1. Pleochroism, Interference Colors,
Optical Mineralogy
Lab 12 - Fall, 2012
Pleochroism, Interference Colors,
and Extinction Angles

2. Pleochroism
The ability of a mineral to show different colors when viewed along differing crystallographic orientations

3. Pleochroism Video Double click to play movie
Watch the grain in the very center
Video: Pleochroism.wmv

4. 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
Source:

5. 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
Source:

6. 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
Source:

7. 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
Source:

8. 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
Source:

9. 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
Source:

10. Michel-Lévy Color Chart
Source:
First order colors are to the left, with orders increasing to the right

11. No Birefringence Garnet is isometric Birefringence is zero
Therefore the interference color is black
This is also known as being “in extinction”

12. Weak Birefringence Quartz has very low birefringence
Colors are very muted
1° grays and white

13. 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

14. High Birefringence Muscovite shows high interference colors
Highest colors (4° and beyond) are pastels

15. 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

16. 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

17. 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
Source:
Video: extprism.wmv

18. Example Movie 2 Double click to play movie
Source:
File was converted from .mov format to .wmv format so it would play in PowerPoint

19. 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
Source:
Video: extrecta.wmv
Double click to play the movie