Optical mineralogy
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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


  • 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


  • 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


  • 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