Iceland spar, formerly called Iceland crystal ( , ) and also called optical calcite, is a transparent variety of

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

, or crystallized

calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

, originally brought from

Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...

, and used in demonstrating the polarization of

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

Formation and composition

Iceland spar is a colourless, transparent variety of calcium carbonate (CaCO₃). It crystallizes in the trigonal system, typically forming rhombohedral crystals.Hughes, H. Herbert., Iceland spar and optical fluorite: ''U. S. Bureau of Mines, Information Circular'' 6468 (1931) 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 Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and fetal c ...

and

evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as oce ...

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

but can occur in different parts of the world with suitable geological conditions.

Characteristics and optical properties

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 Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefrin ...

, where the crystal's

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

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 In geometry, two geometric objects are perpendicular if they intersect at right angles, i.e. at an angle of 90 degrees or π/2 radians. The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', � ...

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.Hughes, H. Herbert., Iceland spar and optical fluorite: ''U. S. Bureau of Mines, Information Circular'' 6468 (1931) 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 can be used to determine a mineral's refractive index. Additionally, Iceland spar is optically active, meaning it can rotate the

plane of polarization For light and other electromagnetic radiation, the plane of polarization is the plane (geometry), plane spanned by the direction of propagation and either the electric vector or the magnetic vector, depending on the convention. It can be defined ...

of light passing through it, a property resulting from its asymmetrical atomic arrangement. 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. 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 Christiaan Huygens, Halen, Lord of Zeelhem, ( , ; ; also spelled Huyghens; ; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor who is regarded as a key figure in the Scientific Revolution ...

Isaac Newton Sir Isaac Newton () was an English polymath active as a mathematician, physicist, astronomer, alchemist, theologian, and author. Newton was a key figure in the Scientific Revolution and the Age of Enlightenment, Enlightenment that followed ...

, and Sir George Stokes studied this phenomenon and contributed to the 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 Augustin-Jean Fresnel (10 May 1788 – 14 July 1827) was a French civil engineer and physicist whose research in optics led to the almost unanimous acceptance of the wave theory of light, excluding any remnant of Isaac Newton, Newton's c ...

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 Polarized light microscopy can mean any of a number of optical microscopy techniques involving polarized light. Simple techniques include illumination of the sample with polarized light. Directly transmitted light can, optionally, be blocked wi ...

, 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 Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation fr ...

. 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 The Sonoran Desert () is a hot desert and ecoregion in North America that covers the northwestern Mexican states of Sonora, Baja California, and Baja California Sur, as well as part of the Southwestern United States (in Arizona and California). It ...

region, in Santa Eulalia, Chihuahua,

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

, and

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

, United States. The clearest specimens, as well as the largest, have been from the Helgustaðir mine in Iceland. Surveying tools and techniques are combined to reduce the risk and cost of exploration to identify deposits.

Geological map A geological map or geologic map is a special-purpose map made to show various geological features. Rock (geology), Rock units or stratum, geologic strata are shown by color or symbols. Bed (geology), Bedding planes and structural features such ...

s and remote sensing techniques, such as

satellite imagery Satellite images (also Earth observation imagery, spaceborne photography, or simply satellite photo) are images of Earth collected by imaging satellites operated by governments and businesses around the world. Satellite imaging companies sell im ...

and

aerial photography Aerial photography (or airborne imagery) is the taking of photographs from an aircraft or other flight, airborne platforms. When taking motion pictures, it is also known as aerial videography. Platforms for aerial photography include fixed-wi ...

, are used for initial exploration and regional assessment to identify potential areas for further exploration. Geophysical surveys, including magnetometry, gravity surveys, and electromagnetic surveys, are then employed to detect anomalies indicating mineralization. Field mapping of surface geology and mineralogy also plays a role in identifying potential mineralization zones.Hughes, H. Herbert., Iceland spar and optical fluorite: ''U. S. Bureau of Mines, Information Circular'' 6468 (1931) The mining process for Iceland spar varies based on the specific geological conditions of the deposit.

Open-pit mining Open-pit mining, also known as open-cast or open-cut mining and in larger contexts mega-mining, is a surface mining technique that extracts rock (geology), rock or minerals from the earth. Open-pit mines are used when deposits of commercially ...

or quarrying is common for surface deposits.Hughes, H. Herbert., Iceland spar and optical fluorite: ''U. S. Bureau of Mines, Information Circular'' 6468 (1931) 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.

