Interference colour and interference figures

Interference colour and interference figures

1. DR. HARISINGH GOUR VISHWAVIDYALAYA SAGAR,M.P (A CENTRAL UNIVERSITY) DEPARTMENT OF APPLIED GEOLOGY Topic- Interference colours and Interference figures Under Guidance Of- Presented by- Prof . R.K. Trivedi Dipankar Sonowal Reg No.Y18251011

2. Contents Interference colour Interference figures : Centered optic axis interference figure Uniaxial interference figures Determination of optic sign from uniaxial interference figures Uniaxial positive mineral : Insertion of length fast gypsum plate, mica plate and quartz wedge Uniaxial negative mineral : Insertion of length fast gypsum plate , mica plate and quartz wedge Biaxial interference figures: Centered acute bisectrix interference figure Biaxial positive mineral: Insertion of length fast gypsum plate, mica plate and quartz wedge Biaxial negative mineral : Insertion of length fast gypsum plate, mica plate and quartz wedge Reference

3. INTERFERENCE COLOUR When an anisotropic mineral in thin section is viewed under cross polars of a petrological microscope, it shows some colours apart from four positions of darkness (extinction positions) with a complete rotation of the microscope stage. This colour is called ‘ INTERFERENCE COLOUR ’ ’.

4. An anisotropic mineral section has only two mutually perpendicular vibration directions. The polarized light on striking the lower part of the section gets resolved into mutually perpendicular polarized rays, vibrating along the vibration directions of the mineral. These rays travel through the length of the microscope tube and reach the analyzer, which has only one polarizing direction. The two rays again get resolved in the plane of polarization of the analyzer. The interference colour is generated as the resultant of the two superimposed polarized light emerging out of analyzer

5. The maximum interference between two rays takes place when the mineral thin section is placed 45° from its extinction position. Therefore, maximum interference colour for a particular mineral is observed when it is viewed 45° from its extinction position. This is the standard procedure to determine interference colour of a mineral in thin section.

6. INTERFERENCE FIGURES When certain sections of anisotropic minerals is observed under convergent polarized light between two polarizers and by using high power objective and bertrand lens an image is formed called interference figures.

7. Under orthoscopic view of the petrological microscope, we can at best know whether the mineral in thin section is ‘ Isotropic ’ or ‘ Anisotropic ’. But in case of anisotropic minerals, whether it is ‘ Uniaxial ’ or ‘ Biaxial ’ we can not ascertain under orthoscopic view of the microscope. However, under conoscopic view of the microscope, both uniaxial and biaxial minerals show different as well as distinctive types of interference figures, from which we can very easily ascertain whether the mineral is ‘ Uniaxial ’ or ‘ Biaxial ’. Moreover, with the help of ‘ Accessory Plates ’, it can be found out whether the uniaxial or the biaxial mineral is ‘ + ve ’ or ‘- ve ’ from their respective interference figures, and this determination is known as the ‘ DETERMINATION OF OPTIC SIGN OF MINERALS’

8. UNIAXIAL INTERFERENCE FIGURES Centered Optic axis interference figure : A mineral section cut normal to the optic axis of uniaxial mineral produce the simplest and most important interference figure . The interference figure consists of isogyres forming a dark cross superimposed on circular rings called isochromes . The point in the centre where the isogyres cross is called as melatope , which represents the optic axis itself. Isochromes are visible only in case of minerals having moderate to high birefringence. In case of minerals with low birefringence, isochromes are not at all visible.

9. Determination of optic sign from uniaxial interference figures For optic sign determination, ‘ Accessory Plates ’ are used. • ‘ Gypsum Plate ’; ‘ Mica Plate ’ & ‘ Quartz Wedge ’ are the three commonly used accessory plates. • In optic sign determination, minerals with isochromes are treated differently than minerals without isochromes . • Gypsum plate is used both in case of the minerals with isochromes and the minerals without isochromes . • Mica plate is used only in case of the minerals without isochromes . • Quartz wedge is used only in case of the minerals with isochromes .

