Anisotropic minerals/interference

1. Anisotropic minerals/interference

2. Isotropic minerals: velocity of light same in all directions. 1 index of refraction • Isometric crystal system • Anisotropic minerals: velocity of light varies. • Light split into two rays; vibrate perp. to each other • 2 velocities, 2 indices of refraction

3. west (left) east (right) Only the component of light vibrating in E-W direction can pass through lower polarizer – light intensity decreases 1) Light passes through the lower polarizer Unpolarized light Plane polarized light PPL=plane polarized light

4. north (back) south (front) Black!! 2) Insert the upper polarizer west (left) east (right) Now what happens? What reaches your eye? Why would anyone design a microscope that prevents light from reaching your eye??? XPL=crossed nicols (crossed polars)

5. Light and colors reach eye! 3) Now insert a thin section of a rock west (left) Unpolarized light east (right) Light vibrating E-W Light vibrating in many planes and with many wavelengths How does this work??

6. Conclusion has to be that minerals somehow reorient the planes in which light is vibrating; some light passes through the upper polarizer Minerals act as magicians!! But, note that some minerals are better magicians than others (i.e., some grains stay dark and thus can’t be reorienting light)

7. 4) Note the rotating stage Most mineral grains change color as the stage is rotated; these grains go black 4 times in 360° rotation-exactly every 90o These minerals are anisotropic Glass and a few minerals stay black in all orientations These minerals are isotropic

8. All anisotropic minerals can resolve light into two plane polarized components that travel at different velocities and vibrate in planes that are perpendicular to one another Some light is now able to pass through the upper polarizer fast ray slow ray mineral grain • When light gets split: • velocity changes • rays get bent (refracted) • 2 new vibration directions • usually see new colors plane polarized light W E lower polarizer

9. Calcite double refraction • Two rays • Two velocities • Each anisotropic mineral has one orientation so that it behaves as if isotropic • Optic axis

10. Birefringence • Difference between two indices of refraction •  = Nslow-nfas • Fixed number • Quartz  = 0.009

11. Retardation • Amount that slow ray lags behind fast ray • Depends on thickness of mineral •  = d(ns-nf)

12. Interference colors • 1 wavelength • Slow and fast rays exit mineral, resolved into vibration direction at upper polar • If in phase, vector perpendicular to polarizer, so cancel

13. Interference colors • 1 wavelength • Rays constructively interfere, light passed through upper polar

14. Scopes: polychromatic light • Some colors destructively interfere, some color constructively interfere • What we see if color that passes upper polar

15. Mineral properties: interference colors/birefringence • Colors one observes when polars are crossed (XPL) • Color can be quantified numerically: d = nhigh - nlow Now do question 4 More on this next week…

16. Estimating birefringence 1) Find the crystal of interest showing the highest colors (D depends on orientation) 2) Go to color chart thickness = 30 microns use 30 micron line + color, follow radial line through intersection to margin & read birefringence

17. w 1.544 1.553 e Example: Quartz w = 1.544 e = 1.553 Data from Deer et al Rock Forming Minerals John Wiley & Sons

18. Example: Quartz w = 1.544 e = 1.553 Sign?? (+) because e > w e - w = 0.009 called the birefringence (d) = maximum interference color (when seen?) What color is this?? Use your chart.

19. Color chart Colors one observes when polars are crossed (XPL) Color can be quantified numerically: d = nhigh - nlow

20. Example: Quartz w = 1.544 e = 1.553 Sign?? (+) because e > w e - w = 0.009 called the birefringence (d) = maximum interference color (when see this?) What color is this?? Use your chart. For other orientations get e' - w ® progressively lower color Rotation of the stage changes the intensity, but not the hue Extinct when either privileged direction N-S (every 90o) and maximum interference color brightness at 45o 360o rotation ® 4 extinction positions exactly 90o apart

21. What interference color is this? If this were the maximum interference color seen, what is the birefringence of the mineral?

22. Extinction • Anisotropic minerals go dark (extinct) every 90° in cross polars

23. parallel extinction inclined extinction Extinction angle Extinction behavior is a function of the relationship between indicatrix orientation and crystallographic orientation

24. XN Extinction angle – parallel extinction • All uniaxial minerals show parallel extinction • Orthorhombic minerals show parallel extinction (this is because xtl axes and indicatrix axes coincide) orthopyroxene PPL

25. extinction angle Extinction angle - inclined extinction • Monoclinic and triclinic minerals: • indicatrix axes do not coincide with crystallographic axes • These minerals have inclined extinction • (and extinction angle helps to identify them) clinopyroxene

26. Parallel extinction • Inclined extinction • Symmetrical extinction (if two cleavage directions) • Undulatory extinction