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Megascopic Study of Common Metamorphic Rocks

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Megascopic Study of Common Metamorphic Rocks EXPERIMENT 9 MEGASCOPIC STUDY OF COMMON METAMORPHIC ROCKS Outline of Experiment____________________________ 9.1 Introduction Gneiss Expected Learning Skills 9.5 Non-foliated Metamorphic 9.2 Requirements Rocks 9.3 Basic Concepts Marble 9.4 Foliated Metamorphic Rocks Quartzite Slate 9.6 Laboratory Exercises Phyllite 9.7 References Schist 9.8 Learning Resources 9.1 INTRODUCTION You have learnt to identify igneous and sedimentary rocks both in hand specimens and thin sections in the previous experiments. In this experiment you will learn to identify megascopic characters of metamorphic rocks of foliated metamorphic rocks such as slate, phyllite, schist and gneiss and non- foliated metamorphic rocks such as marble and quartzite. 162 Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. 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; ❖ understand the facies and protolith of slate, phyllite, schist, gneiss, marble and quartzite; and ❖ discuss uses and Indian occurrences of slate, phyllite, schist, gneiss, marble and quartzite. 9.2 REQUIREMENTS You will require the following to perform this experiment successfully: • Hand specimens of slate, phyllite, schist, gneiss, marble, quartzite and hand lens. • Laboratory file, pen/ pencil and eraser. • You are instructed to draw the sketch of the hand specimen observed in the laboratory given by your instructor. Note: • Do not use pen/pencil/marker pen to mark the hand specimen of the rock. • Please do not attempt to break the specimens in the laboratory. Rock specimens you are examining cannot be readily replaced. • You are advised to identify the rock hand specimen provided to you at your study centre with the help of megascopic characters provided in the tables and photograph of the hand specimen. 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. Instructions to perform the experiment and document in your laboratory file 163 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….….............….…....... In the laboratory, you will make observations and identify the given hand specimens and describe the megascopic characters as given in Table 9.1. • Take the rock specimen in your hand and observe it carefully. • Identify the megascopic characters of rock as mentioned in Table 9.1. • Examine the minerals in the rock with the help of a magnifying lens to determine its texture and mineralogical composition. • Identify whether the metamorphic rock you are examining is foliated and non-foliated on the basis of texture and structure. • Name the given rock on the basis of texture and mineral composition. • Find out the protolith or parent rock and facies and write in your laboratory file. • Write uses and occurrences in India. • You are instructed to draw the sketch of the hand specimen observed in the laboratory given by your instructor. Remember!! Descriptions of the hand specimens 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 9.3 BASIC CONCEPTS Let us introduce metamorphic rocks as a type of rock which has undergone a substantial change from their original igneous, sedimentary, or earlier metamorphic form. Thus, every metamorphic rock has a parent or a precursor rock (the rock from which it was derived) known as protolith. The protolith can be of igneous, sedimentary or even metamorphic origin. Metamorphic rocks are those whose original texture, mineralogy and composition have undergone changes as a result of recrystallisation under the new set of high pressure and temperature (higher than the conditions existed at the time of formation) conditions. Metamorphism refers to the changes in the parent rock or protolith in the solid state and not by melting the whole rock. Metamorphic rocks evidenced by the changes that occur mainly in the mineral lattice structures as well as textures. Moreover, changes in chemical composition may also take place. Metamorphic minerals are the new minerals that grow in place of previously existing minerals within the solid rock only under the new regime of pressures and temperatures. They are classified on the basis of texture (foliated vs. non-foliated), grain-size, and mineral composition. While examining the hand specimens you have to observe colour, grain size, texture 164 Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. and structure and finally to infer the name of the rock. You have to find out the metamorphic facies and parent rock or protolith. All the observations made by you have to be documented in the laboratory file in the manner given in Table 9.1. Table 9.1: Megascopic Characters of ………………. 1. Colour : 2. Grain Size : 3. Hardness : 4. Texture/Structure : • Foliation : • Cleavage : • Structure : 5. Mineral Composition : • Essential : • Accessory : 6. Metamorphic Facies : 7. Inference/Name : 8. Parent Rock : 9. Important Uses : 10. Indian Occurrences : Before discussing about the types of metamorphic rock, let us discuss the above-mentioned points: 1. Colour: Metamorphic rocks exhibit different colours depending on their mineral composition. Brown or black colour is possibly due to the dominance of biotite. Presence of chlorite gives green colour to the rock. Dominance of calcite renders white or off-white colour to the rock. If the metamorphic rock is rich in muscovite, quartz and feldspar and its colour will be silvery off-white. You have to mention colour of the rock by observing its appearance in the light. Thus, the colour of rock in hand specimen will be documented in your laboratory file as per visualization of your eyes. 2. Grain size: It reflects pressure and temperature conditions of metamorphism. Generally, high temperature and pressure conditions resulted in the formation of high-grade of metamorphic rocks, favouring formation of larger grains. Whereas, low pressure and temperature conditions favour smaller grains. You have to observe the grain size of the 165 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….….............….…....... rock on the basis of the size of the majority of grains present in the rock. The grain size is categorised as given below: • fine-grained < 0.75 mm • medium grained 0.75-1 mm • coarse-grained 1-2 mm • very coarse-grained > 2 mm However, in hand specimen terms such as fine, medium-, coarse- grained are used for categorising all metamorphic rocks except slates and phyllites, where a fine grain size is implied by the name. You have to observe grain size for both matrix and porphyroblasts/porphyroclasts, if you find them in the hand specimen. 3. Hardness: Hardness is the measure of resistance to a smooth surface that offers to abrasion. In other words, it is the resistance offered by a rock on breaking when certain amount of force is applied. It is described in terms of hard, compact, brittle and soft. 4. Texture and Structure: It includes porphyroblasts/porphyroclasts, foliation and layering, small folds or wrinkles or kink folds, cumulates, equigranular, and inequigranular. Metamorphic rocks either bear foliated (layered) or non-foliated texture. Foliated texture is a pervasive layering caused by compositional layering or by the parallel orientation of platy (e.g., mica) or elongate (e.g., amphibole) mineral grains. Foliation is caused by recrystallisation under directed (compressional) stress. If rock lacks in the platy or elongated mineral grains and does not exhibit foliation even though it recrystallized under great pressure then it is said to be non-foliated texture commonly found in rocks such as marble and quartzite. Foliated textures show a distinct planar character in which the mineral grains of the rock are aligned with each other in a definite direction. Non- foliated textures do not exhibit alignment of mineral grains. Non-foliated textures display random orientation of the mineral grains. Generally, the term foliation is used as non-genetic term for layering. Foliation refers to any layering in a metamorphic rock. This term is applied to surfaces possibly of relict bedding or to surfaces that are purely formed as a result of deformation and/or recrystallisation. There are several types of foliation: i) Compositional layering: The foliation is defined by alternate layers composed of different mineral grains. You can recognise it easily by difference s in the colour of the layers. ii) Gneissosity: It refers to compositional layering in which granoblastic layers of roughly equidimensional grains (such as quartz, feldspar), 166 Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. alternate with more schistose layers of platy or elongate grains, or with other granoblastic layers. iii) Schistosity: It refers to foliation which is represented by a "preferred orientation" of platy or flaky minerals. It signifies that the rock is made up of parallel or subparallel bands or layers of platy and flaky minerals. The bands may comprise
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