Geology 2142

1. Geology 2142 Extinction and Extinction Angles and the Sign of Elongation

2. Birefringence and Extinction If an optic axis of an anisotropic mineral is vertical (ie parallel to the propagation direction of the light in your �scope) C is equal in all direction The mineral is dark If the optic axis is NOT vertical All anisotropic minerals go dark every 90� of stage rotation They go into extinction

3. Extinction Extinction happens when the vibration directions of the light passing through the mineral are parallel to the vibration directions of both the upper and lower polarizer No vector component of the incident plane polarised light can be resolved into the mineral vibration direction that is parallel to the upper polariser

4. Extinction - 2 This means that all the E-W vibrating light from the lower polariser passes through the mineral and exits with the same E-W vibration direction Light with an E-W vibration direction is completely absorbed by the N-S oriented analyser and the mineral goes dark

5. Extinction Angles We can measure the extinction angle ie. The angle between a cleavage (or direction of elongation) and extinction A useful tool in identification of some minerals Micas always have parallel extinction Clinopyroxenes have inclined extinction Orthopyroxene has straight extinction in prismatic sections

6. Cleavage Cleavage appears as straight lines or cracks in thin section When describing cleavage: Number of cleavage planes Orientation to outer shape of crystal (if any) Angle between planes if more than one Pervasiveness (ie is it found inall the crystals?

8. Cleavage

9. Types of Extinction Parallel Mineral dark parallel to cleavage Biotite, muscovite Inclined Mineral extinct at some angle to cleavage Clinoamphibole, clinopyroxene Symmetrical Go extinct at angles symmetrical with cleavage Calcite

10. Categories of Extinction

11. Muscovite (ppl) This section consists of tabular muscovite grains in a matrix of anhedral quartz The photo makes it appear as if muscovite has lower relief than quartz � this is an optical illusion. Muscovite shows its perfect basal cleavage in this image Also it is non-pleochroic

12. Muscovite (xpl - extinct) In this section you can see the high order interference colours of muscovite Most grains show second order colours though the small grain on the far right shows typical third order The muscovite grain in the center is showing the typical parallel extinction

13. Muscovite (xpl, max birefringence) The grain in the center is now showing its maximum interference colour � second order red on one side of the twin plane and second order yellow on the other

14. Hornblende (E-W, ppl) Typical long sections of green hornblende in ppl The one in the middle shows one extreme of pleochroism None of the sections show the typical cleavage intersection but all show one cleavage

15. Hornblende (N-S, ppl) Notice the distinct change in colour now that the stage has been rotated through 90 degrees

16. Hornblende (xpl) showing absorbtion Hornblende is a clinoamphibole and so has inclined extinction Most of the grains show second order colours that are to some extent affected by the body colour of the grains

17. Pleochroism in biotite (ppl) Biotite oriented E-W showing a deep brown colour The perfect basal cleavage is easily seen in this section

18. Pleochroism in biotite (ppl) Same grain after rotation. Note that is is now a straw yellow / brown colour You can see the perfect cleavage planes quite easily in this picture

19. Birefrigence in biotite Biotite showing third order interference When biotite is near extinction you will see a characteristic mottled appearance (similar to that of pyrophyllite and muscovite) Note that like the other micas biotite shows straight extinction

20. Straight extinction in biotite

21. Orthopyroxene + olivine (xpl) Orthopyroxene showing characteristic straight extinction. Magnesian olivine shows upper second order interference colours

22. Orthopyroxene + olivine (xpl) Orthopyroxene showing first order yellow interference colours

23. Measuring Extinction Angles Rotate stage so that length or cleavage is N-S Record angle from the stage goniometer Rotate stage until mineral is extinct Means that either the fast or slow ray is parallel to N-S If already extinct then no more rotation needed Rotate stage clockwise to extinction and record angle The extinction angle is the difference between the two If measured with anticlockwise rotation the extinction angle is 90�-measured value Smaller value is the one reported

24. Measuring Extinction Angles To be diagnostic Must specify the grain orientation In practise we measure on the grain that shows maximum retardation

25. Sign of Elongation Some minerals have a prismatic (elongate) habit Due to crystal structure or cleavage Polarised light passing through elongate fragments of anisotropic grains with polarization parallel to the long dimension will not travel at the same speed as light polarised in other directions

26. Sign of Elongation Prismatic minerals divided into Length fast Fast wave vibrates parallel to elongation Length slow Slow wave vibrates parallel to long direction CANNOT be used on minerals that form equidimensional fragments

27. Problems Easy for tetragonal and hexagonal minerals Only one optic axis Problematic or impossible for orthorhombic, monoclinic and triclinic minerals

28. Determining the Vibration Directions of the Light Rays When a mineral is in extinction the vibration directions must be N-S and E-W. Use an accessory plate to figure out which ray is fast and which is slow. Accessory plates are made of gypsum, mica or quartz.

29. Accessory Plates Gypsum, mica and quartz are anisotropic Split light into a slow and fast ray Plates produce a known retardation Mounted so the slow ray vibrates across the plate Fast ray vibrates parallel to the length of the plate

30. Accessory Plates -2 Mounted in a slot aligned NW-SE Slow ray vibrates NE-SW Fast ray vibrates NW-SE

31. Gypsum Plate AKA: Fullwave, 1-wavelength or quartz senstive tint or first order red. Can be marked as gips, gyps, rot I 1l, D=550 nm or D= 537 nm. Gives either 500 or 537 nm of retardation. Gives a very distinctive colour in XPL. Seen at the first to second order transition on the chart.

32. Mica Plate AKA 1/4 wavelength plate or quarter wave plate Marked as mica, glimmer, 1/2l or D=147 nm Gives a first order white interference colour Produces 147 nm of retardation

33. Slow Mineral Parallel to Slow Plate Light from lower polariser enters Split into fast and slow Slow ray is retarded relative to the fast one Insert accessory plate Slow direction parallels slow direction of mineral

34. Slow Mineral Parallel to Slow Plate Slow wave retarded by an additional amount The amount equals the retardation of the plate (Da) Total retardation is that of the mineral PLUS that of the plate In this case the interference colour INCREASES

35. Example Quartz: birefringence = 0.009 nm Retardation = 270 nm First order white interference colour Gypsum plate retardation = 550 nm When the fast direction of the plate and fast direction of the mineral coincide the total retardation is 270 + 550 = 820 nm Second order yellow interference colour

36. Slow Mineral on Fast Plate Plane light enters the mineral splits into a fast and slow ray Slow ray retarded by dm The slow and fast ray enter the plate The fast ray from the mineral is retarded by an amount Da Slow wave from the mineral is not retarded

37. Slow Mineral on Fast Plate This allows the slow wave to catch up so the total retardation decreases DT=dm-Da Example Quartz D=270 Gypsum D = 500 So DT=280 Gives first order white with a very weak yellow tint

38. Determination of Sign of Elongation Put mineral into extinction with the length or cleavage trace less than 45� from the N-S cross hair Rotate stage 45� clockwise This places the vibration direction closest to length or cleavage in the NE-SW direction Insert accessory plate If retardations ADD the ray with vibration closest to the length is the slow ray - LENGTH SLOW Retardations subtract - ray with vibration closest to length is fast - LENGTH FAST