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Optical Mineralogy. Lab 12 - Fall, 2012 Pleochroism, Interference Colors, and Extinction Angles. Pleochroism. The ability of a mineral to show different colors when viewed along differing crystallographic orientations. Pleochroism Video. Double click to play movie

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optical mineralogy

Optical Mineralogy

Lab 12 - Fall, 2012

Pleochroism, Interference Colors,

and Extinction Angles

pleochroism
Pleochroism
  • The ability of a mineral to show different colors when viewed along differing crystallographic orientations
pleochroism video
Pleochroism Video
  • Double click to play movie
  • Watch the grain in the very center
interference colors
Interference Colors
  • Interference colors are the colors that you see when a rock thin section is placed between two pieces of polarizing film
  • This phenomenon is a side effect of  light slowing down as it passes through different substances
  • You observe another effect of light slowing down when you see a straw in a glass of water appear to bend where it passes into the water
index of refraction
Index of Refraction
  • The slowing down of light as it passes through a substance is measured in a number called the index of refraction
  • Interference colors are an effect produced by the fact that most solids have more than one index of refraction
retardation
Retardation
  • As the two beams of polarized light pass through a crystal, they travel at different speeds and get out of phase
  • The slow ray is said to be retarded and the phase difference is called retardation
integral retardation
Integral Retardation
  • If the retardation is a whole number of wavelengths, the beams recombine with the same orientation as when they entered the crystal
  • These wavelengths will be blocked by the upper polarizer
half integral retardation
Half-integral Retardation
  • If the retardation is a whole number of wavelengths plus one-half, the beams recombine with an orientation perpendicular to the original direction of polarization
  • These wavelengths will be fully transmitted by the upper polarizer
birefringence
Birefringence
  • The greater the difference between the indices of refraction, the more intense are the interference colors produced
  • The difference in the index of refraction in two viewing directions is called the birefringence
michel l vy color chart
Michel-Lévy Color Chart
  • First order colors are to the left, with orders increasing to the right
no birefringence
No Birefringence
  • Garnet is isometric
  • Birefringence is zero
  • Therefore the interference color is black
  • This is also known as being “in extinction”
weak birefringence
Weak Birefringence
  • Quartz has very low birefringence
  • Colors are very muted
  • 1° grays and white
low birefringence
Low Birefringence
  • Kyanite shows some color
  • Kyanite grain in center is surrounded by muscovite, biotite, and opaque minerals
  • Lower image is in pp – note that kyanite is colorless
high birefringence
High Birefringence
  • Muscovite shows high interference colors
  • Highest colors (4° and beyond) are pastels
extinction angle
Extinction Angle
  • The angle formed by one line of the crystal with the extinction position
    • Either the longest dimension of the mineral or the system of cleavage lines are generally used as this line of reference
determination of extinction angle
Determination of Extinction Angle
  • Initially, view the crystal in pp
  • The reference line is rotated so it coincides with the direction of the polarizer (E-W)
  • The analyzer is inserted
  • The stage is rotated and turned slowly until extinction occurs
  • The rotation angle is the extinction angle
example movie 1
Example Movie 1
  • Double click to play the movie
  • The angle is actually determined by noting the position of the stage before and after rotation
example movie 2
Example Movie 2
  • Double click to play movie
parallel extinction
Parallel Extinction
  • When the analyzer is inserted, if the crystal is already dark, the extinction angle is 0°
  • The mineral is said to have a straight, or parallel extinction
  • Double click to play the movie
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