Английская Википедия:Iceland spar

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Шаблон:Short descriptionIceland spar, formerly called Iceland crystal (Шаблон:Lang-is Шаблон:IPA-is, Шаблон:Lit) 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.

Formation and Composition

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Calcite rhombohedral crystal structure

Iceland spar, or calcite, is a colourless, transparent variety of calcium carbonate (CaCO₃). It crystallizes in the trigonal system, typically forming rhombohedral crystals. 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.

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

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

Characteristics and Optical Properties

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Calcite crystal birefringence

Iceland spar is characterized by its large, readily cleavable crystals, easily divided into parallelepipeds. This feature makes it easily identifiable and workable. One of the most remarkable properties of Iceland spar is its birefringence, where the crystal's refractive index differs for light of different polarizations. 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.

Iceland spar possesses several optical properties other than double refraction and birefringence. It is highly transparent to visible light, allowing light to pass through with minimal absorption or scattering, which is ideal for optical applications requiring clarity. Iceland spar can produce vivid colours when viewed under polarized light due to its birefringent nature. This effect is known as the "Becke line" and is used to determine the refractive index of the mineral. Additionally, Iceland spar is optically active, meaning it can rotate the plane of polarization of light passing through it, a property resulting from its asymmetrical atomic arrangement. These optical properties contribute to the mineral's scientific importance and aesthetic appeal.

Historical Significance

Iceland spar holds historical importance in optics and the study of light. One of its most notable properties is its ability to exhibit double refraction. This phenomenon was first described by the Danish scientist Erasmus Bartholin in 1669, who observed it in a specimen of Iceland spar.

The study of double refraction in Iceland spar played a role in developing the wave theory of light. Scientists such as Christiaan Huygens, Isaac Newton, and Sir George Stokes studied this phenomenon and contributed to our understanding of light as a wave. Huygens, in particular, used double refraction to support his wave theory of light, in contrast to Newton's corpuscular theory. Augustin-Jean Fresnel published a complete explanation of double refraction in light polarization in the 1820s.

The understanding of double refraction in Iceland spar also led to the development of polarized light microscopy, which is used in various scientific fields to study the properties of materials. Iceland spar has been used historically in optical instruments like polarizing microscopes and navigation equipment.

Mining

Mines producing Iceland spar include many mines producing related calcite and aragonite. Iceland spar occurs in various locations worldwide, historically named after Iceland due to its abundance on the island. Other productive sources include China and the greater Sonoran Desert region, in Santa Eulalia, Chihuahua, Mexico, and New Mexico, United States. The clearest specimens, as well as the largest, have been from the Helgustaðir mine in Iceland.

Uses

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Iceland spar, possibly the Icelandic medieval sun stone used to locate the sun in the sky when obstructed from view

It has been speculated that the sunstone (Шаблон:Lang-non, 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. 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.

William Nicol (1770–1851) invented the first polarizing prism, using Iceland spar to create his Nicol prism.

See also

• Spar sunstone

Cultural impact

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