Extinction Angle and Pleochroism

1. Extinction Angle and Pleochroism IN THIS LECTURE • Extinction Angle • Sign of Elongation • Categories of Extinction • Extinction in Uniaxial Minerals • Extinction in Biaxial Minerals • Pleochroism in Isotropic Minerals • Pleochroism in Uniaxial Minerals • Pleochroism in Biaxial Minerals

2. Extinction Angle • The angle between the length or a prominent cleavage in a mineral and a vibration direction is a diagnostic property called the extinction angle. • To determine the extinction angle • Rotate the stage of the microscope until the length or cleavage of the mineral is aligned with the north-south cross hair. • Record the reading from the stage goniometer at this point (g1) • Rotate the stage until the mineral goes extinct (dark) • Record the new reading from the goniometer (g2) • The extinction angle is the difference between G1 and G2. • If the extinction angle measured by rotating the stage clockwise is EA, then the extinction angle measured by rotating the stage anticlockwise is 90°-EA. • Normally the smaller angle is reported.

3. Extinction Angle

4. Extinction Angle • The extinction angle measured on a specific mineral in thin-section depends on exactly how the grain happens to be oriented in the sample. • Hence it is necessary to specify the orientation of the mineral grain on which the measurement is made. • In many cases the diagnostic extinction angle is measured on grains that display maximum retardation (optic axes horizontal) • In others, extinction angles are specified for certain orientations, such as resting on a cleavage surface or a cut through the mineral parallel to a specific crystallographic plane

5. Sign of Elongation • Sign of elongation refers to whether the mineral is length fast or length slow • Length fast means that the fast ray vibrates more or less parallel to the length of an elongate mineral • Length slow means that the slow ray vibrates more or less parallel to the length of an elongate mineral. • Length fast is also called negative elongation whereas length slow is also called positive elongation

6. Determining Elongation Direction • Start with the mineral extinct and with the mineral elongation or trace of cleavage so that it is less than 45° from the N-S cross hairs. This generally is the position after rotating to measure the extinction angle • Rotate the stage 45° clockwise. This places the vibration direction closest to the length or prominent cleavage NE-SW. • Insert an accessory plate. • If the retardations add, the ray whose vibration direction is closet to the length or cleavage is the slow ray and the mineral is length slow. If the retardations subtract, the ray whose vibration direction is closest to the length or cleavage is the fast ray, and the mineral is length fast.

7. Function of Accessory Plates • The primary function of the accessory plates is to determine which of the rays coming through a minerals is the fast ray and which is the slow ray. • The information is used to determine the sign of elongation as we have just seen and the optic sign which we will look at later. • In addition the accessory plates may help to distinguish between different orders of interference colors. • The common accessory plates are gypsum and mica plates. • Accessory plates are carefully prepared so that they produce a known amount of retardation and the slow ray vibration direction is across the width of the holder and the fast ray vibration direction is across the length of the holder. • In most microscopes the accessory plates slide into the optical path in a slot aligned NW-SE so that the accessory slow ray vibrates NE-SW and the fast ray vibrates NW-SE.

8. Accessory Plates • The gypsum plate produces 537 or 550 nm of retardation and yields a distinct magenta interference color seen at the transition from first to second order. • The mica plate produces around 150 nm of retardation and yields a first-order white interference color. • When looking at minerals under the microscope, if the slow ray vibration direction is parallel to the slow ray vibration direction of the accessory plate, then the slow ray relative to the fast ray will be retarded a distance of Dm + Da giving a total retardation of Dt. • If the mineral produces 250 nm of retardation (first-order white) and the gypsum plate is used (Da = 550 nm) then the total retardation is 800 nm and the interference color observed will increase to second order yellow. • Therefore retardations add = slow on slow • If the fast ray vibration direction of the mineral is parallel to the slow ray vibration direction of the accessory plate the opposite will occur and hence retardation subtract = slow on fast

9. Categories of Extinction • There are four categories of extinction • Parallel extinction • Inclined extinction • Symmetrical extinction • No extinction angle

10. 1. Parallel Extinction • With parallel extinction the mineral is extinct when the cleavage or length is aligned parallel to one of the cross hairs. • The extinction angle is 0°. • Either the slow ray or fast ray vibration direction is parallel to the trace of cleavage or length of the mineral.

