 (1931)
56. ↑ "Pynchon's First Novel in 10 Years Has Sex, Explosives (Update1)". 2007-09-30. https://www.bloomberg.com/apps/news?pid=20601088&sid=ab6WLEn2ciGU&refer=muse.

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