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.

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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

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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),

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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 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. Gneisses may possess schistosity. iv) Cleavage: It is an important feature of metamorphic rocks. It is the capacity of the rock to split along certain directions parallel or subparallel to the smooth surfaces. Cleavage refers to schistosity surfaces that are more or less planar. Cleavage is usually defined by aligned fine-grained mica or chlorite grains. It is therefore most common in aluminous, low-grade metamorphic rocks. Cleavage is of many types. But here discussion is restricted to slaty and crenulation cleavages. Slaty cleavage is a perfectly planar type of cleavage and defined by extremely fine-grained mica and/or chlorite in slates and phyllites. The slate exhibits slaty cleavage or excellent rock cleavage, i.e tendency to break into thin and flat slabs or sheets. Crenulation cleavage is a cleavage defined by alignment of mica and/or chlorite grains on the limbs of cm-to mm-scale periodic folds or crenulations. v) Granulation: Typically, this metamorphic rock structure of is characterised by an essentially granular character of the constituent minerals. The individual grains may be irregular in outline but are interlocking.

Let us recall few terms related to texture that you have read in Unit 14 of BGYCT-135 course.

• Porphyroblastic textures are associated with rocks where the large size recrystallised mineral grains known as porphyroblasts embedded in the fine-grained groundmass. The metamorphic mineral such as garnet and staurolite tend to recrystallise and form large, individual crystals while other minerals such as mica and biotite form masses composed of small interlocked grains. Typically, rock contains large crystals of one mineral embedded in a matrix of small crystals of the other, e.g. large garnets are often embedded in the mass of fine-grained groundmass of muscovite or biotite. Metamorphic rocks displaying such type of texture is termed as porphyroblastic texture. It is quite similar to the porphyritic texture found in 167 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... the igneous rocks. The term augen is used for a large, eye-shaped porphyroblasts that commonly occurs in a few gneisses.

Fig. 9.1: Garnet porphyroblasts in muscovite schist.

• Megacryst is a generic term used to call a large crystal. • Granoblastic texture is produced in the metamorphic rocks in which mineral grains exhibit equidimensional shape and form a welded mosaic of recrystallised mineral grains. • Porphyroclastic texture refers to a coarse, relict, deformed crystals (porphyroclasts) set in a fine grained, sometimes mylonitic matrix. Porphyroclasts may be a relict type of porphyroblast, phenocryst or clastic grains. Porphyroclasts may be augen or may have asymmetric "tails". • Crystalloblastic texture in which the crystal faces of recrystallized. minerals assumed resemblance which is quite similar to the holocrystalline texture of an igneous rock. • Xenoblastic texture is the texture in which crystal grains have poorly developed crystal outline. • Idioblastic texture is used for those rocks in which the crystal faces are relatively sharp and well-developed. • Poikiloblastic texture is exhibited in a rock exhibits larger mineral grains enclosed within the smaller mineral grains.

On the basis of texture, several structures are present in the metamorphic rocks (Fig. 9.2). We will discuss here a few of them.

i) Phyllitic structure shows a strong parallel arrangement or foliation of fine- grained minerals. The phyllitic texture commonly exhibits wrinkles over

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Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. the surfaces. The parallelism of fine-grained mica gives rise to silky lustre sheen also known as phyllitic lustre. ii) Slaty structure has a strong parallelism in the foliation of fine-grained clay minerals and platy minerals (e.g. micas) which imparts a strong slaty cleavage (Fig. 9.2a). iii) Schistose structure or schistosity is strongly foliated texture, produced by the growth of minerals. Schist has a foliation or mineral alignment of medium to coarse-grained minerals. Such type of texture is associated with the mica schist, chlorite schist and hornblende schist (Fig. 9.2b). iv) Gneissose structure or gneissosity shows discontinuous banding of light- coloured medium to coarse-grained (such a quartz and feldspar) and dark- coloured minerals (e.g. pyroxene and hornblende) with granulose texture. The light and dark bands differ in composition and arranged alternately, thus produced by the segregation of minerals. This compositional layering or gneissic banding gives rise to gneiss with a striped appearance, e.g. gneiss (Fig. 9.2c). v) Granulose texture is produced by an abundance of equidimensional minerals in the form of welded interlocking mosaic of crystals, e.g. quartzite and marble (Fig. 9.2d).

Fig. 9.2: Diagrams showing a) Slaty; b) Schistose; c) Gneissose; and d) Granulose structures.