Environmental issues

Some potential environmental issues associated with Iceland spar mining include

habitat destruction Habitat destruction (also termed habitat loss or habitat reduction) occurs when a natural habitat is no longer able to support its native species. The organisms once living there have either moved elsewhere, or are dead, leading to a decrease ...

water pollution Water pollution (or aquatic pollution) is the contamination of Body of water, water bodies, with a negative impact on their uses. It is usually a result of human activities. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and ...

air pollution Air pollution is the presence of substances in the Atmosphere of Earth, air that are harmful to humans, other living beings or the environment. Pollutants can be Gas, gases like Ground-level ozone, ozone or nitrogen oxides or small particles li ...

soil degradation Soil retrogression and degradation are two regressive evolution processes associated with the loss of equilibrium of a soil health, stable soil. Retrogression is primarily due to soil erosion and corresponds to a phenomenon where succession revert ...

, and visual impact. Mining activities can destroy natural habitats, mainly if the mining site is located in ecologically sensitive areas, leading to the loss of

biodiversity Biodiversity is the variability of life, life on Earth. It can be measured on various levels. There is for example genetic variability, species diversity, ecosystem diversity and Phylogenetics, phylogenetic diversity. Diversity is not distribut ...

and disrupting local ecosystems. Water sources can be contaminated through the discharge of chemicals used in the extraction and processing of minerals, impacting aquatic life and water quality. Mining activities can also lead to soil erosion and degradation, mainly if proper land reclamation measures are not implemented after mining ceases. Open-pit mining operations can have a significant visual impact on the landscape, altering the natural scenery of an area. These measures may include

erosion control Erosion control is the practice of preventing or controlling wind or water erosion in agriculture, land development, coast, coastal areas, Bank (geography), river banks and construction. Effective erosion controls handle surface runoff and are ...

, environmentally friendly mining techniques, and the reclamation of mined areas to restore them to a natural state.

Health concerns

Mining, including Iceland spar mining, poses various health risks to workers and nearby communities. Some key health concerns associated with mining activities include respiratory issues,

noise-induced hearing loss Noise-induced hearing loss (NIHL) is a Hearing loss, hearing impairment resulting from exposure to loud sound. People may have a loss of perception of a narrow range of Frequency, frequencies or impaired perception of sound including hyperacusi ...

, chemical exposure,

musculoskeletal disorders Musculoskeletal disorders (MSDs) are injuries or pain in the human musculoskeletal system, including the joints, ligaments, muscles, nerves, tendons, and structures that support limbs, neck and back. MSDs can arise from a sudden exertion (e.g., l ...

, injuries and accidents, and mental health issues. Dust generated during mining operations can contain harmful particles, leading to respiratory problems. The high noise levels generated by mining activities can cause hearing loss over time if proper protective measures are not in place. Miners may also be exposed to harmful chemicals used in the extraction and processing of minerals, which can cause various health issues. The physical demands of mining work, such as heavy lifting and repetitive motions, can result in musculoskeletal disorders. Injuries and accidents are also common risks in mining, including falls, equipment-related incidents, and mine collapses. The demanding nature of mining work, along with long hours and isolation, can contribute to mental health issues such as stress, anxiety, and depression. Mining companies must implement health and safety measures to mitigate these risks to protect workers and nearby communities, including

personal protective equipment Personal protective equipment (PPE) is protective clothing, helmets, goggles, or other garments or equipment designed to protect the wearer's body from injury or infection. The hazards addressed by protective equipment include physical, elect ...

, dust control measures, and health and safety training. Regularly monitoring air quality, noise levels, and other potential hazards is essential to ensure a safe working environment.

Uses

Iceland spar has been historically used in telecommunications due to its unique optical properties. One of its key features, birefringence, made it worthwhile in early optical technologies, such as developing optical instruments like polarizing microscopes and constructing optical rangefinders and gunsights. While uncommon, Iceland spar has historically been used in navigation as a polarizing filter to determine the sun's direction on overcast days. It has been speculated that the

sunstone Sunstone is a microcline or oligoclase feldspar, which when viewed from certain directions exhibits a aventurescence, spangled appearance. It has been found in Southern Norway, Sweden, various United States localities and on some beaches along ...

(, a different mineral from the gem-quality

) mentioned in medieval Icelandic texts, such as Rauðúlfs þáttr, was Iceland spar, and that

Vikings Vikings were seafaring people originally from Scandinavia (present-day Denmark, Norway, and Sweden), who from the late 8th to the late 11th centuries raided, pirated, traded, and settled throughout parts of Europe.Roesdahl, pp. 9� ...

used its light-polarizing property to tell the direction of the sun on cloudy days for

navigational Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...

purposes. The polarization of sunlight in the

Arctic The Arctic (; . ) is the polar regions of Earth, polar region of Earth that surrounds the North Pole, lying within the Arctic Circle. The Arctic region, from the IERS Reference Meridian travelling east, consists of parts of northern Norway ( ...

