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Iceland Spar
spar 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.[1][2] Formation and composition[edit] 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 it also forms 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[edit] Iceland spar is characterized by its large, readily cleavable crystals, easily divided into parallelepipeds.[11][12] This feature makes it easily identifiable and workable.
[35] Once extracted, the calcite is processed to remove impurities, prepared for various applications, including optical instruments and jewelry, and used as a source of calcium carbonate in industries like construction and agriculture. ISBN 978-87-7709-010-3.^ 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. 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.
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