The metamorphic rocks are mainly: foliated and non-foliated types.

169 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... i) Foliated Metamorphic Rocks: These metamorphic rocks have suffered a great deal of directed pressure during their genesis. As a result of the pressure, the development of foliation surfaces or foliation planes takes place. Formation of foliation planes takes place due to enough alignment of platy minerals in the rock which eventually leads to the formation of weak planes and the rock becomes easily cleavable along the foliation surfaces. The lists of foliated metamorphic rocks arranged in increasing grade of metamorphism are; slate, phyllite, schist and gneiss types.

ii) Non-Foliated Metamorphic Rocks: These metamorphic rocks have not suffered any shear stress during their formation near the surface. No foliation plane or surface is developed during the course of rock formation. Marble and quartzite have suffered a great deal of stress and still foliated textures have not been developed as these two rocks do not contain platy minerals. No development of foliation surfaces and no schistosity in the rock make the rock hard and compact.

Note: You are advised to refer to Unit 14 of BGYCT-135 for understanding of the textures and structures of metamorphic rocks before performing this experiment.

5. Mineral Composition: Common minerals present in the metamorphic rocks include: quartz, feldspar, mica, calcite, and hornblende. You have read about index minerals in Block 4 of BGYCT-135 course. Index minerals are those minerals in the metamorphic rocks which indicate the degree of metamorphism the original rock has suffered. Index minerals form at certain pressure and temperature conditions, therefore help in the identification of grade of metamorphism. Index minerals also provide important evidences about the nature of the protolith and the metamorphic conditions which have produced it. Chlorite, muscovite, biotite, garnet, and staurolite are index minerals, representing a sequence with respect to low-to-high grade rocks.

You have read about essential and accessory minerals while identifying igneous rocks megascopically and microscopically in Experiments 1 to 6. Similarly, you have to identify essential and accessory minerals in metamorphic rocks. You have to briefly mention about physical properties of the mineral(s) identified.

6. Metamorphic Facies: On the basis of mineral assemblage, you have to mention the facies. You have read about the metamorphic facies in Unit 13 of BGYCT-135 course. 170

Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. 7. Inference/Name: On the basis of textural and mineral composition, you have to infer name of the rock identified in hand specimen.

8. Parent Rock: You have to mention about the parent rock or protolith

9. Important Uses: You have to list the uses of rock you have studied.

10. Indian Occurrences: Give here the important Indian occurrences of the rock.

Now you will identify foliated and non-foliated metamorphic rocks such as slate, phyllite, schist, gneiss, quartzite and marble.

9.4 FOLIATED METAMORPHIC ROCKS

As discussed above, foliation of the rocks shows development of conspicuous parallelism in the mineralogical and structural constitution. The features which show parallelism include: slaty cleavage, schistosity and gneissose structures. The foliated metamorphic rocks in accordance with their metamorphic grade are; slate, phyllite, schist and gneiss types. Let us discuss them one by one in the following sections.

9.4.1 Slate

Slate is a fine-grained foliated metamorphic rock (Fig. 9.3), formed by low- grade regional metamorphism of shale or mudstone. The metamorphism of Al rich minerals in shale to micaceous minerals in slate and the parallel alignment of these platy minerals give rise to foliation in slate which renders slate to break along the foliation planes smoothly. Slate is most often formed by the low-grade metamorphism of shale, mudstone, or siltstone. The colour of the slate depends on the mineral constituents. Often black (carbonaceous) slate contains organic material; red slate gets its colour from iron oxide; and green slate usually contains chlorite. Slate formation is favoured by the compression due to horizontal forces along with a minor amount of heating in a sedimentary basin which is now a part of or is involved in the convergent plate boundary.

You are advised to identify hand specimen of slate with the help of megascopic characters given in the Table 9.2 and photograph and sketch of the hand specimen given in Figures 9.3a and 9.3b.

171 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…......

Fig. 9.3: Slate: a) Photograph of a hand specimen showing slaty cleavage; and b) Sketch.

Table 9.2: Megascopic Characters of Slate. 1. Colour : Variable with the shades of grey, black, green and red. 2. Grain Size : Very fine grained (less than 0.1mm), crystals not visible with unaided eye. 3. Hardness : Hard, but brittle.

4. Texture/Structure: • Foliation : Well foliated (mm scale).

• Cleavage : Perfect slaty cleavage with smooth surface. It breaks easily long the planes parallel to the sheet silicates, causing a slaty cleavage. • Structure : Slaty structure.

5. Mineral Composition: • Essential : Mica minerals (biotite, chlorite, muscovite) which impart a sheen on foliation surfaces. • Accessory : Apatite, graphite, magnetite, tourmaline, or zircon as well as feldspar may be present, but cannot be identified with unaided eyes. 6. Metamorphic : Greenschist facies. Facies

7. Inference/Name : SLATE

8. Parent Rock : Mudstone / shale (pelitic).

9. Important Uses : Used in the construction. It is used as roof and floor tiles, and blackboards; standard material for the beds of pool / snooker /

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Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. billiard tables due to its sturdiness and appealing look. 10. Indian Occurrences : Associated with the Delhi and Aravalli Supergroup of rocks in parts of Ajmer, Alwar, Jaipur, Sawai Madhopur, Bharatpur, Bundi, Udaipur and Chittaurgarh districts. 9.4.2 Phyllite

Phyllite is a foliated metamorphic rock formed by the metamorphism of slate (Fig. 9.4). The pressure and temperature conditions involving formation of phyllite are more or less comparable to that of the low-grade regionally metamorphosed It forms in areas which have undergone regional metamorphism, where sedimentary rock strata have been subjected to moderate heat and compression aaccompanied by the collision of continental plates and mountain-building events. Formation of phyllite corresponds to preferred orientation of micaceous mineral grains in the slate. The fissile property of the protolith is not vanished off and is retained in the metamorphosed rocks so formed. Phyllite is characterised by silky sheen due to the presence of fine-grained mica grains which is also called as phyllitic lustre (Fig. 9.3) at the cleavage surfaces. You are advised to identify hand specimens of the phyllite with the help of megascopic characters given in the Table 9.3. Photographs and sketch of the hand phyllite specimen are given in Figures 9.4a, 9.4b and 9.4c.

Table 9.3: Megascopic Characters of the Phyllite. 1. Colour : Variable shades of grey with sheen. 2. Grain Size : Fine to medium grained (0.1-1.00 mm). 3. Hardness : Hard but less compact. 4. Texture/Structure: • Foliation : Foliated (mm scale); Well-developed schistosity. Surface of the phyllite is typically lustrous and sometimes wrinkled. Presence of mica (biotite and muscovite) flakes gives rise to satin luster. • Cleavage : Cleavage is poor as surfaces are coarse and rough. • Structure : Phyllitic structure. 5. Mineral Composition: • Essential : Muscovite, biotite, quartz and plagioclase. • Accessory : Sericitic mica, graphite and chlorite.

173 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... 6. Metamorphic Facies : Low grade (higher than the slate). 7. Inference/Name : PHYLLITE 8. Parent Rock : Shale or mudstone. 9. Important Uses : Phyllite is used as decorative aggregates, floor tiles, and other interior home decorations, exterior building or facing stone, and garden decorations. 10. Indian Occurrences : Aravalli range, western part of the Shillong Plateau and Sonbhadra, Singrauli and Sidhi areas.

Fig. 9.3: a) and b) Hand specimens of a phyllite showing foliated surface, satiny lustre and crinkled appearance; and c) Sketch of a hand specimen.

9.4.3 Schist

Schist is a medium to coarse-grained metamorphic rock possess schistosity and marked with the preferred orientation of predominately large mica flakes (platy minerals). These platy minerals commonly include micas (such as muscovite and biotite), which displays a planar alignment that gives rise to schistose structure of the rock. Schist also contains a small amount of other minerals, often such as quartz and feldspar. Apart from describing texture, the term schist refers to the rocks having a wide variety of chemical compositions. In general, schist refers to a strongly foliated, medium grained metamorphic rock in general formed on the continental side of a convergent plate boundary. 174

Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. It is formed by the metamorphosis of rocks having grade higher than slate. Schist is composed of platy mineral grains that are large enough to be discerned with naked eye. The alignment of these platy minerals led to the formation of foliation surfaces. The foliation surfaces so formed are much distinct and coarse due to high degree of crystallisation of micaceous minerals. The name “schist” just needs enough alignment of platy minerals to be called so. Occurrence of other minerals in schist gives rise to a rock with a new name viz. hornblende-schist, garnet mica schist etc.

You are advised to identify hand specimens of the schist with the help of megascopic characters given in the Table 9.4. Photographs and sketch of the hand specimen are given in Figures 9.4a, 9.4b and 9.5, respectively.

Fig. 9.4: Garnet porphyroblasts in: a) Biotite schist; and b) Chloritic schist.

Table 9.4: Megascopic Characters of Schist. 1. Colour : Variable; with alternate dark and light bands often with shine. 2. Grain Size : Fine to medium grained, often crystals can be seen by unaided eyes. 3. Hardness : Hard and compact.

4. Texture/Structure: • Foliation : Well-developed foliation (scale: mm to cm scale) due to preferred orientation of sheet silicates (mainly biotite and muscovite). Quartz and feldspar grains show no preferred orientation. It is smooth in touch. • Cleavage : Flaky minerals form roughly parallel layers. Rock splits into thin pieces due to the presence of mica. • Structure : Schistose structure.

175 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... 5. Mineral Composition: • Essential : Mica, quartz, plagioclase. Garnet porphyroblasts garnet is common. Chlorite schist consists of chlorite. • Accessory : Quartz, feldspars, kyanite, chlorite, garnet, staurolite and sillimanite may be present. 6. Metamorphic Facies : Intermediate/medium metamorphic grade between phyllite and gneiss. 7. Inference/Name : SCHIST

8. Parent Rock : Shale and mudstone.

9. Important Uses : Schist is strong and durable; thus, it is used inbuilding construction or decorative rock walls. 10. Indian Examples : Northern part of Sikkim, southern Son valley, north of Nahan Himachal Pradesh. Kallamala range, Kolar, Raichur, Tungabhadra valley in southern India.

Fig. 9.5: a) and b) Sketches of hand specimen of a schist presented in Fig. 9.4a and 9.4b.

9.4.4 Gneiss

Gneiss is a high-grade, foliated metamorphic rock. The rock is medium to coarse grained. It is characterised by alternate bands of light and dark minerals which render the rock with compositional and/or structural variation. Lighter minerals are felsic whereas dark minerals are mafic in composition. The banding is not continuous are observed in the schist. Foliation is distinct, but the rock is not cleavable along the foliation planes as in case of slate and schist. Gneiss is formed by regional metamorphism from variety of protoliths.

176

Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. You are advised to identify hand specimens of gneiss with the help of megascopic characters given in the Table 9.5. Photographs and sketch of the hand specimen are given in Figures 9.6a, 9.6b and 9.7, respectively.

Fig. 9.6: a) Garnet bearing gneiss; and b) Feldspar bearing gneiss.

Table 9.5: Megascopic Characters of Gneiss. 1. Colour : Variable with alternate dark and light bands. 2. Grain Size : Medium to coarse grained. 3. Hardness : Hard and compact. 4. Texture/Structure: • Foliation : Foliated, banded appearance and made up of granular mineral grains. Typically, it is marked with the foliations, but generally with the rough surface. • Cleavage : Poor. • Structure Gneissose structure. 5. Mineral Composition: • Essential : Quartz, feldspars, mica and amphiboles with alternate light and dark coloured bands giving rise to gneissic bands to the rock. Light and dark bands of felsic (such as feldspar and quartz) and mafic minerals (biotite, pyroxene, amphibole, garnet etc). • Accessory : Hornblende, garnet. 6. Metamorphic Facies : High grade metamorphic rock. 7. Inference/Name : GNEISS 8. Parent Rock : Shale, mudstone, or felsic igneous rocks. 9. Important Uses : It is used as dimension stone in construction of building facings, paving.

177 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... 10. Indian Occurrences : Gneiss is confined to small pockets in the Central India and in Karnataka, Banded gneissic complex in Aravalli. Bundelkhand craton, Chhotanagpur gneissic complex.

Fig. 9.7: Sketch of gneiss in the hand specimen.

In the above section you have learnt the identification of foliated metamorphic rocks. Now, in the following section let us discuss megascopic characters of non-foliated metamorphic rocks i.e. marble and quartzite.

9.5 NON-FOLIATED ROCKS

As discussed in section 9.3, the non-foliated rocks are those metamorphic rocks which are characterised by total or nearly total absence of foliation or parallelism of mineralogical constituents. Typical examples of non-foliated rocks are marbles, quartzites etc. Let us discuss megascopic characters of marble and quartzite in the following section.

