Pleochroism
Pleochroism is an intriguing optical property of certain substances, particularly crystals, that causes them to exhibit different colors when viewed from various angles. This phenomenon, also known as polychroism, occurs most prominently in crystals under polarized light, which restricts light waves to a single direction. The crystal's unique structure and atomic composition play a crucial role in determining whether it will display pleochroism. Different crystal systems, such as hexagonal or tetragonal, exhibit varying degrees of pleochroism, categorized as dichroic (showing two colors) or trichroic (showing three colors).
Gemologists and mineralogists often leverage this characteristic as a valuable tool for identifying gemstones and minerals. Notably, certain crystals, like tourmaline and alexandrite, are well-known for their pleochroic properties. Historically, it is believed that ancient Vikings utilized pleochroic crystals, potentially cordierite (or iolite), known as "sunstones," to navigate on cloudy days. Pleochroism reflects the intricate relationship between light and crystal structures, offering insights into both mineral identification and historical navigation methods.
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Subject Terms
Pleochroism
Pleochroism is a property that makes some substances appear to be different colors when viewed from different angles. Also referred to as polychroism, pleochromatism, and polychromatism, this optical illusion occurs most often in crystals that are exposed to polarized light. The structure of a crystal determines whether it will display pleochroism. As a result, gemologists and mineralogists often use pleochroism as a tool to help identify types of gemstones or minerals. Researchers believe that ancient Vikings used a type of crystal with pleochroism as a navigational tool on cloudy days.
Background
“Pleochroism” comes from the Greek words pléōn, meaning “more,” and khrôma, meaning “color.” It most commonly occurs in crystals. The atoms within crystals align in a very definite, symmetrical, three-dimensional pattern that repeats. These patterns are geometric and fall into one of seven systems of classification that define the crystal’s structure. These systems are isometric, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and trigonal. Each classification is based on the geometric shape of the crystal’s cells. For example, an isometric crystal is cube-shaped, with all sides equal and forming right angles where they meet.
The type of atoms that comprise a crystal determines its structure. The atom type also determines how quickly light waves pass through a crystal and whether these waves change direction in a process called refraction. This process occurs in the same way for all crystals of the same type, which scientists refer to as a crystal’s refractive index and is one way that experts categorize crystals.
Most crystals form through natural processes. Gems and minerals form in the earth because of heat, pressure, and other geological processes. Other crystals can be manufactured or may form through organic processes. Kidney stones form in this way. However, crystals of an organic nature do not exhibit pleochroism.
Overview
Pleochroism occurs when a substance, most often a crystal, has a structure that absorbs light of various wavelengths differently. Pleochroism most commonly occurs when the light waves are polarized. Light waves are forms of electromagnetic energy that move through space and air. Some, like the light waves generated by the sun, vibrate in an S-shaped pattern. As the light vibrates away from the source, the S-shaped waves travel at angles that can be vertical, horizonal, or any angle in between. These are called transverse waves.
Crystals display pleochroism best when they are exposed to polarized light, which has light waves that move only in one direction. Such light waves are called longitudinal waves. Light waves become polarized when they are exposed to something that restricts the directions in which the waves can move. This can be something natural, like the surface of a pond or lake, or something humanmade, such as a polarizing filter. This type of filter only allows light waves traveling in one direction to pass through, which reduces the intensity of the light. For example, water on a road will partially polarize the reflected light into horizontal waves, so polarized sunglass lenses are designed to filter vertically. This reduces the shine and glare from the wet road.
However, not all crystal structures display pleochroism and appear differently when exposed to transverse light waves versus polarized light. Isometric crystals do not display pleochroism. Hexagonal, tetragonal, and trigonal crystal structures display two colors and are known as dichroic crystals. Crystals with monoclinic, orthorhombic, and triclinic structures show three colors and are called trichroic crystals.
A crystal’s optical axis may determine whether it will demonstrate pleochroism. An optical axis is a virtual line drawn through a lens, a crystal, or another object that corresponds to the path that light takes through that object. In crystals, the optical axis is a series of imaginary parallel lines defining the direction of the light as it passes through all parts of the crystal without experiencing birefringence. Also known as double refraction, birefringence occurs when the light encounters something that causes it to split and begin moving into two slightly different directions.
Crystals capable of double refraction and changing the direction of light are called anisotropic crystals. These crystals allow light to travel through them in different directions and at different speeds. They display pleochroism and give the appearance of being more than one color when they are viewed from different angles, depending on the speed and wavelength of the light that is visible at that angle. Some crystals are especially known for pleochroism, such as tourmaline, corundum, and alexandrite.
Pleochroism is also a way for experts to identify crystals by type. By examining a thin slice of the mineral with a special microscope under polarized light, experts can determine the number of colors its structure produces. Crystals that are similar in other ways will display a different pleochroism color pattern when examined, which can help identify them. In some cases, crystals can be handled in a way that either enhances their pleochroic colors or diminishes them. This can happen when a crystal is cut or set into jewelry.
Experts believe that the pleochroic colors of one type of crystal may have helped ancient Viking sailors. Referred to as sunstones in manuscripts, these crystals were said to allow navigation even when the sun was hidden behind clouds. Scientists have determined that the crystal cordierite (also known as iolite) changes color from yellow to dark blue when it is at a right angle to the sun’s rays. It is possible that cordierite is the mythical sunstone. The discovery of one of these crystals in the underwater wreckage of a Viking ship seemingly confirmed that ancient sailors used pleochroism to help them sail on the ocean.
Bibliography
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“Iolite—Gem of the Vikings.” International Colored Gemstone Association, www.gemstone.org/iolite. Accessed 18 Aug. 2022.
“Pleochroism.” Gem Lab, www.gemlab.co.in/gemology/pleochroism/. Accessed 18 Aug. 2022.
Rigby, Sara. “What Is Polarised Light?” BBC Science Focus, 26 April 2021, www.sciencefocus.com/science/what-is-polarised-light/. Accessed 18 Aug. 2022.
Smith, Kiona M. “Mysterious Sunstones in Medieval Viking Texts Could Really Have Worked.” Ars Technica, 6 April 2018, arstechnica.com/science/2018/04/mysterious-sunstones-in-medieval-viking-texts-could-really-have-worked/. Accessed 18 Aug. 2022.
“What Is Gemstone Pleochroism?” International Gem Society, www.gemsociety.org/article/what-is-gemstone-pleochroism/. Accessed 18 Aug. 2022.