1. Lab 13 – Fall, 2012 Uniaxial Interference Figures
Optical Mineralogy
Lab 13 – Fall, 2012
Uniaxial Interference Figures
Source:

2. Conoscopic Observation
In order to observe an interference figure the microscope must be used in the conoscopic mode
Conoscopic refers to the cone-shaped illumination obtained when the condenser lens is near the thin section
This requires that the following conditions be met

3. Conoscopic Technique
A. Analyzer inserted and crossed with respect to polarizer (CN)
B. Objective lens with a numerical aperture (N.A.)  0.65 must be used
C. The condensing lens must be moved (or swing-out lens inserted) to focus the light on a small area
D. The Bertrand lens must be inserted

4. Choosing a Grain
Choose a grain that stays in extinction or has very low colors
You are trying to locate a grain with its optic axis perpendicular to the slide
You want to be looking along the optic axis, or as close as you can possibly get – this produces a “centered” optic axis figure
How close that is depends on the birefringence of the mineral

5. Choosing a Grain, II
For quartz, the grain must be almost black at all times, for olivine, first-order gray will do
For calcite, any recognizable interference color will probably work
Try to be at least in the lower 10% of the mineral's color range
Sometimes you just can't do it with a given thin section, especially if the mineral you're dealing with has only tiny grains or very few of them

6. Conoscopic vs. Orthoscopic Observation
Diagram compares the two types of viewing
Source:

7. Conoscopic Procedure
Select a grain whose interference you wish to check
Make sure the cover slip is facing up
Move the grain to the center of the stage
Be sure you are in CN (are the polars crossed?)
Focus at low power
Make sure you are not focused on a crack or impurity in the grain

8. Conoscopic Procedure, II
Increase to medium power, double check focus
Move up to high power and double check focus
Be sure to raise or flip in the auxiliary condenser lens

9. Bertrand Lens Insert the Bertrand lens
If your scope does not have a Bertrand lens, remove the eyepiece and look down the microscope tube
An interference figure should appear – rotate the stage to see if there is any change

10. No Interference Figure?
Check that the microscope is in the correct configuration
Check that the grain on high power is not focused on a crack or impurity
Also check that the high power objective is properly centered

11. Uniaxial Minerals
The optical class uniaxial has minerals from two mineral systems:
Tetragonal – A4
Hexagonal
Rhombohedral division A3
Hexagonal division A6
Each system has a unique high order axis, as shown – this is the optic axis

12. Quadrant Labels
The quadrants are labeled starting in the upper right and going counterclockwise
Roman numerals are used to designate quadrants

13. Optic Axis
The optic axis is designated as the crystallographic “Z” axis
When a thin section of a mineral is cut perpendicular to the optic axis, and then viewed perpendicular to the thin section, light is traveling along the optic axis
Light traveling in this direction experiences a single index of refraction, ω (omega)

14. Optic Axis Figures The isogyre has 1º color; the area between the
Figure Source:
The isogyre has 1º color; the area between the
isogyre arms is 1º white, unless isochromes are present

15. Low vs. High Birefringence
Source:
Quartz, low birefringence
Calcite, high birefringence

16. Origin of Isogyres
Figure 21, page 28, W. W. Moorhouse, The Study of Rocks in Thin Section – S marks the slow ray (for the + case)
In conoscopic view, ω always vibrates ║ to the z axis and tangential to the isochromes, whereas ε ́ always vibrates  to the isochromes
Whenever one of these vibration directions is parallel to the polarizer (i.e., E-W), extinction occurs

17. Origin of Isogyres, II
The two bands of extinction form a centered cross for an optic axis section
The point where the isogyres meet is called the melatope and represents the optic axis itself
Melatope comes from Greek words meaning “dark” and “place”

18. Origin of Isochromes
Light which travels along the optic axis is not split into two rays, nepsilon' = nomega, and exits the mineral to form the melatope
No retardation "between" rays
Source:

19. Origin of Isochromes, II
Light following paths 2 & 4 experience moderate retardation nepsilon' < nomega ~ 550 nm
Light following paths 3 & 5 experience moderate retardation nepsilon' << nomega ~ 1100 nm because light makes a larger angle with optic axis and must take a longer path through the sample
Source:

20. Photomicrograph of High-Birefringence Mineral
The colored rings are isochromes
Calcite – highly birefringent
Picture:

21. Accessory Plates
Accessory plates are plates of anisotropic minerals ground to a thickness that gives a particular retardation of light
When inserted into the light path, they change the retardation of light coming through the thin section by a specific amount and the resultant interference color helps to identify the mineral

22. 1o Red Accessory Plate
This is the compensator you will encounter most frequently
The lab microscopes are equipped with one, and we will use it extensively
The full wave plate is also called a gypsum plate, 1l plate, 550 nm plate, or 1o red plate (1o rot, in German) because it is usually made of gypsum and produces a 550 nm or 1o red retardation
Source:

23. Quarter Wave Plate
This plate is found on your microscopes in lab, but we do not use it extensively
As the name implies it produces a retardation of ¼l
It is also called a mica plate, 150 nm plate, and 1o gray plate,  because it is usually made of muscovite (glimmer in German) and produces a retardation of 150 nm, or 1o gray
Source:

24. Quartz Wedge This is a crystal of quartz cut into a wedge shaped
Since its thickness varies along the wedge, it can produce a range of retardations that correspond to interference colors from 0 (1o black) up to about 3800nm (5o green)  - this varies from wedge to wedge 
The wedge, like all compensators usually has its slow direction clearly marked, and is inserted into the microscope tube such that slow direction in the compensator is at a 45o angle to the polarizing direction
Source:

