# Iceland spar

Iceland spar, formerly called Iceland crystal (Icelandic: silfurberg, 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.

## Formation and composition

Iceland spar is a colourless, transparent variety of calcium carbonate (CaCO$_3$). 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

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 can be used to determine a mineral's refractive index. 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 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, Isaac Newton, 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 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.
Surveying tools and techniques are combined to reduce the risk and cost of exploration to identify deposits. Geological maps and remote sensing techniques, such as satellite imagery and aerial photography, 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.
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. 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, water pollution, air pollution, soil degradation, 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 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, 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, chemical exposure, musculoskeletal disorders, 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, 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 (Old Norse: sólarsteinn, 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.

### 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, 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 and concrete. Its high purity and brightness make it an ideal filler in paints and coatings. In metallurgy, calcite acts as a flux to lower the melting point of metals during smelting and refining. Additionally, it is used in agriculture as a soil conditioner 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 (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 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. 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.

## Cultural impact

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