EXPERIMENT 6

USE OF POLARISING MICROSCOPE

Structure______

6.1 Introduction 6.4 Optical Properties of Minerals Expected Skills Under Plane Polarised Light 6.2 Requirements Between Cross Nicol 6.3 Basic Concepts 6.5 Laboratory Exercises Parts and Functioning of Polarising Microscope 6.6 References Adjustments of Microscope 6.7 Learning Resources

6.1 INTRODUCTION

In the previous two experiments, you have identified minerals based on their physical properties. Now in the next two experiments, you would learn to identify the same minerals based on their optical properties. The optical properties are studied under the polarising/petrological microscope, using transmitted light. So, prior to that get familiarised with a polarising microscope you will be using for mineral identification. You will be handling and working on polarising microscope for the first time. In this experiment, you will learn the use of polarising microscope for the study of minerals. In this experiment we will give a brief account of polarising microscope and its functioning in the identification of minerals. You will also learn about the optical properties used for the identification of minerals both under polarised light and cross nicol condition. With the help of these optical properties you will examine and identify the minerals in the subsequent Experiments 7 and 8.

Expected Skills______

After performing this experiment, you should be able to  describe polarising microscope;  discuss functioning and parts of the polarising microscope;  make necessary adjustments in the microscope for its usage;  list optical properties of the minerals under plane polarised light, and ……………………………..…………………….…………………………………………………………………………………………………………………………………………….BGYCL-134 Crystallography, Mineralogy and Economic Geology: Laboratory  list optical properties of the minerals in cross nicol condition.

6.2 REQUIREMENTS

You need to have a polarising microscope in order to know its handling and use. You will get this microscope from your study centre. If the number of microscopes is limited in the study centre, you are advised to work in groups. You would also require thin section of the mineral.

Requirements Polarising microscope Mineral thin sections

Instructions: You are required to study Unit 4 Polarising Microscope of BGYCT-133 course (Crystallography, Mineralogy and Economic Geology) before performing this experiment. Bring this practical manual along with Block 3 Optical Mineralogy of BGYCT-133 course while attending the Practical Counselling session.

6.3 BASIC CONCEPTS

A polarising microscope, also known as petrological microscope, is a type of microscope that uses polarised and transmitted light for study of optical properties of the crystalline material such as minerals and rocks. The polarising microscopes are of two types:  transmitted light microscope used for the study of rocks and minerals; and  reflected light microscope used for the study of opaque minerals, mostly ores. In this experiment, we will discuss about transmitted light polarising microscope in details.

6.3.1 Parts and Functioning of Polarising Microscope A transmitted light polarising microscope is different from other microscopes as it is equipped with two polars (i.e., nicol prisms) that are oriented at right angles to each other so that their polarisation directions are perpendicular to one another (Fig. 6.1 and 6.2). The lower polarising nicol is called as the ‘polariser’ and the upper as ‘analyser’ as it aids analysis. Light source located below the stage of the microscope is initially unpolarised. It first passes through the lower polariser (usually, just called the polariser), where it gets polarised in the

94 Experiment……………………………..…………….………………………………………………………………………………………………………………………………………………… 6 Use of Polarising Microscope manner that the light starts vibrating from (the users) right to left. These directions are referred to East (right) and West (left). This is called Plane Polarised Light (abbreviated as PPL). The analyser is similar to polariser, but is oriented at right angle to polariser. It has a polarisation direction exactly perpendicular to that of the lower polariser. These directions are usually referred to North-South (Fig. 6.2a and b).

Fig. 6.1: Polarising microscope and important components. When the analyser is inserted in the microscope, without a thin section of mineral specimen then the field of view becomes dark, provided polariser and analyser must be at 90º to each other. Small difference in the angle will little amount of light to pass through and field of view will not be perfectly dark. In cross nicol condition the analyser receives light vibrating in an East-West direction from the polariser, but because of orientation cannot transmit it, as it is absorbed. The above arrangement of analyser to polariser is referred to as cross polars or cross nicol condition and is abbreviated as XPL. Let us discuss different parts of the polarising microscope and their functioning.  Concave mirror: Near the base of the polarising microscope, there is a concave mirror which reflects the ordinary light upwards (Fig. 6.1). The source of light may be natural like sunlight or an artificial, for example, an electric bulb.

