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Wikipedia

Pleochroism

January 01, 1970

Pleochroism is an optical phenomenon in which a substance has different colors when observed at different angles, especially with polarized light.[1]

Pleochroism of cordierite shown by rotating a polarizing filter on the lens of the camera
Pleochroism of tourmaline shown by rotating a polarizing filter on the lens of the camera

Etymology edit

The roots of the word are from Greek (from Ancient Greek πλέων (pléōn) 'more', and and χρῶμα (khrôma) 'color'). It was first made compound in the German term Pleochroismus by mineralogist Wilhelm Haidinger in 1854, in the journal Annalen der Physik und Chemie.[2] Its first known English usage is by geologist James Dana in 1854.[3]

Background edit

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 double refraction of light where light of different polarizations is bent different amounts by the crystal, and therefore follows different paths through the crystal. The components of a divided light beam follow different paths within the mineral and travel at different speeds. When the mineral is observed at some angle, light following some combination of paths and polarizations will be present, each of which will have had light of different colors absorbed. At another angle, the light passing through the crystal will be composed of another combination of light paths and polarizations, each with their own color. The light passing through the mineral will therefore have different colors when it is viewed from different angles, making the stone seem to be of different colors.

Tetragonal, trigonal, and hexagonal minerals can only show two colors and are called dichroic. Orthorhombic, monoclinic, and triclinic crystals can show three and are trichroic. For example, hypersthene, which has two optical axes, can have a red, yellow, or blue appearance when oriented in three different ways in three-dimensional space.[5] Isometric minerals cannot exhibit pleochroism.[1][6] Tourmaline is notable for exhibiting strong pleochroism. Gems are sometimes cut and set either to display pleochroism or to hide it, depending on the colors and their attractiveness.

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 edit

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 edit

Purple and violet edit

Blue edit

  • 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
  • Cordierite (aka 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 edit

  • 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 edit

  • 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 edit

  • 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 edit

See also edit

References edit

  1. ^ a b "Pleochroism in minerals". Webmineral.
  2. ^ Oxford English Dictionary, 2006.
  3. ^ Merriam Webster
  4. ^ Bloss, F. Donald (1961). An Introduction to the Methods of Optical Crystallography. New York: Holt, Rinehart and Winston. pp. 147–149.
  5. ^ Bloss, F. Donald (1961). An Introduction to the Methods of Optical Crystallography. New York: Holt, Rinehart and Winston. pp. 212–213.
  6. ^ "The Pleochroic Minerals". galleries.com.
  7. ^ Rogers, Austin F.; Kerr, Paul F. (1942). Optical Mineralogy (2 ed.). McGraw Hill Book Company. pp. 113–114.
  8. ^ What is gemstone pleochroism? International Gem Society, retrieved 28-Feb-2015

January 01, 1970
pleochroism, optical, phenomenon, which, substance, different, colors, when, observed, different, angles, especially, with, polarized, light, cordierite, shown, rotating, polarizing, filter, lens, camera, tourmaline, shown, rotating, polarizing, filter, lens, . Pleochroism is an optical phenomenon in which a substance has different colors when observed at different angles especially with polarized light 1 Pleochroism of cordierite shown by rotating a polarizing filter on the lens of the camera Pleochroism of tourmaline shown by rotating a polarizing filter on the lens of the camera Contents 1 Etymology 2 Background 3 In mineralogy and gemology 4 List of pleochroic minerals 4 1 Purple and violet 4 2 Blue 4 3 Green 4 4 Yellow 4 5 Brown and orange 4 6 Red and pink 5 See also 6 ReferencesEtymology editThe roots of the word are from Greek from Ancient Greek plewn pleōn more and and xrῶma khroma color It was first made compound in the German term Pleochroismus by mineralogist Wilhelm Haidinger in 1854 in the journal Annalen der Physik und Chemie 2 Its first known English usage is by geologist James Dana in 1854 3 Background editAnisotropic 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 double refraction of light where light of different polarizations is bent different amounts by the crystal and therefore follows different paths through the crystal The components of a divided light beam follow different paths within the mineral and travel at different speeds When the mineral is observed at some angle light following some combination of paths and polarizations will be present each of which will have had light of different colors absorbed At another angle the light passing through the crystal will be composed of another combination of light paths and polarizations each with their own color The light passing through the mineral will therefore have different colors when it is viewed from different angles making the stone seem to be of different colors Tetragonal trigonal and hexagonal minerals can only show two colors and are called dichroic Orthorhombic monoclinic and triclinic crystals can show three and are trichroic For example hypersthene which has two optical axes can have a red yellow or blue appearance when oriented in three different ways in three dimensional space 5 Isometric minerals cannot exhibit pleochroism 1 6 Tourmaline is notable for exhibiting strong pleochroism Gems are sometimes cut and set either to display pleochroism or to hide it depending on the colors and their attractiveness 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 lt Y lt Z with the left most having the least absorption and the rightmost the most 7 In mineralogy and gemology editPleochroism 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 editPurple and violet edit 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 Spodumene Kunzite strong purple clear pink Tourmaline strong pale purple purple Putnisite pale purple bluish grey Blue edit 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 Cordierite aka 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 edit 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 edit 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 edit 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 edit 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 brownSee also editBirefringence Medieval sunstoneReferences edit 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 Retrieved from https en wikipedia org w index php title Pleochroism amp oldid 1224869022, wikipedia, wiki, book, books, library,

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