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Study of Optical Properties of Common Rock Forming

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Study of Optical Properties of Common Rock Forming EXPERIMENT 7 STUDY OF OPTICAL PROPERTIES OF COMMON ROCK FORMING MINERALS-I Structure______________________________________ 7.1 Introduction 7.4 Optical Properties of Minerals Expected Skills Quartz 7.2 Requirements Orthoclase 7.3 Basic Concepts Plagioclase Microcline Muscovite Biotite 7.5 Laboratory Exercises 7.6 References 7.7 Learning Resources 7.1 INTRODUCTION The chemical composition, crystal systems, physical properties and optical properties of minerals are used to identify a particular mineral. You have studied physical properties of some of the common rock-forming minerals in the previous two experiments i.e., in Experiment 5 and 6. You have also studied polarising microscope and optical properties of minerals in Experiment 4. Now, in this experiment, you will examine optical properties of four rock-forming minerals, namely, quartz, orthoclase, plagioclase and microcline under a polarising microscope. Expected Skills_______________________________________________ After performing this experiment, you should be able to: recognise optical properties of quartz under plane polarized light and under cross nicol conditions; identify feldspar group of minerals such as orthoclase, plagioclase and microcline; and get acquainted with the optical properties of mica group of minerals such as muscovite and biotite. BGYCL……………………………..…………….…………..………………………………………………………………………………………………………………………-134 Crystallography, Mineralogy and Economic Geology: Laboratory 7.2 REQUIREMENTS To perform this experiment successfully you would require the following: Polarising microscope with light source Thin sections of quartz, orthoclase, plagioclase and microcline, muscovite, biotite minerals Pen, pencil, eraser, scale, sharpener, colour pencils and drawing compass Laboratory file Instructions: You are required to study Units 8, 9 and 10 of BGYCT-133 course (Crystallography, Mineralogy and Economic Geology) before performing this experiment. Bring this practical manual along with Block 3 of BGYCT-133 course while attending the Practical Counselling session. 7.3 BASIC CONCEPTS You have read about basic concepts related to the optical properties of minerals studied under the polarising microscope, both under plane polarised light and cross nicol. Microscopic study of minerals helps in the precise identification. It is the cheapest and fastest method for the identification of minerals. The properties which you will observe under plane polarised light are: Colour Pleochroism Form/habit Cleavage Relief Refractive Index Twinkling Optical properties to be studied between cross nicol condition are: Isotropism/anisotropism Interference colours/Polarisation colours Extinction/extinction angle Twinning Apart from the above optical properties, there are some other properties such as inclusions and pleochroic haloes, alteration of minerals and zoning, etc about which you have read in Unit 9 and 10 of the course BGYCT-133. How to write Experiments 7 and 8 in the laboratory file? You are instructed to follow the following steps: To describe optical properties of the mineral you will observe under the polarising microscope in thin section, draw a circle (using drawing compass) in your laboratory file Divide the circle drawn by you into two equal halves (as shown in Fig. 7.1) 112 ……………………………..…Experiment 7 ..……….……………………………………………………………………………………………………………………………….Study of Optical Properties of Common Rock-Forming Minerals-I You will draw the sketch of the characters of a mineral grain as observed under the plane polarised light (abbreviated as PPL) In one half of the circle In the other half of the circle, you will have to draw optical properties of mineral observed under the cross nicol condition (abbreviated as XPL) Make use of coloured pencils to represent colour(s) of the mineral observed as observed under the microscope. You are instructed to draw the sketch of the optical properties observed in plane polarised light in one half of the circle and crossed polars/cross nicol in another half as shown on Fig. 7.1. Fig. 7.1: Circle representing field of view under the microscope in plane polarised light (PPL) and crossed polars (XP). After writing about the observed optical properties of the minerals and making their sketches both under the plane polarised light and cross nicol you will have to mention the diagnostic optical property of the mineral in your laboratory file. Diagnostic property forms the basis of mineral identification. Please note that it is important for you to learn the diagnostic optical properties of a mineral given to you in order to identify it under the microscope. Before examining the optical properties of some common rock-forming minerals such as quartz, orthoclase, plagioclase and microcline, you are advised to recall the Experiment 4. 