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Microscopic Study of Common Metamorphic Rocks

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Microscopic Study of Common Metamorphic Rocks EXPERIMENT 10 MICROSCOPIC STUDY OF COMMON METAMORPHIC ROCKS Outline of Experiment____________________________ 10.1 Introduction Gneiss Expected Learning Skills 10.5 Non-foliated Metamorphic 10.2 Requirements Rocks 10.3 Basic Concepts Marble 10.4 Foliated Metamorphic Quartzite Rocks 10.6 Laboratory Exercises Slate 10.7 References Phyllite 10.8 Learning Resources Schist 10.1 INTRODUCTION You have learnt to identify metamorphic rocks in hand specimens in the previous experiment. In this experiment you will learn to identify microscopic characters of metamorphic rocks of foliated metamorphic rocks such as slate, 184 Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. phyllite, schist and gneiss and non-foliated metamorphic rocks such as marble and quartzite. Expected Learning Skills__________________________ After performing this experiment, you should be able to: ❖ identify texture and mineral composition of foliated metamorphic rocks such as slate, phyllite, schist and gneiss; ❖ recognise texture and mineral composition of non-foliated metamorphic rocks such as marble and quartzite; and ❖ list the facies and associated protolith of slate, phyllite, schist, gneiss, marble and quartzite. 10.2 REQUIREMENTS To perform this experiment successfully, following are the requirements: • Polarising microscope with light source • Thin sections of slate, phyllite, schist, gneiss, marble and quartzite • Pen, pencil, eraser, scale, sharpener, coloured pencils and drawing compass • Laboratory file • You are instructed to draw the sketch of the hand specimen observed in the laboratory given by your instructor. Instructions: You are required to study Block 3 of BGYCT-135 course (Petrology) before performing this experiment. Bring this practical manual along with Block-3 of BGYCT-133 course while attending the Practical Counselling session. You have read in Block 3 of BGYCT-133 course that the optical properties of minerals are helpful in their identification. These properties of minerals are useful in the identification of rocks. In this experiment, you will have to utilise your knowledge and experience about the optical properties of rock-forming minerals in the identification of rocks in thin section. You have been instructed to follow the steps mentioned below: 1. In order to describe the petrographic characters of the rock in thin section you will observe under the microscope, draw a circle with the help of 185 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….….............….…....... drawing compass in your laboratory file as done in the previous experiments. 2. You will draw sketches of the petrographic characters of the rock as seen under plane polarised light (abbreviated as PPL) in one half of the circle 3. In the other half of the circle, you must draw the observed petrographic characters of the rock observed the cross nicols or crossed polars (abbreviated as XP). 4. You have read that the rock is an aggregate of minerals. Therefore, to understand optical properties of rock forming minerals is essential which you had learnt in Experiments 7 and 8 of BGYCL-134 course. Such studies done earlier will help you in microscopic studies of rocks. 5. Make use of the coloured pencils to represent colour(s) of the minerals as seen under the microscope in thin sections. After writing about observed petrographic characters of the rocks and draw their sketches both under plane polarised light and crossed nicols, you have to mention name and protolith of the rock in your laboratory file. Remember!! Descriptions of the petrographic characteristics in the tables and the sketch given in this experiment are generalized. You have to document your own observations and draw the sketch of the hand specimen in the laboratory file. 10.3 BASIC CONCEPTS You are repeatedly being instructed that your knowledge of recognising minerals acquired from BGYCT-133 and BGYCL-134 courses will be helpful in the experiments related to microscopic study of rocks. It is utmost important that the optical properties of the minerals present in the rock should be observed and identified carefully. You have to document the following microscopic characters as given in Table 10.1 in your laboratory file. We had discussed these characters in the previous experiment, related to the identification of rocks in hand specimens. You are instructed to read this section very carefully to understand different aspects of the microscopic/petrographic character to be identified. Table 10.1: Microscopic Characters of ………………. 