Iceland spar

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Iceland spar,

colorless variety of crystallized calcitecalcite
, very widely distributed mineral, commonly white or colorless, but appearing in a great variety of colors owing to impurities. Chemically it is calcium carbonate, CaCO3, but it frequently contains manganese, iron, or magnesium in place of the calcium.
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, characterized by its properties of transparency and double refraction. It is used chiefly in the manufacture of Nicol prismsNicol prism
, optical device invented (1828) by William Nicol of Edinburgh. It consists essentially of a crystal of calcite, or Iceland spar, that is cut at an angle into two equal pieces and joined together again with Canada balsam.
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, which are essential parts of polarizing microscopes and other optical instruments. The principal deposit is in Iceland, but small quantities are found in other countries, including the United States and Mexico. Iceland spar is believed by some to have been used by the Vikings as a sunstonesunstone.
1 Crystal mineral thought by some to have been used by the Vikings as an aid to navigation, especially in conditions of low visibility due to clouds or fog when the position of the sun was uncertain.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Iceland Spar


(limestone spar), a variety of the mineral calcite consisting of transparent, colorless, or slightly tinted crystals. It is used in constructing polarizing prisms for various instruments producing polarized light. There is a large deposit near Helgustadir at Eskifjordur in Iceland (hence the name), where optical-quality crystals have been extracted from cavities in basalt. In the USSR deposits of Iceland spar have been found in the Nizhniaia Tunguska Basin, the Yakut and Tuva ASSR’s, the Northern Caucasus, Transcaucasia, Middle Asia, and the Far East.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

Iceland spar

[′īs·lənd ′spär]
A pure, transparent form of calcite found particularly in Iceland; easily cleaved to form rhombohedral crystals that are doubly refracting. Also known as Iceland crystal.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Iceland spar is common in Scandinavia, and a device containing a crystal of the mineral was discovered in a 16th century shipwreck.
Therefore, and as X-raying, as well as heating up to 360[degrees]C in C[O.sup.2] atmosphere, changed the intensity of this signal by only 2-3%, the observed OSA spectra of Iceland spar and calcites are in essence the excitation spectra of [Mn.sup.2+] ions fluorescence.
(I know of no way that Iceland spar can work, since neither the color nor intensity of light seen through it changes when it's rotated.) Nevertheless, in those observing conditions the Sun's position can be directly seen - making it unclear what advantage the polarizer offers those lost at sea.
In 1808 a French physicist, Etienne-Louis Malus (1775-1812), was playing idly with a crystal of Iceland spar and found that the sunlight reflected from a window produced only a single ray of light after passing through the crystal.
If, however, the polarized light, en route from one piece of Iceland spar to the other, was made to pass through some organic liquid, the second piece of Iceland spar had to be turned for the polarized light to shine brightly again.
In 1829 the Scottish physicist William Nicol (1768-1851) devised a useful instrument in this connection by cementing two crystals of Iceland spar together with Canadian balsam.