7.4 OPTICAL PROPERTIES OF MINERALS We have studied the optical properties of minerals both under plane polarised light and cross nicol in Units 9 and 10 of BGYCT-133 course and Experiment 4 of this course. Now, we will identify quartz, orthoclase, plagioclase and microcline, muscovite, biotite minerals in thin sections on the basis of their optical properties. 7.4.1 Quartz Quartz (SiO2) belongs to tectosilicate group of the silicate minerals and is one of the dominant minerals of the group. It occurs in all the three types of rocks such as igneous, sedimentary and metamorphic. Let us recall physical properties of quartz that we have learnt in Unit 10 of BGYCT-133 of this course. Observe optical properties of quartz in the polarising microscope as discussed below (as shown in Figure 7.2a, b and c). 113 BGYCL……………………………..…………….…………..………………………………………………………………………………………………………………………-134 Crystallography, Mineralogy and Economic Geology: Laboratory Under Plane Polarised Light 1. Colour : Colourless in thin section 2. Pleochroism : Non - pleochroic 3. Form/habit : Quartz commonly displays anhedral form but euhedral prismatic forms are also common 4. Cleavage : Absent 5. Relief : Very low 6. Refractive Index : 1.55 Between Cross nicol 1. Isotropism/anisotropism : Anisotropic 2. Interference colours : First order white and gray 3. Extinction : Undulose extinction 4. Twinning : Not visible in thin section 5. Inclusions : Present Diagnostic properties: Quartz in thin section is colourless with low relief recognized by lack of cleavage and alteration in PPL. First order white or weak yellow interference colours, and undulatory extinction in XP. Quartz shows all the negative optical characters. (a) (b) 114 ……………………………..…Experiment 7 ..……….……………………………………………………………………………………………………………………………….Study of Optical Properties of Common Rock-Forming Minerals-I (c) Fig. 7.2: Quartz in thin section: a) Sketch of quartz with low relief in PPL and XP; b) Photomicrograph of quartz under PPL and between XP (Source: http://minerva.union.edu/hollochk/c_petrology/ig_minerals.htm); and c) Wavy extinction in quartz. 7.4.2 Orthoclase Orthoclase, plagioclase and microcline are minerals belonging to the feldspar group of the silicate minerals. Feldspar group of minerals are the most abundant and are found nearly in most of igneous, sedimentary and metamorphic rocks. These minerals display more or less similar physical properties, but they display distinctive optical properties. Let us recall physical properties of orthoclase that we learnt in Unit 10 of BGYCT-133 of this course. Observe the optical properties of orthoclase in polarising microscope as discussed below (as shown in Figures 7.3a and b). Under Plane Polarised Light 1. Colour : Colourless in thin section, but also shows pale brown 2. Pleochroism : Non-pleochroic 3. Form/habit : Orthoclase displays subhedral to euhedral forms 4. Cleavage : Two sets of cleavages intersecting each other at an angle of 90o; one set of perfect cleavage and one set of imperfect 5. Relief : Low 6. Refractive Index : 1.52 Between Cross nicol 1. Isotropism/anisotropism : Anisotropic 2. Interference colours : First order, white and light gray 3. Extinction : Oblique extinction, angle varies from 0 to 12o 4. Twinning : Simple Carlsbad twinning Diagnostic properties: Orthoclase is colourless or turbid due to alteration; low relief; one or two sets distinct cleavages intersecting at an angle at 90o. It displays first-order interference colours and usually exhibits simple Carlsbad twinning. 115 BGYCL……………………………..…………….…………..………………………………………………………………………………………………………………………-134 Crystallography, Mineralogy and Economic Geology: Laboratory (a) (b) Fig.7.3: Orthoclase in thin section: a) Sketch of orthoclase with two set cleavage in PPL and XP; b) Photomicrograph of orthoclase in PPL and in XP. (Source: www.gsi.gov.in) 7.4.3 Plagioclase Plagioclase is sodic-calcic feldspar (NaAlSi3O8 to CaAl2Si2O8) in composition. Let us recall physical properties of plagioclase that we learnt in Unit 10 of BGYCT-133 of this course. Observe optical properties of plagioclase in the polarising microscope as discussed below (as shown in Figures 7.4a, b and c). Under Plane Polarised Light 1. Colour : Colourless in thin section 2. Pleochroism : Non-pleochroic 3. Form/habit : Normally subhedral to anhedral, lath shape 4. Cleavage : Two set of cleavages intersecting at 90o, one set is perfect cleavage and the other one is poor 5. Relief : Low 6. Refractive Index : 1.58 116 ……………………………..…Experiment 7 ..……….……………………………………………………………………………………………………………………………….Study of Optical Properties of Common Rock-Forming Minerals-I Between Cross nicol 1. Isotropism/anisotropism : Anisotropic 2. Interference colours : First order light grey to
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