95 ……………………………..…………………….…………………………………………………………………………………………………………………………………………….BGYCL-134 Crystallography, Mineralogy and Economic Geology: Laboratory  Polariser: The ordinary light which is initially unpolarised, becomes polarised after passing through the polariser. The light leaves the polariser and starts vibrating parallel to the short diagonal of the nicol prism. This light first passes through the lower polariser (also known as polariser), where it becomes polarised and vibrates from the users right to left. These directions are referred to as East (right) and West (left).  Sub-stage Diaphragm(s): One or two diaphragms may be located below the stage. Function of the diaphragms is to reduce the area of light entering the thin section.

(a) (b) Fig. 6.2: Polarising Microscope: a) Parts and functioning; and b) Optical path of transmitted light.

 Mineral Plate on Graduated Rotating Disc or Stage: The graduated rotatable disc, on which thin section of the mineral or rock is placed, lies in between the two nicols. The microscopic stage or disc is capable of rotation and can be locked at any point. It is employed for precise angular measurement. The light entering from the polariser is resolved into two vibration directions at right angles to each another. They are parallel to the vibration direction of the mineral. Hence, two rays leave the mineral plate.  Objective lens: It is placed at lower end of the microscope tube. Its function is to produce a sharp and clear image. The light passing through a hole in 96 Experiment……………………………..…………….………………………………………………………………………………………………………………………………………………… 6 Use of Polarising Microscope the rotatable stage of the microscope enters the lower lens, called an objective lens. For mineralogical work, there are three objectives: low, medium and high power for image magnification as may be required. These objectives are mounted on nose piece and can be successively rotated into position.  Microscope tube: The microscope is focused by moving the tube up and down. The focusing is done by adjusting both the coarse and fine knobs.  Analyser: It is the second polariser called analyser mounted within the microscope tube. It can be pushed in and out so that it can be in the light path (inserted position) or out of the light path (analyser out position or without analyser). If the analyser is out such that it is not in the light path then the polarised light is transmitted through the ocular lens. If the analyser is in, then the plane polarised light coming from the lower polariser is blocked and no light is transmitted though the ocular lens above. We have discussed this phenomenon in the earlier section. We have discussed earlier that the rays leaving the mineral plate are broken into two vibrations. One ray is parallel to the long diagonal, which is reflected out. And the other ray is parallel to the short diagonal of the analyser that reaches the eye piece.  Ocular/eye piece: It is placed at the upper end of the microscope tube. The eye piece merely enlarges the image including any imperfection resulted from poor quality objective. The eye piece in the microscope contains cross- wires.  Bertrand Lens: It is used to study minerals in convergent light or under conoscopic condition. It is inserted into the upper microscope tube.  Condensing Lens: It is also called condenser or convergent lens. It is a small hemispherical lens attached to a bar so that it can be inserted. This lens is used when Bertrand lens is inserted. Both condensing lens and Bertrand lens are used in case of conoscopic illumination. The polarising microscope may be assembled either for orthoscopic or conoscopic illumination. The orthoscopic arrangement provides the eye with a realistic virtual image with a flat field, showing mineral thin section on the microscopic stage. If the observation is made under an orthoscopic condition, either with the polariser alone (in case of plane polarised light) or with the analyser and polariser both (in case of cross nicol) are used. If the observations are made in conoscopic arrangement, Bertrand lens and the condenser both are used. With the help of polarising microscope, it is possible to study and identify the minerals and rocks in detail. We hope that now you are familiar with the parts and working of the polarising microscope.

6.3.2 Adjustments of Microscope It is important to note that several types of adjustments that need to be made before examining a thin section of a mineral/rock under the microscope. The main purpose of the adjustments is to bring all parts of the microscope at a suitable position so that the thin section can be studied properly.