25. Uniaxial Positive Sign
In a uniaxial mineral, the two principle indices of refraction are denoted ε (epsilon) and ω (omega)
If ε > ω, the mineral is uniaxial positive
Figure source:

26. Uniaxial Negative Sign
Figure source:
If ε < ω, the mineral is uniaxial negative

27. Determination of the Optical Sign
Accessory plates may be used to determine the optical sign
Minerals with isochromes are usually treated differently than minerals without isochromes

28. Uniaxial Mineral, No Isochromes
The 1º red (Rot 1) plate is inserted
On most microscopes, this will be from the SE
The slow direction of the accessory plate (N) should be aligned NE-SW
A blue color appears in quadrants I & III, which indicates addition
A yellow color in quadrants II & IV indicates subtraction
This is a uniaxial positive mineral with low birefringence
Figure Source:

29. Uniaxial Positive with 1º Red Plate
Uniaxial positive mineral, with 1º red plate
Note blue in quadrants I & III, yellow in quadrants II & IV
The isogyres show the 1º red color of the accessory plate
Image source:

30. Uniaxial Mineral, No Isochromes
Figure 24b, page 30, W. W. Moorhouse, The Study of Rocks in Thin Section
A mica or quarter λ plate may be used for minerals with low to moderate birefringence
It produces a pair of black dots in quadrants where subtraction occurs

31. Uniaxial Mineral, with Isochromes
The isochromes in quadrants I &III move inward, and those in quadrants II & IV move outward
This is a uniaxial positive mineral with moderate to high birefringence
Figure Source:

32. Multiple Isochromes
If the interference figure displays numerous isochromes, color changes produced with the 1º red plate become difficult to detect
In this case the quartz wedge is used
Inserting the Quartz wedge results in the movement of the isochromes about the isogyres
Source:

33. Use of the Quartz Wedge
In quadrants where the colors subtract, the isochromes move outward as lower order colors form near the melatope and displace higher order colors
In quadrants where the colors add, the isochromes move inwards, towards the melatope
The isogyre, on insertion of the accessory adopts the interference color corresponding to the retardation of the accessory
Source:

34. Uniaxial Mineral, with Isochromes, using Quartz Wedge
Source:
Left, positive; right, negative

35. Uniaxial Mineral, No Isochromes
A blue color appears in quadrants II & IV, which indicates subtraction
A yellow color in quadrants I & III indicates addition
This is a uniaxial negative mineral with low birefringence

36. Uniaxial Mineral, with Isochromes
The isochromes in quadrants I &III move outward, and those in quadrants II & IV move inward
This is a uniaxial negative mineral with moderate to high birefringence

37. Uniaxial Negative with 1º Red Plate
Uniaxial negative mineral, with 1º red plate
Note blue in quadrants II & IV, yellow in quadrants I & III
The isogyres show the 1º red color of the accessory plate
Image source:

38. Summary of Uniaxial Sign Determination
The diagram summarizes the determination of uniaxial signs using a 1o red plate

39. Off-Center Figures
Finding a grain with the optic axis oriented exactly perpendicular to the stage will sometimes be very difficult
It would be much more common to find one wherein the optic axis is at a slight angle to being perpendicular to the microscope stage
Figure source:

40. Off-Center Figure Properties
Such a grain will exhibit the following properties:
It is a grain that shows w refractive index and an e' refractive index that is close the w refractive index
It would also show very low order (1o gray interference colors between extinction positions if the analyzer is inserted in orthoscopic mode 

41. Off-Center Figure Diagram
On rotation of the stage, the melatope would rotate in a circle around the perimeter of the field of view, and the bars of the isogyres would remain oriented E-W and N-S
Figure source:

42. Rotation of an Off-center Figure
Figure 22, page 29, W. W. Moorhouse, The Study of Rocks in Thin Section

43. Off-Center Orientation Diagram
The melatope lies outside the field of view
The vibration direction of the ordinary ray is tangential to the isochromes
The vibration direction of the extraordinary ray is radial from the melatope

44. Photomicrographs of Off-Center Figures
Source:
Thick Quartz – Left 15º off center; Right 30º off center

45. Positive Off-Center Figure
For an optically positive crystal, all NE and SW quadrants will turn blue and the NW and SE quadrants will turn yellow, both colors replacing the 1ogray color present before insertion of the compensator
Figure source:

46. Negative Off-Center Figure
For an optically negative crystal, all NE and SW quadrants will turn yellow and all NW and SE quadrants will turn blue, both colors replacing the 1ogray color present before insertion of the compensator
Figure source:

47. Flash Figure
A mineral grain is oriented with it's optic axis horizontal
This orientation exhibits the maximum birefringence, for this mineral in the thin section, and produces a flash figure
Source:
Possible additional Source:

48. Flash Figure II
The flash figure results because the vibration directions, of the indicatrix, within the field of view are nearly parallel to polarization directions of the microscope
extraordinary rays vibrate parallel to optic axis
ordinary rays vibrate perpendicular to optic axis

49. Flash Figure III
With the grain at extinction, the optic axis is oriented either EW or NS in the resulting interference figure
The interference figure produced occupies most if not all of the field of view and consists of a very broad, fuzzy isogyres cross
Upon rotating the stage, < 5° rotation, the isogyres will split and move out of the field of view in opposite quadrants

50. Flash Figure Diagram
Diagram showing flash figure orientation, and a flash figure image
Source:

51. Flash Figure after Small Rotation
The isogyre splits and quickly leaves the field of view
The optic axis lies along the line connecting the isogyres
Source: