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Interference figures Very important tool to determine optical characteristics. They will tell you: Very important tool to determine optical characteristics.

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Presentation on theme: "Interference figures Very important tool to determine optical characteristics. They will tell you: Very important tool to determine optical characteristics."— Presentation transcript:

1 Interference figures Very important tool to determine optical characteristics. They will tell you: Very important tool to determine optical characteristics. They will tell you: Uniaxial vs biaxial Uniaxial vs biaxial Optic sign Optic sign 2V angle – for biaxial 2V angle – for biaxial Used for estimating chemistry of mineral Used for estimating chemistry of mineral

2 Technique Technique Focus with highest power objective Focus with highest power objective Be certain observation is conoscopic light Be certain observation is conoscopic light Flip in conoscope on some microscopes Flip in conoscope on some microscopes Most of them have to raise light sources as high as possible Most of them have to raise light sources as high as possible Insert Betrand lens or remove ocular Insert Betrand lens or remove ocular Interference figure forms on top of objective lens Interference figure forms on top of objective lens Betrand lens required to refocus the image Betrand lens required to refocus the image

3 Slightly more modern version conoscope Bertrand lens

4 Fig Isogyres Melatope Isochromes One type of Uniaxial Interference Figure

5 Figure consist of isogyres and isochromes Figure consist of isogyres and isochromes Isochromes: patterns of interference colors Isochromes: patterns of interference colors Isogyres: dark bands (extinction) Isogyres: dark bands (extinction) Nature of interference figure and patterns as stage rotated determines optical property Nature of interference figure and patterns as stage rotated determines optical property Types of figures controlled by cut of the grain Types of figures controlled by cut of the grain

6 Uniaxial Interference Figure Three types: Three types: Optic axis figure Optic axis figure Off-center optic axis figure Off-center optic axis figure Flash Figure Flash Figure Note – these correspond with the principle cuts of the indicatrix Note – these correspond with the principle cuts of the indicatrix

7 Optic Axis Figure Forms when optic axis perpendicular to stage Forms when optic axis perpendicular to stage Grain exhibits low interference color (extinct) Grain exhibits low interference color (extinct)

8 Figure Figure Black cross of isogyres Black cross of isogyres Circular isochromes Circular isochromes Melatope - location of optic axis Melatope - location of optic axis Isochromes are increasingly higher order colors outward Isochromes are increasingly higher order colors outward

9 Isochromes (and isogyres) result from ray paths of conoscopic light: Isochromes (and isogyres) result from ray paths of conoscopic light: Light traveling along optic axis (melatope) has no retardation Light traveling along optic axis (melatope) has no retardation Light near melatope has low retardation (d and  little higher Light near melatope has low retardation (d and  little higher Light far from melatope have higher retardation (d and  increase more) Light far from melatope have higher retardation (d and  increase more) Thicker and high birefringent minerals have more isochromes Thicker and high birefringent minerals have more isochromes E.g. calcite (large  ) vs quartz (small  ) E.g. calcite (large  ) vs quartz (small  )

10 Fig Thin section Indicatrix (1)Longer path length, greater d, higher interference colors (2)Perpendicular to light ray is section of indicatrix, larger 

11 Origin of Isochromes and Isogyres  rays vibrate tangent to isochromes  rays vibrate tangent to isochromes  rays vibrate perpendicular to isochromes  rays vibrate perpendicular to isochromes Isogyres are where vibration directions are N-S and E-W, extinct Isogyres are where vibration directions are N-S and E-W, extinct

12 Fig Thin section Indicatrix Note: each ray path has its own section of indicatrix; Each has unique n  ’, so has increasing  outward

13 Off-center OA figure Figure forms when OA is not perpendicular to stage Figure forms when OA is not perpendicular to stage Correct grain will have intermediate interference colors Correct grain will have intermediate interference colors Use of figure similar to centered OA figure Use of figure similar to centered OA figure

14 Off-center OA figure If OA < ~30º to stage, melatope in field of view If OA < ~30º to stage, melatope in field of view Isogyres swing around center of cross hairs Isogyres swing around center of cross hairs If melatope is out of field of view, difficult but possible to determine optic sign If melatope is out of field of view, difficult but possible to determine optic sign