97 ……………………………..…………………….…………………………………………………………………………………………………………………………………………….BGYCL-134 Crystallography, Mineralogy and Economic Geology: Laboratory  Focusing: There are two types of focusing adjustments in microscope: coarse and fine, which are carried out by turning the coarse and fine focus knobs mounted at the left and right sides of the microscope tube. Rotating these knobs moves the microscope stage up and down. This adjustment will help you in focusing the microscope in accordance with the lenses so that the thin section is clearly visible through the eyepiece.  Ocular adjustment: We know that human beings have variable eyesight. For clear examination of thin section under the microscope, it is required that cross-wires of the ocular(s) should be focused as per the eyesight of the users. The cross-wires of ocular(s) can be brought into focus by adjusting the height of the eye lens by rotating the eye lens.  Illumination adjustment: To study a thin section under the microscope, the thin section must be illuminated by a light source. There are mostly two kinds of light source for microscope, namely, an external light and a built-in (i.e. internal) light. Generally, microscopes use an external light source, in which a thin section is illuminated by directing the light from the Sun or from a bulb/lamp. This light is directed via a small movable circular (flat or concave) mirror attached to the base of the microscope, which further passes upward through a system of lenses and mirrors and illuminates the thin section. You may fix the intensity of the illumination by moving the mirror. The built-in light source is located in the modern microscopes to illuminate thin section. Here the intensity of light is adjusted by inserting or removing the filters.  Centering the objectives: In general, centering of the microscope involves all the optical components such as light source, polars, condenser, objective, ocular as well as the rotatable microscope stage must be aligned on a common central axis. This should coincide with the vertical axis of the microscope, i.e. the direction of vertical light rays in the microscope (Raith et al., 2012). It is important to center a microscope before we proceed to study thin section. If centering of the microscope is not done prior to its uses, it could be possible that the mineral under study may not be visible in the center of the field of view of the microscope. It may also not remain at a constant position while rotating the stage. Therefore, it becomes difficult for us to study the optical properties of the mineral. Hence, it would become necessary for us that the lens axis of the objective must be aligned or coincided with the axis of the rotation of the stage. For centering the objectives, the simple procedure is as follows: a) Insert a thin section onto the rotatable microscope stage and select a small grain in the thin section while viewing at ocular. b) Rotate the stage; if the selected grain or object remains stationary in its central position, it indicates that the objective is centered. c) If the selected grain does not remain stationary, it indicates that the objective is not centered and it needs to be centered. d) The centering of the objectives can be done by adjusting the centering screws (provided by the manufacturer) located in the objective casing or the nose piece using the centering tools available with the microscope. While centering, if you find that the centre of rotation of the circular path of the selected grain coincides with the intersection cross-wires of the 98 Experiment……………………………..…………….………………………………………………………………………………………………………………………………………………… 6 Use of Polarising Microscope ocular while rotation of the stage, it indicates that the objectives is centered.

6.4 OPTICAL PROPERTIES OF MINERALS

We have discussed polarising microscope as well as its working and adjustments. Let us now study optical properties of minerals that you will study under the microscope. Minerals in thin sections are examined in the following two positions:  Under Plane Polarised Light (PPL).  Between Cross Nicol / Cross Polar (XP).  Let us list optical properties of minerals studied under plane polarised light and under cross nicol condition (Table 6.1). Table 6.1: Optical properties of minerals under plane polarised light and in cross nicol condition.

S.No. Plane Polarised Under Cross Nicol Light condition (PPL) - Analyser Out (XP) - Analyser In

1. Colour Isotropism / Anisotropism

2. Pleochroism Interference colours

3. Form/ Habit Extinction/ extinction angle 4. Cleavage Twinning 5. Relief/ Refractive Index

6. Twinkling

6.4.1 Under Plane Polarised Light 1. Colour: In the thin section, a mineral can appear as opaque or non-opaque.  Non-opaque minerals: If a mineral is transparent or translucent, first its colour is required to be determined. The coloured minerals in thin section are much less diverse than those in hand specimen. Many minerals that appear pink, green, yellow, blue or even black may be completely colourless or nearly so in thin sections. Some minerals appear colourless such as quartz, feldspar, whereas minerals like hornblende, biotite are strongly coloured.  Opaque minerals: Mostly, metallic minerals are opaque such as

hematite (Fe2O3), magnetite (Fe3O4), pyrite (FeS2). Their grains have sharp boundaries and appear black or brownish black under PPL.

99 ……………………………..…………………….…………………………………………………………………………………………………………………………………………….BGYCL-134 Crystallography, Mineralogy and Economic Geology: Laboratory 2. Pleochroism: Change in quality and quantity of the colour is observed in some minerals on rotation of the stage. Pleochroism is the variation in colour resulting from differential absorption of wavelength in different directions. This property is exhibited by some of the coloured anisotropic minerals, e.g., biotite shows light yellowish brown to greenish and dark brown (Fig. 6.3a). Hornblende exhibits light green to dark green (Fig. 6.3b).