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 The Elizabethan era is the epoch in the Tudor period of the history of England during the reign of Queen Elizabeth I (1558–1603). Historians often depict it as the golden age in English history. The Roman symbol of Britannia (a female ...

that sank in 1592 off

Alderney Alderney ( ; ; ) is the northernmost of the inhabited Channel Islands. It is part of the Bailiwick of Guernsey, a British Crown Dependencies, Crown dependency. It is long and wide. The island's area is , making it the third-largest isla ...

suggests that this navigational technology may have persisted after the invention of the magnetic

compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with No ...

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

Modern applications

Despite being historically significant, Iceland spar still holds an essential place in modern applications. Due to its optical properties, Iceland spar is still used in instruments like polarizing microscopes,

lenses A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

, and filters. Iceland spar is also used in optical instruments for geological and biological microscopy as its birefringence helps to reveal material structure. It is also a practical tool used in education and research to demonstrate optical principles. Though its applications are less widespread than in the past, Iceland spar continues to contribute to various scientific and technological endeavours. As a type of calcite, Iceland spar can be used in construction as a building material in

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...

and

concrete Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...

. Its high purity and brightness make it an ideal filler in paints and coatings. In metallurgy, calcite acts as a

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications in physics. For transport phe ...

to lower the melting point of metals during

smelting Smelting is a process of applying heat and a chemical reducing agent to an ore to extract a desired base metal product. It is a form of extractive metallurgy that is used to obtain many metals such as iron-making, iron, copper extraction, copper ...

and refining. Additionally, it is used in agriculture as a

soil conditioner A soil conditioner is a product which is added to soil to improve the soil’s physical qualities, usually its fertility (ability to provide nutrition for plants) and sometimes its mechanics. In general usage, the term "soil conditioner" is often ...

and neutralizer to adjust soil pH levels and improve crop yields. Calcite also contributes to environmental remediation efforts, treating acidic water and soil by neutralizing acidity and removing heavy metals.

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 Diagenesis () is the process of physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a role as sedi ...

(compaction and cementation). Studying Iceland spar can provide valuable information about past environmental conditions, such as the presence of ancient seas and marine life, as carbonate rocks like

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

often form in marine environments. The presence of Iceland spar can also indicate the presence of hydrothermal activity, as calcite can form in hydrothermal veins.

Conservation and protection

Due to their scientific and historical significance, conservation efforts related to Iceland spar primarily focus on preserving specimens and mining sites. One of the challenges in preserving Iceland spar specimens is the risk of damage during extraction, handling, and storage.Hughes, H. Herbert., Iceland spar and optical fluorite: ''U. S. Bureau of Mines, Information Circular'' 6468 (1931) Mining sites that yield high-quality Iceland spar specimens are also of interest for conservation. These sites may be designated protected areas to prevent

overexploitation Overexploitation, also called overharvesting or ecological overshoot, refers to harvesting a renewable resource to the point of diminishing returns. Continued overexploitation can lead to the destruction of the resource, as it will be unable to ...

Cultural impact

The

Thomas Pynchon Thomas Ruggles Pynchon Jr. ( , ; born May 8, 1937) is an American novelist noted for his dense and complex novels. His fiction and non-fiction writings encompass a vast array of subject matter, Literary genre, genres and Theme (narrative), th ...

novel '' Against the Day'' uses the doubling effect of Iceland spar as a theme.

References

{{refs, refs= {{WD1913, title=Polarimetry {{OED, Iceland spar {{WD1828, title=Birefringence Miers, Henry A., ''Mineralogy: an introduction to the scientific study of minerals''. Nabu Press. {{ISBN, 1-177-85127-X Chap. 6, p. 128. 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
2016. {{Cyclopaedia 1728, title=Isaac Newton 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. Russell, Daniel E . 17 February 2008. Retrieved December 31, 2010.
Helgustadir Iceland Spar Mine
''

mindat.org Mindat.org is a non-commercial interactive online database covering minerals around the world. Originally created by Jolyon Ralph as a private project in 1993, it was launched as a community-editable website in October 2000. it is operated by ...

'' 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" Kelley, Vincent C. 1940. Retrieved December 31, 2010.
Iceland Spar in New Mexico
. ''

American Mineralogist ''American Mineralogist: An International Journal of Earth and Planetary Materials'' is a peer-reviewed scientific journal covering the general fields of mineralogy, crystallography, geochemistry, and petrology. It is an official journal of the Mi ...

'', Volume 25, pp. 357-367 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.
CNKI:SUN:KJQB.0.2009-33-061
/ref>
from Polarization.net. Retrieved February 8, 2007. Karlsen, Leif K. 2003. ''Secrets of the Viking Navigators''. One Earth Press. {{ISBN, 978-0-9721515-0-4, 220 pp. 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. 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 ccessed December 5, 2011 Whittaker, E. T., '' A History of the Theories of Aether and Electricity''. Dublin University Press, 1910. Calcium minerals Carbonate minerals Medieval history of Iceland Optical materials Polarization (waves) Transparent materials Trigonal minerals