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Understanding Petrographic Microscope Techniques and Concepts

Explore the fundamentals of petrographic microscope usage, including polarization, pleochroism, and birefringence. Learn about the Indicatrix, optical anisotropy, and more. Enhance your skills in identifying minerals in thin sections.

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Understanding Petrographic Microscope Techniques and Concepts

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  1. LightinMinerals 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: • 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. • Check optic class/sign of unknown minerals High Power - Conoscope

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

  33. Thin Section of Gabbro

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