(a) (b) Fig. 6.3: Minerals showing strong pleochroism: a) Biotite shows light yellowish brown to greenish and dark brown shades (Source: http://minerva.union.edu/hollochk/c_petrology/ig_minerals.htm); and b) Hornblende shows light green to dark green shades. (Photo credit: Mageswari Gayu)

3. Form: We shall consider two aspects in this study i.e. shape and habit. A) Shape: We can observe following fundamental shapes of the minerals under the microscope:  Euhedral: Complete outline or boundary of the mineral grain is seen such as, hexagonal or rectangular (Fig. 6.4a).  Subhedral: Only partial outline or boundary of the mineral grain is observed (Fig. 6.4b).  Anhedral: The minerals grains are irregular in shape and the grain boundaries are not visible (Fig. 6.4c).

(a) (b) (c) Fig. 6.4: Fundamental shapes of mineral grains: a) Euhedral; b) Subhedral; and c) Anhedral.

100 Experiment……………………………..…………….………………………………………………………………………………………………………………………………………………… 6 Use of Polarising Microscope B) Habit: It reflects natural state of growth of the minerals. This is very useful in recognition of those minerals that are characteristically well developed, such as garnet, zircon and sphene. Many minerals like hornblende usually exhibit a distinctive prismatic habit; zeolites (only natrolite, not all) have radiating acicular crystals. Tabular habit is characteristic of feldspars. Mica is flaky in nature. Let us now list the commonly recognised habits.  Equant: The length and width of a crystal are nearly equal (Fig. 6.5a).  Prismatic or columnar: The length is more than the width of the crystal (Fig. 6.5b).  Acicular: They exhibit needle shape crystals which are arranged radially (Fig. 6.5c).  Lath shaped: They are prismatic but very small in size (Fig. 6.5d, for example, plagioclase in basalt.

(a) (b)

(c) (d) Fig. 6.5: Common minerals habits: a) Equant; b) Prismatic; c) Acicular; and d) Lath shape.

4. Cleavage: It is an ability of the mineral to crack along well-defined crystallographic planes within the lattice structure. Under the microscope the cleavages appear as parallel lines in the mineral grain, which may be distinct, faint or absent. If there are more than one set of cleavages, then the angle between the cleavages is measured to identify the minerals. The set of cleavages depends upon the chance in which the section is cut. Prismatic sections of hornblende, augite will show only one direction of cleavage, whereas their basal sections contain two sets of cleavages. Let us illustrate the method to find out the cleavage angle. Keep one set of cleavage parallel to the cross-wire and reading ‘a’ is taken on the scale of the disc on the stage. Now rotate the stage till the second set of cleavage becomes parallel to the same cross-wire. In this position, reading ‘b’ is 101 ……………………………..…………………….…………………………………………………………………………………………………………………………………………….BGYCL-134 Crystallography, Mineralogy and Economic Geology: Laboratory taken. The difference between the two readings ‘a’ and ‘b’ is the cleavage angle. Some of the important minerals and the cleavage sets exhibited by them include:  One set: Present in minerals, such as biotite (Fig. 6.3a) and muscovite (Fig. 6.6a).  Two sets: Present in minerals such as hornblende intersecting at an angle of 56o and 124o (Fig. 6.3b), in orthoclase intersecting at an angle of 90o (Fig. 6.6b) and augite with 2 sets of cleavages intersecting at an angle of 87o and 93°.  Three sets: Calcite with 3 sets of perfect rhombohedral cleavage.  Absent: In minerals such as quartz or olivine, cleavage is absent. Minerals may have even four (fluorite) or six (sphalerite) sets of cleavages but number of sets visible under the microscope depend on the orientation of the section.

(a) (b) Fig. 6.6: Cleavages under microscope: a) One set of cleavage in muscovite or biotite; and b) Two sets of cleavages at 900 in orthoclase, plagioclase or microcline.

5. Relief: Certain minerals stand out more sharply as compared to others when observed under the microscope. Relief is the distinctiveness with which a mineral stands out from the embedding medium when observed in plane polarised light under the microscope. Most commonly, Canada Balsam having refractive index of 1.54 is used as the mounting medium. Relief is directly related to the refractive index (RI). It depends on the difference between the refractive index (RI) of the mineral and the RI of the enclosing cement (the mounting medium) of the thin section. The term negative relief is used when the refractive index of the mineral is lower than the mounting medium. Conversely, the term positive relief is used when the mineral has higher refractive index than the mounting medium. Greater the difference in refractive index between the mineral and mounting medium, higher is the relief. Relief may be categorised as:

102 Experiment……………………………..…………….………………………………………………………………………………………………………………………………………………… 6 Use of Polarising Microscope  low or faint relief (Fig. 6.7a), e.g., quartz  moderate relief (Fig. 6.7b), e.g., augite  high or strong relief (Fig. 6.7c), e.g., zircon We have discussed in Unit 9 of BGYCT-133 course that the Becke line test is a technique in optical mineralogy which helps us to determine the relative refractive index of two materials. It is done by lowering the stage (increasing the focal distance) of the petrographic microscope and observing which direction the light appears to move.

(a) (b) (c) Fig. 6.7: Sketches showing various categories of relief: a) low; b) moderate; and c) high. 6. Refractive Index: You have learnt that the relief depends upon the RI of the mineral and the medium in which it is embedded/ mounted. Now we shall discuss how to determine refractive index using polarising microscope. If the mineral has more refractive index than Canada Balsam, it will appear to be raised up. It will indicate that the mineral has positive relief and higher refractive index. On the other hand, if the mineral appears to be depressed it is said to have negative relief and lower refractive index than the embedded medium, i.e. Canada Balsam. 7. Twinkling: You can observe twinkling in minerals such as calcite and dolomite. The refractive index of an ordinary ray in calcite is 1.66 and for an extraordinary ray is 1.49. Refractive index of Canada Balsam is 1.54. Thus, calcite shows double reflection; it possesses two vibration directions for transmitted light. When the stage is rotated the calcite shows a rough surface and inconspicuous cleavages in one position. While in the other position well defined borders, a smooth surface and conspicuous cleavages are visible. These extremes of relief are exhibited when the two vibration directions of calcite in the section are parallel in turn with the vibration direction of the light emerging from the polariser. This means that each of the two vibration directions has its own refractive index. A rapid rotation of the stage produces a rapid change of the relief which is described as twinkling. You may compare this effect with that of the stars.

6.4.2 Between Cross Nicol Now, let us move on to the optical properties observed between cross nicol condition. In order to bring the microscope in this position, please insert analyser in the microscopic tube. We have read earlier subsection 6.3.1 that in this position the short diagonal of the lower nicol (polariser) is at right angle to the inserted upper nicol (analyser). While making observations between cross

103 ……………………………..…………………….…………………………………………………………………………………………………………………………………………….BGYCL-134 Crystallography, Mineralogy and Economic Geology: Laboratory nicol, if the thin section is not kept on the microscopic stage between the two nicols, the field of view will be completely dark. We shall now discuss about the optical properties which you would observe between cross nicol: 1. Isotropism/ Anisotropism Minerals may be either isotropic or anisotropic. We shall learn about these terms here: a) Isotropic: Some minerals become dark when viewed under the cross nicol and remain completely dark on the rotation of the stage. These minerals are said to be isotropic minerals. The minerals crystallising in the cubic/isometric system and amorphous substances are isotropic and the phenomenon is called isotropism. For example, garnet, glass, leucite, etc. including the Canada Balsam (Fig. 6.8).

(a) (b) Fig. 6.8: View of isotropic mineral between cross nicols: a) Sketch; and b) Photomicrograph of garnet (in black colour) showing isotropism. (Photo credit: Dr. Meenal Mishra) b) Anisotropic: Those minerals which transmit light even though under the cross nicol conditions are called as anisotropic minerals and phenomenon is called anisotropism. Anisotropic minerals become completely dark four times on one complete rotation of the microscopic stage by 360o. They include, the minerals belonging to orthorhombic, monoclinic, triclinic, tetragonal, trigonal and hexagonal systems (Fig. 6.9).

(a) (b) Fig. 6.9: Anisotropism in different minerals: a) Sketch and microphotograph of plagioclase showing black and white bands (polysynthetic twinning). It becomes dark four times in one complete rotation; and b) Photomicrograph showing quartz (shades of gray) and colourful zircon (high relief) between cross nicol exhibiting interference colours.

