Iceland spar, formerly called Iceland crystal ( , ) 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. 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 also 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
thumb|Calcite crystal birefringence
Iceland spar is characterized by its large, readily cleavable crystals, easily divided into parallelepipeds.
- It is birefringent, whereby its refractive index differs for light of different polarizations. This effect is known as the "Becke line" and can be used to determine a mineral's refractive index.
- 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.
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.
The study of Iceland spar's double refraction by scientists including Christiaan Huygens, Augustin-Jean Fresnel published a complete explanation of double refraction in light polarization in the 1820s.
It is speculated Vikings used its light-polarizing property to tell the direction of the sun on cloudy days for navigational purposes.
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 and prepared for applications including optical instruments and jewelry, and as a source of calcium carbonate for industrial use.
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. Open-pit mining operations can have a significant visual impact on the landscape, altering the natural scenery of an area. It has been used in navigation as a polarizing filter to determine the sun's direction on overcast and foggy days.
William Nicol (1770–1851) used Iceland spar to invent the first polarizing prism, the Nicol prism.
Modern applications
Iceland spar holds an essential place in modern applications including polarizing microscopes, lenses, and filters. Its birefringence in geological and biological microscopy reveals material structure, and in education and research it is a practical tool to demonstrate optical principles.
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. In metallurgy, calcite acts as a flux to lower the melting point of metals during smelting and refining. It is used in agriculture as a soil conditioner and neutralizer to adjust soil pH levels and improve crop yields. Calcite contributes to environmental remediation efforts, treating water and soil by neutralizing acidity and removing heavy metals. 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. The presence of Iceland spar can indicate hydrothermal activity, as calcite can form in hydrothermal veins.
Conservation and protection
Conservation efforts related to Iceland spar primarily focus on preserving specimens and mining sites. One challenge in preserving specimens is the risk of damage during extraction, handling, and storage. Mining sites that yield high-quality specimens are of interest for conservation and may be designated protected areas to prevent overexploitation.
See also
- Spar sunstone