15 Fig Off-center OA, melatope in field of view Off-center OA, melatope in field of view Optic Axis inside field of view

16 Fig Off-center OA figure, melatope outside field of view Off-center OA figure, melatope outside field of view Thin section Indicatrix Optic Axis outside field of view

17 Optic Normal (Flash Figure) Formed when OA is parallel to stage Formed when OA is parallel to stage Grains have highest interference colors Grains have highest interference colors Broad diffuse isogyres, split and leave field of view Broad diffuse isogyres, split and leave field of view Not much use Not much use Determines orientation of OA – e.g. pleochroism Determines orientation of OA – e.g. pleochroism

18 Fig.7-39

19 Determining Optic Sign Orientation of vibration directions known in each quadrant Orientation of vibration directions known in each quadrant Insertion of accessory plate will cause subtraction and addition Insertion of accessory plate will cause subtraction and addition Determines sign Determines sign

20 Fig    ’ always points toward melatope, Orientation of vibration directions from fig AdditionSubtraction + -

21 Biaxial Interference Figures 5 major figures 5 major figures 2 useful ones: 2 useful ones: Acute bisectrix (Bxa) figure Acute bisectrix (Bxa) figure Optic axis figure Optic axis figure 3 worthless ones: 3 worthless ones: Obtuse Bisectrix (Bxo) figure Obtuse Bisectrix (Bxo) figure Optic Normal figure (Flash figure) Optic Normal figure (Flash figure) Off-center figure Off-center figure

22 Acute bisectrix figure Bxa axis (X or Z depending on sign) oriented perpendicular to stage Bxa axis (X or Z depending on sign) oriented perpendicular to stage Biaxial Indicatrix Optically negativeOptically positive Fig. 7-27

23 Acute bisectrix figure Grains have intermediate to low interference colors (depends on 2V) Grains have intermediate to low interference colors (depends on 2V) Isogyres form cross that splits and leaves field of view as stage is rotated Isogyres form cross that splits and leaves field of view as stage is rotated Two melatopes (i.e. two OA) Two melatopes (i.e. two OA) Isochromes are oval or figure 8 around the melatope Isochromes are oval or figure 8 around the melatope

24 Fig Grain at extinction Grain 45º from extinction Acute bisectrix figure Optic Plane

25 Optic Axis Figure Formed when OA is vertical Formed when OA is vertical These grains have zero or small retardation These grains have zero or small retardation If 2V > 30º, only one melatope (OA) in field of view If 2V > 30º, only one melatope (OA) in field of view If 2V very small, looks like an off-center Bxa figure If 2V very small, looks like an off-center Bxa figure

26 Fig V < 30º 2V > 30º

27 Determining Optic Sign Done with Bxa or OA figure Done with Bxa or OA figure Example of Bxa figure: Example of Bxa figure: Two light rays vibrate along Bxa axis (either Z or X, the other must be Y) Two light rays vibrate along Bxa axis (either Z or X, the other must be Y) Y vibration is n , this one is perpendicular to the optic plane Y vibration is n , this one is perpendicular to the optic plane Other depends if mineral is + or – Other depends if mineral is + or – If +, then vibration is X = n  If +, then vibration is X = n  If -, then vibration is Z = n  If -, then vibration is Z = n  Use accessory plate to determine if vibration is fast or slow Use accessory plate to determine if vibration is fast or slow

28 Fig Light from bottom Two vibration directions depend on which axis is Bxa Fast on slow? Slow on slow? Shows if Bxa is Z or X Subtraction Addition Remember: Think about slice of indicatrix to give you vibration directions

29 Fig Determining optic sign with Biaxial OA figure Slow over fast - subtraction Slow over slow - addition Subtraction Addition

30 Determining 2V – several techniques Bxa figure: Bxa figure: Spacing between melatopes relates to 2V Spacing between melatopes relates to 2V Depends on numerical aperture (NA) of objectives Depends on numerical aperture (NA) of objectives Can guess within about 10º Can guess within about 10º

31 Fig Numerical aperture 15º 60º45º 30º

32 Optic axis figure Curvature of the isogyre depends on 2V Curvature of the isogyre depends on 2V If 2V = 90º, the isogyre is a straight line If 2V = 90º, the isogyre is a straight line If 2V = 0º, the isogyre forms a cross – it is uniaxial If 2V = 0º, the isogyre forms a cross – it is uniaxial

33 Fig. 7-52


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