1. Grain Size : 2. Texture : • Foliation : • Lineation : 186 Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. • Structure : 3. Mineral Composition : • Essential Minerals : • Accessory Minerals : 4. Metamorphic Facies : 5. Inference/Name : 6. Parent Rock : Before thin sections examination of foliated and non-foliated rocks, let us discuss their texture, mineral composition, facies and parent rock. 1) Grain Size: Grain size in metamorphic rock is highly variable. It can be broadly described as coarse, medium or fine. Further subdivisions of the grains can be given by standard size scale. You have to measure the grain size with the help of micrometer provided in the eye piece. For this purpose, you have to bring grain below the eye piece in-built with the micrometer. In order to classify the rock in the thin section (as hornblende schist, feldspar bearing gneiss etc.) you have to consider average size of the majority of the grains. 2) Texture: You have read that metamorphic rocks are formed as the result of various types of metamorphic processes the pre-existing igneous and sedimentary rocks which involve changes in the textures, structures and mineralogical compositions. Structures in the metamorphic rocks reflected on large scale or outcrop scale or hand specimen scale or they may be of regional scale also. On the contrary textures are at small scale phenomenon, and vary from microscopic to hand specimen scale. Mutual agreement and arrangement of mineral grains in the rock unit, their shape, size and growth constitute a few important factors responsible for the development of certain texture types. A) Foliation: In general, foliation is a non-genetic term used for layering in a metamorphic rock. Foliated rocks typically appear as if the minerals are stacked like pages of a book. The term is applied to surfaces which represents relict bedding or to surfaces resulted from the deformation and/or recrystallization of pre-existing minerals. There are several types of foliations: • Compositional layering: The foliation is defined as alternate layers composed of different minerals. You can easily recognise it by differences in the colour of the layers. 187 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….….............….…....... • Schistosity: Schistosity refers to foliation which possesses a "preferred orientation" of platy or flaky minerals and signifies parallel to sub-parallel arrangement of flaky or platy or platy minerals forming bands of distinctive characters/nature. The bands composed of minerals such as garnet and hornblende and layers consists of platy minerals such as mica, chlorite, aluminosilicates or prismatic minerals such as amphibole, tourmaline. Gneissose rocks may or may not possess schistosity. • Gneissosity: It refers to compositional layering in which layers of roughly equidimensional grains (such as quartz, feldspar), alternate with more schistose layers of platy or elongate grains, or with other granoblastic layers. B) Lineation: Lineation refers to any linear structure that occurs repeatedly in the rock. It is a linear fabric element of a primary igneous or sedimentary structure or a secondary feature developed due to deformation. Minerals such as hornblende is longer in one direction and linearity is similar to pencil or a needle. These linear minerals are also aligned within rock and are referred as a lineation. Hornblende, tourmaline, or stretched quartz grains are linear crystals and can be arranged as a foliation, a lineation, or foliation/lineation both. Metamorphic textures are categorised into four following groups: i) Palimpsest (Relict) Textures Blasto-Ophitic Texture Blasto-Intergranular Texture Blasto-Porphyritic Texture ii) Typomorphic Textures Porphyroclastic Texture Mylonitic Texture Nodular Texture Granoblastic Texture Crystallization Textures Porphyroblastic Texture Poikiloblastic Texture iii) Reaction Textures Reaction-Rim Texture Kelyphitic Texture Corona Texture iv) Intergrowth Textures 188 Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. Symplectite Texture Let us discuss them in brief. i) Palimpsest Texture: Palimpsest texture or relict refers to the texture inherited from the texture of a protolith preserved even after the metamorphism of the protolith. Low-grade metamorphic rocks show good preservation of relict textures and are formed in such a way that deformation gets limited. Palimpsest textures include; ophitic texture, intergranular texture, porphyritic texture and cumulate texture. • Blasto-Ophitic Texture: Plagioclase laths (Fig.10.1) enclosed within the pyroxene crystal (sometimes olivine). When plagioclase laths are fully surrounded in the matrix, it is said to be ophitic and if partially surrounded, it is called as subophitic texture. Fig. 10.1: Plagioclase laths enclosed by augite crystal under XP. (Source: www.alexstrekeisen.it)
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