104 Experiment……………………………..…………….………………………………………………………………………………………………………………………………………………… 6 Use of Polarising Microscope 2. Interference colours or polarisation colours and birefringence: It is an important property of a mineral, when we study thin sections under the microscope. The interference colours of an anisotropic mineral under cross polars vary depending upon the orientation of the mineral/rock thin section (Fig. 6.9b). When white light passes through an anisotropic mineral all wavelengths are split into two polarised rays vibrating at 90o to each other. Different colours have different wavelengths, so when the rays leave the crystal, some colours may be retarded. Anisotropic minerals in the intermediate position between the positions of extinction show different colours which are the outcome of the interference of the two rays of lights with the same vibration plane (after their emergence from analyser). Out of which one is retarded with reference to the other and these colours are known as interference colours. Interference colours reach their maximum intensity midway between two extinction positions, i.e., 45o position. The colours depend upon the thickness, birefringence and crystallographic orientation of the section. Polarisation colours of mineral range from light grey to dark grey, light yellow, dark yellow to brown, pink, purple and blue shades. A standard colour chart designed by Michel-Levy is used to measure and compare polarisation colour of various minerals that range from first, second, third or even higher orders. Standard polarisation colours of common rock-forming minerals such quartz, plagioclase, clinopyroxene (augite) and olivine are also shown in this Table 6.2. Birefringence is a measure of the difference between the maximum and minimum refractive indices of a particular mineral. In other words, it is the difference between the refractive indices of two rays, i.e., extraordinary ray and ordinary ray. Calcite shows highest birefringence. In a thin section of quartz with a standard thickness of 0.03mm, the interference colours may vary from grey to white. While, olivine shows a wide range of colours from Newton`s scale of interference colours. Let us now examine and list (Table 6.2) the order of the colours with brief reference to prominent colours. Interference colour chart is being shown in Fig. 6.10. Table 6.2: List of orders of interference colours. Spectrum Colours Examples First Order Grey, white, light yellow or Quartz, plagioclase, sometimes light orange orthoclase, hypersthene, chlorite Second Order VIBGYOR- sharp, distinct rainbow Augite, hornblende, colours, i.e. violet, indigo, blue, green, yellow, orange and red Third Order VIBGYOR-repeated but faint Muscovite, biotite rainbow colours Fourth and Pale green and pink- very faint Calcite zircon Higher Orders

105 ……………………………..…………………….…………………………………………………………………………………………………………………………………………….BGYCL-134 Crystallography, Mineralogy and Economic Geology: Laboratory

(a)

(b) Fig. 6.10: a) Michel Levy chart showing the order of interference colours for minerals under microscope; and b) Chart showing standard polarisation colours of common rock forming minerals. [Source: Cornelis Klein (2007) Mineral Science, John Wiley and Sons]

3. Extinction: When the vibration direction of an ordinary and an extraordinary ray of isotropic minerals are parallel to the vibration directions of nicols in a polarising microscope, no light reaches the eye and the mineral is said to be in the extinction position. This is because of the light passing through the polariser also passes through the mineral but is stopped by the analyser, as it has a vibration direction perpendicular to that of the polariser. This phenomenon occurs four times during the rotation of the stage of a microscope at an angle of 360° between the cross nicol. The isotropic mineral is always in the position of extinction between the cross nicol. In case of an anisotropic mineral, the two adjacent extinction positions are separated from each other by 90°. 106 Experiment……………………………..…………….………………………………………………………………………………………………………………………………………………… 6 Use of Polarising Microscope Extinction angle is the angle between crystallographic directions and the position of maximum extinction. This angle is measured with reference to cleavage, crystal outline, prominent crack or twin plane. Types of Extinction In order to determine the extinction angle, we have to keep the cleavage or crack parallel to the cross-wire between the cross nicols. On the rotation of the stage, the mineral becomes dark which is called as extinction position. Following four types of extinctions have been recognised: a) Straight or parallel extinction: You will observe that the mineral becomes dark parallel to the cross-wire on rotation of the microscopic stage. It is called as straight or parallel extinction (Fig. 6.11). Example: hypersthene.