11. 2. Inclined Extinction • With inclined extinction the mineral is extinct when the cleavage of length is aligned parallel to one of the cross hairs. • The extinction angle will be greater than 0°. • Neither vibration direction is aligned parallel to the trace of the cleavage of the length of the mineral. • If the slow ray vibration direction is closest to the length or trace of cleavage, the mineral is length slow. If the fast ray is closest the mineral is length fast.

12. 3. Symmetrical Extinction • Symmetrical extinction may be observed in minerals that display either two cleavages or two distinct crystal faces. • If the extinction angles EA1 and EA2 measured from the two cleavage or crystal faces to the same vibration direction, are the same, extinction is symmetrical.

13. 4. No Extinction Angle • Many minerals lack distinct cleavages or do not display an elongation or crystal faces. • Although they go extinct once every 90° of stage rotation, there is no cleavage, elongation or crystal face from which to measure an extinction angle. • In these situations we say that the mineral has no extinction angle.

14. Other Types of Extinction • In addition to the before mentioned types of extinction, some minerals may not go totally extinct at any stage position. • Usually this is the result of • strain in the crystal lattice • chemical zoning • In many deformed rocks, the mineral grains are bent or distorted so that different parts of the grain go extinct at different times. If the extinction follows an irregular or wavy pattern it is called undulose extinction. • Many minerals grow so that they are compositionally zoned. Because extinction angle may be controlled by chemical composition in monoclinic and triclinic minerals, the extinction angle may vary systematically with composition so that the centre of the grain may display one extinction angle and the rim another.

15. Extinction in Uniaxial Minerals • Tetragonal and many hexagonal minerals are prismatic and either elongate or stubby parallel to the c-axis. • A sample with the highest birefringence will have its c-axis parallel to the microscope stage and will display parallel extinction to prismatic cleavage and inclined or symmetrical extinction to rhombohedral or pyramidal cleavage. • Extinction is parallel to {001} cleavage in all grain orientations.

16. Extinction in Biaxial Minerals • Orthorhombic minerals display parallel or symmetrical extinction in sections cut parallel to (100), (010) and (001) and inclined extinction in random orientations. • Grains cut to yield maximum retardation always display parallel or symmetrical extinction. • Monoclinic minerals display parallel or symmetrical extinction if {010} happens to be vertical and inclined extinction is most other orientations. • Triclinic minerals display inclined extinction in most orientations.

17. Pleochroism • Pleochroic minerals change color as the stage is rotated when the sample is observed in plane light. • The color changes because the fast and the slow rays are absorbed differently as they pass through the mineral and therefore have different colors. • When the fast ray vibration direction is parallel to the lower polariser, all light passes as the fast ray, so the mineral displays that color. • When the slow ray vibration direction is parallel to the lower polariser the minerals displays the color of the slow ray. • If the stage is rotated to allow both the slow and the fast rays to come through, the perceived color is usually an intermediate between the two colors.

18. Isotropic Minerals and Pleochroism • Isotropic minerals are not pleochroic because they do not experience double refraction. • In plane light, isotropic minerals display a uniform color as the stage is rotated.

19. Anisotropic Minerals and Pleochroism • Colored uniaxial minerals are usually pleochroic which can be sufficiently described by identifying the colors of both the ordinary and extraordinary rays (we’ll get to those terms) • To describe the pleochroism of biaxial minerals it is necessary to specify three colors • Light vibrating parallel to X • Light vibrating parallel to Z • Light vibrating parallel to Y