Iceland spar
Template:Short descriptionIceland spar, formerly called Iceland crystal (Template:Langx Template:IPA, Template:Lit) 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.<ref name="1913Websters"/><ref name="OEDIcelandspar"/>
Formation and composition
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Iceland spar is a colourless, transparent variety of calcium carbonate (CaCO3).<ref name=":1">Template:Cite journal</ref> It crystallizes in the trigonal system, typically forming rhombohedral crystals.<ref name=":3">Hughes, H. Herbert., Iceland spar and optical fluorite: U. S. Bureau of Mines, Information Circular 6468 (1931)</ref> 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.<ref name=":1" /><ref>Template:Cite journal</ref>
Iceland spar forms in sedimentary environments, mainly limestone and dolomite rocks, but also in hydrothermal veins and evaporite deposits.<ref name=":4">Template:Cite journal</ref> It precipitates from solutions rich in calcium and carbonate ions, influenced by temperature, pressure, and impurities.<ref name=":4" /><ref>Template:Cite journal</ref>
The most common crystal structure of Iceland spar is rhombohedral, but other structures, such as scalenohedral or prismatic, can form depending on formation conditions.<ref name=":2">Template:Cite journal</ref><ref>Template:Cite journal</ref> Iceland spar is primarily found in Iceland but can occur in different parts of the world with suitable geological conditions.<ref name=":1" /><ref>Template:Cite journal</ref>
Characteristics and optical properties
Iceland spar is characterized by its large, readily cleavable crystals, easily divided into parallelepipeds.<ref name="Websters1828" /><ref name="Miers2010" /> 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.<ref name=":32">Hughes, H. Herbert., Iceland spar and optical fluorite: U. S. Bureau of Mines, Information Circular 6468 (1931)</ref>
- It is birefringent, whereby its refractive index differs for light of different polarizations.<ref name="Websters1828" /><ref name="Miers2010" /> 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.<ref name=":22">Template:Cite journal</ref> This effect is known as the "Becke line" and can be used to determine a mineral's refractive index.<ref name=":12">Template:Cite journal</ref><ref>Template:Cite journal</ref>
- 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.<ref>Template:Cite journal</ref>
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.<ref name=":13">Template:Cite journal</ref><ref>Template:Cite book</ref>
The study of Iceland spar's double refraction by scientists including Christiaan Huygens,<ref name="Huy1690" /> Isaac Newton, and Sir George Stokes played a role in developing the wave theory of light.<ref name="1728Cyclopaedia" /><ref name="Larmor2010" /> Huygens, in particular, used double refraction to support his wave theory of light, in contrast to Newton's corpuscular theory.<ref>Template:Cite book</ref> Augustin-Jean Fresnel published a complete explanation of double refraction in light polarization in the 1820s.<ref name="Whittaker" />
The understanding of double refraction in Iceland spar led to the development of polarized light microscopy, used to study the properties of materials.<ref name=":14">Template:Cite journal</ref><ref name=":23">Template:Cite journal</ref>
It is speculated Vikings used its light-polarizing property to tell the direction of the sun on cloudy days for navigational purposes.<ref name="VikingPolarization" /><ref name="Karlsen2003" />
Mining
Named after Iceland due to its abundance on the island,<ref name="Russell2008" /> 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.<ref name="GraniteGap" /><ref name="Kelly1940" /><ref name="Guizhou" /> The clearest specimens, as well as the single largest, are from the Helgustaðir mine in Iceland.<ref name=":0">Template:Cite web</ref>
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.<ref name=":33">Hughes, H. Herbert., Iceland spar and optical fluorite: U. S. Bureau of Mines, Information Circular 6468 (1931)</ref> 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.<ref name=":33" /><ref name=":17">Template:Cite journal</ref>
Environmental issues
Some potential environmental issues associated with Iceland spar mining include habitat destruction, water pollution, air pollution, soil degradation, and visual impact.<ref name=":03">Template:Cite web</ref><ref name=":18">Template:Cite journal</ref> 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.<ref name=":03" /> Water sources can be contaminated through the discharge of chemicals used in the extraction and processing of minerals, impacting aquatic life and water quality.<ref name=":18" /> Mining activities can also lead to soil erosion and degradation, mainly if proper land reclamation measures are not implemented after mining ceases.<ref>Template:Cite journal</ref> Open-pit mining operations can have a significant visual impact on the landscape, altering the natural scenery of an area.<ref name=":03" /> These measures may include erosion control, environmentally friendly mining techniques, and the reclamation of mined areas to restore them to a natural state.<ref name=":03" />
Uses
Iceland spar's unique optical properties made it historically useful in applications including telecommunications, polarizing microscopes, optical rangefinders, and gunsights.<ref name=":15">Template:Cite journal</ref><ref>Template:Cite journal</ref> It has been used in navigation as a polarizing filter to determine the sun's direction on overcast and foggy days.<ref name="peasoup" /> It is speculated that the sunstone (Template:Langx, 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.<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 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.<ref>Template:Cite web</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>
William Nicol (1770–1851) used Iceland spar to invent the first polarizing prism, the Nicol prism.<ref>Template:Cite web</ref>
Modern applications
Iceland spar holds an essential place in modern applications including polarizing microscopes, lenses, and filters.<ref>Template:Cite journal</ref> Its birefringence in geological and biological microscopy reveals material structure, and in education and research it is a practical tool to demonstrate optical principles.<ref name=":16">Template:Cite journal</ref>
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.<ref name=":5">Template:Cite journal</ref> In metallurgy, calcite acts as a flux to lower the melting point of metals during smelting and refining.<ref>Template:Cite journal</ref> It is used in agriculture as a soil conditioner and neutralizer to adjust soil pH levels and improve crop yields.<ref name=":02">Template:Cite journal</ref> Calcite contributes to environmental remediation efforts, treating water and soil by neutralizing acidity and removing heavy metals.<ref name=":02" />
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).<ref name=":52">Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> The presence of Iceland spar can indicate hydrothermal activity, as calcite can form in hydrothermal veins.<ref name=":42">Template:Cite journal</ref>
Conservation and protection
Conservation efforts related to Iceland spar primarily focus on preserving specimens and mining sites.<ref name=":06">Template:Cite web</ref> One challenge in preserving specimens is the risk of damage during extraction, handling, and storage.<ref name=":35">Hughes, H. Herbert., Iceland spar and optical fluorite: U. S. Bureau of Mines, Information Circular 6468 (1931)</ref> Mining sites that yield high-quality specimens are of interest for conservation and may be designated protected areas to prevent overexploitation.<ref name=":06" />
Cultural impact
The Thomas Pynchon novel Against the Day uses the doubling effect of Iceland spar as a theme.<ref>Template:Cite web</ref>