1. Light in Minerals II

2. Light in Minerals Polarization: Vibration directions
The Petrographic Microscope
Measuring Index of Refraction
Pleochroism
Birefringence: Optical anisotropy
The Indicatrix:
Uniaxial Minerals
Biaxial Minerals

3. Petrographic Microscope

4. Scope

5. Becke Line Method
As you increase the working distance, the white line moves to the medium of higher index.
If dispersion curves cross, liquid has higher slope and yellow moves in and blue moves out.

6. I grain > I oil

7. Igrain > Ioil

8. I grain < I oil

9. I grain = I oil

10. Dispersion The index of refraction (velocity) varies with wavelength.
Blue is bent more than red.

11. Optical Anisotropy Pleochroism: Birefringence:
Different absorption for light vibrating in different directions.
Mineral grains change color on rotation in plane-polarized light.
Birefringence:
Different velocities (index or refraction) for light vibrating in different directions.
Mineral grains change color on rotation between crossed polarizers

12. Pleochroism
Pleochroism is different absorption spectra for light vibrating in different directions
Pleochroism is observed non-cubic Fe-bearing minerals

13. Pleochroism: Tourmaline in PPL

14. Birefringence

15. Birefringence Birefringence is the color of a grain in crossed polars.
Birefringence is the difference between the operative indices of refraction of a grain times the thickness of the grain.
The color observed is controlled by the orientation of the grain and the thickness of the grain.

16. The Optical Indicatrix
The vibration direction of light determines the operative index of refraction in anisotropic media.
The index of refraction is represented by an ellipsoid called the indicatrix.
The elliptical section perpendicular to the ray (propagation direction) determines the operative indices of refraction.

17. The Optical Indicatrix
For Hexagonal, Tetragonal and Trigonal crystals the indicatrix is an ellipsoid of rotation. There is one circular section, and these crystals are said to be uniaxial.
For Orthorhombic, Monoclinic, and Triclinic crystals the indicatrix is a general ellipsoid. There are two circular sections. These crystals are said to be biaxial.

18. The Uniaxial Indicatrix

19. Orthoscope Normal imaging setup Parallel light from below
Condenser out
Bertrand out

20. Conoscope Convergent light Condenser in High power objective
Analyzer in
Bertrand in or ocular out

21. Conoscope Find a grain near its optic axis
Gives a figure on back of objective lens
Figure gives optic class and optic sign
Optic class = isotropic, uniaxial, or biaxial
Optic sign = positive or negative

22. Uniaxial Optic Axis Figure

23. Uniaxial Sign

24. The Biaxial Indicatrix

25. Biaxial Figures

26. Estimate 2V from Optic Axis Figure

27. Thin Section of Gabbro

28. What to do when you look at a rock in thin section
Low Power, Plane-Polarized Light
High-Power Plane-Polarized light
Crossed Polarizers, orthoscope
Conoscope

29. Low Power, Plane-Polarized Light
Count the number of minerals present.
For Each Mineral, Note:
Grain size
Opacity
Color/Pleochroism
Relief and Approximate Indices of Refraction (work in from the edge)
Grain Habit or Shape

30. Low Power, Crossed Polarizers
Birefringent or Isotropic ?
Maximum birefringence
Extinction (parallel, inclined, symmetric, or asymmetric)
Twinning

31. High Power - Orthoscope
Check for oriented inclusions.
Look at accessory or other fine-grained phases.
High Power - Conoscope
Check optic class/sign of unknown minerals

32. Reflected Light Use reflected light to examine opaques
Color and contrast
Bireflectance
Inclusion textures

33. Thin Section of Gabbro