10. Accessory Plates are mounted in a slot aligned NW-SE in the microscope tube

11. With fast ray of the mineral parallel to fast ray of the accessory plate – retardation increases ( ADDITION) With fast ray of the mineral parallel to slow ray of the accessory plate – retardation decreases ( SUBTRACTION) With slow ray of the mineral parallel to fast ray of the accessory plate – retardation decreases ( SUBTRACTION ) With slow ray of the mineral parallel to slow ray of the accessory plate – retardation increases ( ADDITION )

12. Insertion of Length Fast GYPSUM PLATE UNIAXIAL POSITIVE MINERAL With the insertion of length fast Gypsum plate, yellow color appears in quadrants II & IV , which indicates subtraction. Yellow colour means subtraction and this subtraction of colour or decrease of retardation results because slow ray of the mineral i.e. E rays or (e) is parallel to fast ray of the gypsum plate. Similarly, blue color appears in quadrants I & III indicating addition. Blue colour means addition and this addition of colour or increase of retardation results because slow ray of the mineral i.e. E ray or (e) is parallel to slow ray of the gypsum plate. Therefore, E ray is slower than O ray

13. INSERTION OF LENGTH FAST MICA PLATE UNIAXIAL POSITIVE MINERAL

14. INSERTION OF LENGTH FAST QUARTZ WEDGE UNIAXIAL POSITIVE MINERAL

15. Insertion of Length Fast GYPSUM PLATE UNIAXIAL NEGATIVE MINERALS • With the insertion of length fast gypsum plate, blue color appears in quadrants ii & iv , which indicates addition . • Blue colour means addition and this addition of colour or increase of retardation results because fast ray of the mineral i.e. E rays or (e) is parallel to fast ray of the gypsum plate . • Similarly, yellow color appears in quadrants i & iii indicating subtraction . • Yellow colour means subtraction and this subtraction of colour or decrease of retardation results because fast ray of the mineral i.e. E ray or (e) is parallel to slow ray of the gypsum plate . • Therefore, e ray is faster than o ray or e < w i.E. Uniaxial negative.

16. INSERTION OF LENGTH FAST MICA PLATE UNIAXIAL NEGATIVE MINERALS

17. INSERTION OF LENGTH FAST QUARTZ WEDGE UNIAXIAL NEGATIVE MINERALS

18. Centered Acute Bisectrix Interference Figure The Centered Acute interference figure consists of: A black cross consisting of two ‘ ISOGYRES ’ of unequal thickness; N-S isogyre is thick & E-W isogyre is thin. The intersecting point of the two isogyres coincides with the centre of the microscope cross hairs. On the E-W isogyre , there are two ‘ MELATOPES ’, which represent the two optic axes themselves. The two melatopes are encircled by concentric coloured rings called ‘ ISOCHROMES ’. Isochromes are visible only in case of minerals having moderate to high birefringence. In case of minerals with low birefringence, isochromes are not at all visible. The interference figure changes its overall configuration with a slight rotation of the microscope stage from the extinction position and the isogyres split into two hyperbolic curves of equal sizes with clockwise or anti-clockwise 450 rotation of the microscope stage.

19. INSERTION OF LENGTH FAST GYPSUM PLATE BIAXIAL POSITIVE MINERALS With the insertion of length fast Gypsum plate, blue color appears in the concave sides of both the hyperbolic curves while yellow color appears in the convex sides of both the hyperbolic curves. Blue colour means addition and this addition of colour or increase of retardation results because slow vibration direction of the mineral, which is the acute bisectrix vibration direction is parallel to the slow ray of the gypsum plate. Yellow colour means subtraction and this subtraction of colour or decrease of retardation results because fast vibration direction of the mineral, which is the obtuse bisectrix vibration direction is parallel to the slow ray of the gypsum plate. In case of biaxial minerals , X > Z; so, Z is the acute bisectrix and X is the obtuse bisectrix .Hence the mineral is BIAXIAL POSITIVE.

20. INSERTION OF LENGTH FAST MICA PLATE

21. INSERTION OF QUARTZ WEDGE

22. INSERTION OF LENGTH FAST GYPSUM PLATE BIAXIAL NEGATVE MINERALS With the insertion of length fast Gypsum plate, yellow color appears in the concave sides of both the hyperbolic curves while blue color appears in the convex sides of both the hyperbolic curves. Yellow colour means subtraction and this subtraction of colour or decrease of retardation results because fast vibration direction of the mineral, which is the acute bisectrix vibration direction is parallel to the slow ray of the gypsum plate. Blue colour means addition and this addition of colour or increase of retardation results because slow vibration direction of the mineral, which is the obtuse bisectrix vibration direction is parallel to the slow ray of the gypsum plate. In case of biaxial minerals , X < Z; so, X is the acute bisectrix and Z is the obtuse bisectrix . Hence the mineral is BIAXIAL NEGATIVE .

23. INSERTION OF LENGTH FAST MICA PLATE

24. INSERTION OF QUARTZ WEDGE

25. Introduction to mineralogy by William D.Nesse P.O. Alexander, A handbook of minerals, crystals ,rocks and ores Wikipedia Image Courtesy- www.google.com Reference

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