1. Lecture 15 (11/15/2006) Analytical Mineralogy Part 2: Optical Properties of Uniaxial Minerals

2. Anisotropy Indicies of refraction can vary in all minerals (except those in the isometric system) depending on the orientation of light ray. Such minerals are said to be anisotropic. Isometric minerals, glass, liquids and gasses have a single refraction index value regardless of the orientation of light rays. Such substances are said to be isotropic.

3. Optical Indicatrices A 3-d map of the indices of refraction for various vibration directions of light rays Orientation of the indicatrix within a mineral is symmetrical with the crystallographic axis IsotropicIsometric Anisotropic – Uniaxial TetragonalHexagonal Anisotropic-BiaxialOrthorhombicMonoclinicTriclinic

4. Isotropic Indicatrix n A sphere whose radius corresponds to the characteristic refraction index- n n Diagram shows change in n for different wavelengths of light in same mineral 4861ÅBlue5893ÅYellow 6563Å Red n n=c/v =c/

5. Optical Recognition of Isotropic Minerals From Bloss (1961) Total Extinction under X-polars Slowing of ray = shortening of wavelength, but no change in polarity

6. Anistropic Minerals All randomly oriented anisotropic minerals cause double refraction (splitting) of light resulting in mutually perpendicular-polarized light rays. n One ray has a higher n (slow ray, or the ordinary ray) than the other ray (the fast ray, or extraordinary ray) Fast ray Slow ray

7. Birefringence (  ), Retardation(Δ), and Interference Colors nn Δ = d*  = n slow ray – n fast ray Δ = d* 

8. Uniaxial Indicatrix Optic Axis = C axis in tetragonal and hexagonal crystals

9. Sections of Uniaxial Indicatrices  = ω-ω = 0 (circular section)  = ε’- ω (random section) = ε - ω (principal section) maximum birefringence Total extinction in x-polar light

10. Double Refraction through Random Sections of Uniaxial Minerals Single Refraction through Circular and Principal Sections of Uniaxial Minerals

11. Re-Polarization of Light through a Non-circular Section of the Uniaxial Indicatrix

12. Extinction of Uniaxial Minerals

13. Conoscopic Interference Figures of Uniaxial Minerals Orthoscopic Conoscopic Isochromes – zones of equal retardation equal retardation Isogyres – represent the areas where the ω and ε’ vibration directions are oriented N-S, E-W

14. Uniaxial Optic Axis (OA) Figure Circular section parallel to stage  = 0

15. Off-centered OA Figure Random section parallel to stage,  < 0, « max 

16. Very Off-centered OA Figure Random section parallel to stage,  « 0, < max 

17. Flash Figure Principal section parallel to stage,  = max 

18. Determining the Optic Sign of Uniaxial Minerals Connect the quadrants that go down in color (to yellow), compare with slow direction of gypsum plate for sign +

19. Next Lecture 11/20/06 Optical Properties of Biaxial Minerals Read: Klein p. 303-309 Perkins and Henke, p. 22-27