Pleochroism
Pleochroism is an optical phenomenon in which a substance has different colors when observed at different angles, especially with polarized light.[1]
Etymology
The roots of the word are from Greek (from
Background
Anisotropic crystals will have optical properties that vary with the direction of light. The direction of the electric field determines the polarization of light, and crystals will respond in different ways if this angle is changed. These kinds of crystals have one or two optical axes. If absorption of light varies with the angle relative to the optical axis in a crystal then pleochroism results.[4]
Anisotropic crystals have
The pleochroic colors are at their maximum when light is polarized parallel with a principal optical vector. The axes are designated X, Y, and Z for direction, and alpha, beta, and gamma in magnitude of the refractive index. These axes can be determined from the appearance of a crystal in a conoscopic interference pattern. Where there are two optical axes, the acute bisectrix of the axes gives Z for positive minerals and X for negative minerals and the obtuse bisectrix gives the alternative axis (X or Z). Perpendicular to these is the Y axis. The color is measured with the polarization parallel to each direction. An absorption formula records the amount of absorption parallel to each axis in the form of X < Y < Z with the left most having the least absorption and the rightmost the most.[7]
In mineralogy and gemology
Pleochroism is an extremely useful tool in mineralogy and gemology for mineral and gem identification, since the number of colors visible from different angles can identify the possible crystalline structure of a gemstone or mineral and therefore help to classify it. Minerals that are otherwise very similar often have very different pleochroic color schemes. In such cases, a thin section of the mineral is used and examined under polarized transmitted light with a petrographic microscope. Another device using this property to identify minerals is the dichroscope.[8]
List of pleochroic minerals
Purple and violet
- Amethyst (very low): different shades of purple
- Andalusite (strong): green-brown / dark red / purple
- Beryl (medium): purple / colorless
- Corundum (high): purple / orange
- Hypersthene (strong): purple / orange
- Kunzite) (strong): purple / clear / pink
- Tourmaline (strong): pale purple / purple
- Putnisite: pale purple / bluish grey
Blue
- Aquamarine(medium): clear / light blue, or light blue / dark blue
- Alexandrite(strong): dark red-purple / orange / green
- Apatite (strong): blue-yellow / blue-colorless
- Benitoite (strong): colorless / dark blue
- Iolite) (orthorhombic; very strong): pale yellow / violet / pale blue
- Corundum (strong): dark violet-blue / light blue-green
- Tanzanite See Zoisite
- Topaz (very low): colorless / pale blue / pink
- Tourmaline (strong): dark blue / light blue
- Zoisite (strong): blue / red-purple / yellow-green
- Zircon (strong): blue / clear / gray
Green
- Alexandrite(strong): dark red / orange / green
- Andalusite (strong): brown-green / dark red
- Corundum (strong): green / yellow-green
- Emerald (strong): green / blue-green
- Peridot (low): yellow-green / green / colorless
- Titanite (medium): brown-green / blue-green
- Tourmaline (strong): blue-green / brown-green / yellow-green
- Zircon (low): greenish brown / green
- Kornerupine (strong): green / pale yellowish-brown / reddish-brown
- Hiddenite (strong): blue-green / emerald-green / yellow-green
Yellow
- Citrine(very weak): different shades of pale yellow
- Chrysoberyl (very weak): red-yellow / yellow-green / green
- Corundum (weak): yellow / pale yellow
- Danburite (weak): very pale yellow / pale yellow
- Kasolite (weak): pale yellow / grey
- Orthoclase (weak): different shades of pale yellow
- Phenacite(medium): colorless / yellow-orange
- Spodumene (medium): different shades of pale yellow
- Topaz (medium): tan / yellow / yellow-orange
- Tourmaline (medium): pale yellow / dark yellow
- Zircon (weak): tan / yellow
- Hornblende (strong): light green / dark green / yellow / brown
- Segnitite (weak): pale to medium yellow
Brown and orange
- Corundum (strong): yellow-brown / orange
- Topaz (medium): brown-yellow / dull brown-yellow
- Tourmaline (very low): dark brown / light brown
- Zircon (very weak): brown-red / brown-yellow
- Biotite (medium): brown
Red and pink
- Alexandrite(strong): dark red / orange / green
- Andalusite (strong): dark red / brown-red
- Corundum (strong): violet-red / orange-red
- Morganite(medium): light red / red-violet
- Tourmaline (strong): dark red / light red
- Zircon (medium): purple / red-brown
See also
- Birefringence
- Medieval sunstone
References
- ^ a b "Pleochroism in minerals". Webmineral.
- ^ Oxford English Dictionary, 2006.
- ^ Merriam Webster
- ^ Bloss, F. Donald (1961). An Introduction to the Methods of Optical Crystallography. New York: Holt, Rinehart and Winston. pp. 147–149.
- ^ Bloss, F. Donald (1961). An Introduction to the Methods of Optical Crystallography. New York: Holt, Rinehart and Winston. pp. 212–213.
- ^ "The Pleochroic Minerals". galleries.com.
- ^ Rogers, Austin F.; Kerr, Paul F. (1942). Optical Mineralogy (2 ed.). McGraw Hill Book Company. pp. 113–114.
- ^ What is gemstone pleochroism? International Gem Society, retrieved 28-Feb-2015
- “Pleochroism.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/pleochroism. Accessed 1 Jan. 2024.
- “Pleochroism, N.” Oxford English Dictionary, Oxford UP, July 2023, https://doi.org/10.1093/OED/5173776922. Accessed 1 Jan 2024.