(a) (b) Fig. 6.11: Straight extinction: a) Sketch; and b) Parallel extinction seen in hypersthene. Extinction is parallel to one set of cleavage. Note that hypersthene shows low First order interference colours and 2 sets of cleavages intersecting at 90o. (Source: http://leggeo.unc.edu/Petunia/IgMetAtlas/minerals/opx.EXT.html) b) Oblique or Inclined extinction: You will find a mineral that does not appear dark in the position parallel to the cross-wire. However, when you rotate the microscopic stage, the mineral grain appears dark at a certain angle, it is said to have oblique extinction (Fig. 6.12). In case of oblique extinction, you can measure the extinction angle. Take the initial reading when the cleavage/ crack / twin plane is parallel to the cross wire. Then rotate the stage till the mineral grain becomes dark or extinct. The difference between these two angles will give you the extinction angle. Hornblende and augite show oblique extinction.

(a) (b) Fig. 6.13: Oblique extinction: a) sketch; and b) Note the oblique extinction in plagioclase. (Photo credit: Prof. J. P. Shrivastava)

107 ……………………………..…………………….…………………………………………………………………………………………………………………………………………….BGYCL-134 Crystallography, Mineralogy and Economic Geology: Laboratory c) Symmetrical extinction: This type of extinction is shown by minerals with squarish outline or rhombic cross-section. The mineral section becomes extinct parallel to diagonal of rhombic pattern, e.g., calcite (Fig. 6.14).

Fig. 6.14: Symmetrical extinction. d) Wavy or undulose extinction: This type of extinction appears as a band or series of bands of darkness crossing a single crystal unit during rotation of the stage. This may be due to strain and deformation. Minerals do not becomes extinct completely, e.g. quartz.

5. Twinning: Two or more crystals of the same or related mineral species intergrown to each other. Such crystals are called twinned crystals. Twinning is very commonly seen in feldspar minerals. Following types of twinning are recognised.

a) Carlsbad twinning: This type of twinning shows one set of dark and bright bands (Fig. 6.15). They alternate their position on rotation of the stage, e.g., orthoclase feldspar.

(a) (b) Fig. 6.15: Carlsbad twinning: a) Sketch; and b) Microphotograph of orthoclase mineral showing Carlsbad twinning.

(b) Polysynthetic twinning: It consists of many thin dark and alternating bright bands, e.g. plagioclase feldspars (Fig. 6.16). The bands alternate their position on rotation.

108 Experiment……………………………..…………….………………………………………………………………………………………………………………………………………………… 6 Use of Polarising Microscope

(a) (b) Fig. 6.16: Polysynthetic twinning: a) Sketch; and b) Microphotograph of plagioclase showing polysynthetic twinning. (c) Cross-hatch twinning: This twinning is closely present in two directions at right angles to each other, e.g., in microcline feldspar (Fig. 6.17).

(a) (b) Fig. 6.17: Cross hatch twinning: a) Sketch; and b) Microphotograph of microcline showing cross hatch twinning. (Photo credit: Dr. Meenal Mishra)

6.5 LABORATORY EXERCISES

 Draw the sketch of the polarising microscope and label its various parts in your laboratory file.  Try to handle the microscope and do the necessary adjustments.  See whether the microscope is centred by inserting a thin section of a mineral on the top of the rotatable microscope stage.  If the optical components of the microscope are not centred, bring them all in a centered position.

6.6 REFERENCES

 Cornelis, K. (2007) Mineral Science, John Wiley and Sons.  Raith, M.M., Raase, P. and Reinhardt, J. (2012) Guide to Thin Section Microscopy.

109 ……………………………..…………………….…………………………………………………………………………………………………………………………………………….BGYCL-134 Crystallography, Mineralogy and Economic Geology: Laboratory  http://leggeo.unc.edu/Petunia/IgMetAtlas/minerals/opx.EXT.html  http://minerva.union.edu/hollochk/c_petrology/ig_minerals.htm (Websites accessed between 2nd and 10th August 2019)

6.7 LEARNING RESOURCES

 Introduction to Optical Mineralogy Link: www.youtube.com/watch?v=_ooSuUHGiiw  Microscopic study of basaltic rocks Link: https://www.youtube.com/watch?v=2RGL3XB2x3E&t=2s  Microscopy Link: www.youtube.com/watch?v=8zNQOl3Srr4  Petrological Microscope Link: www.youtube.com/watch?v=a6LJSdofqtc  Petrographic Microscopy - Thin Sections Demonstration Link: www.youtube.com/watch?v=9DkZRQU52A0  Thin Section Making Link: www.youtube.com/watch?v=BR0Lc1n_e2k

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