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Optical Mineralogy

Lab 12 - Fall, 2012

Pleochroism, Interference Colors,

and Extinction Angles

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Pleochroism

• The ability of a mineral to show different colors when viewed along differing crystallographic orientations

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Pleochroism Video

• Double click to play movie

• Watch the grain in the very center

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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

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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

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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

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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

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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

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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

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Michel-Lévy Color Chart

• First order colors are to the left, with orders increasing to the right

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No Birefringence

• Garnet is isometric• Birefringence is zero• Therefore the

interference color is black

• This is also known as being “in extinction”

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Weak Birefringence

• Quartz has very low birefringence• Colors are very muted• 1° grays and white

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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

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High Birefringence

• Muscovite shows high interference colors• Highest colors (4° and beyond) are pastels

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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

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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

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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

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Example Movie 2

• Double click to play movie

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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