9.5.1 Marble

Marble is a medium to coarse grained, crystalline metamorphic rock whose parent rock is possibly limestone or dolostone. During the formation of marble, protolith composed of calcite crystallises, as a result fossil shells, pore spaces and the distinction between grain and cement is disappeared. Marble displays typically a sugary texture known as saccharoidal texture. In general, marble is a non-foliated, regionally thermally metamorphic rock produced by metamorphism of limestone. Marble comprises of dolomite (18 mole% of Mg + Limestone) is called a dolomarble. On metamorphosis or transformation or alteration, the calcite crystals reform themselves in a denser and equigranular form of calcite crystals, thus making the rock harder even tougher than the limestone which corresponds to hardness of 3 on Moh’s Hardness Scale. 178

Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. Marble usually forms at convergent tectonic setting or due to the heating of limestone or dolomite by ascending magma. Unlike quartzite, marble could be scratched with a metal blade.

You are advised to identify the hand specimens of marble with the help of megascopic characters given in the Table 9.6. Photographs and sketch of the hand specimen are given in Figures 9.8a and 9.7b respectively.

Table 9.6: Megascopic Characters of Marble. 1. Colour : Variable, pure marble is white, whereas, marble with traces of iron is reddish or pink. 2. Grain Size : Medium to coarse grained, equigranular.

3. Hardness : Hard and compact.

4. Texture/Structure : Granular, saccharoidal texture. • Foliation : Non-foliated.

• Cleavage : Absent, but fracture is present.

• Structure : Granulose structure; dolomitic marble shows slightly schistose structure. 5. Mineral Composition: • Essential : Calcite or dolomite. Primarily, it is composed of calcium carbonate and it reacts with dilute acid. • Accessory : Clay minerals, mica, quartz, pyrite, iron oxide, and graphite. Gritty to touch. 6. Metamorphic Facies : Variable grade regional or contact metamorphism. 7. Inference/Name : MARBLE

8. Parent Rock : Limestone or dolostone.

9. Important Uses : Crushed stone is used as an aggregate in highways, railroad beds, and construction. Dimension stone in monuments, buildings, sculptures, paving etc. 10. Indian Occurrences : Narmada river valley in Central India in Bhedaghat area in Jabalpur District, of Madhya Pradesh. Yellow marble is found in Jaisalmer district of Rajasthan.

179 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…......

Fig. 9.7: a) Hand specimen of a marble showing saccharoidal texture; and b) Sketch of the hand specimen.

9.5.2 Quartzite

Quartzite is a non-foliated hard metamorphic rock. It is formed by the metamorphism of pure quarzitic sandstone. During metamorphism, pre- existing quartz grains recrystallise, creating new larger grains. Under moderate to high-grade metamorphism, the quartz grains in the sandstone fuse together. The distinction between cement and grains and also open pore spaces disappear and the grains are interlocked with each other. Quartzite is a non- foliated metamorphosed sandstone which forms along the convergent plate boundaries and along the orogens. Quartzite is a common metamorphic rock as its protolith sandstone is also common. Alteration of quartz rich (>90% quartz) sandstone takes place due to the pressure, heat and chemical activity during the metamorphism. The fusion of quartz grains during metamorphism makes the rock denser, equigranular and hard. Unlike marble, quartzite cannot be scratched with a metal blade due to presence of quartz and it essentially gives the rock hardness corresponding to 7 on Moh’s hardness scale.

You are advised to identify the hand specimens of quartzite with the help of megascopic characters given in the Table 9.7. Photographs and sketch of the hand specimen are given in Figures 9.8a and 9.8b, respectively.

Table 9.7: Megascopic Characters of Quartzite. 1. Colour : Variable; Pure quartzite is white, may be reddish due to presence of ferrous mineral. It is sometimes green also (fuchsite, quartzite). 2. Grain Size : Medium grained, equigranular, interlocking grains of quartz; sugary appearance. 3. Hardness : Hard and compact.

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Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. 4. Texture/Structure : Granular.

• Foliation : Absent.

• Cleavage : Absent, fracture is frequently observed.

• Structure : Granulose structure.

5. Mineral Composition: • Essential : Quartz; other mineral impurities such as hematite may be present. • Accessory : Micas, feldspars, garnets and some amphibole. 6. Metamorphic Facies : Grade Variable, resulted from regional or contact metamorphism. 7. Inference/Name : QUARTZITE

8. Parent Rock : Pure sandstone, quartz arenite.

9. Important Uses : Extensive use in building and road construction. 10. Indian Occurrences : Delhi ridge is composed of quartzite, Lesser Himalayas.

Fig. 9.8: a) Hand specimen of a quartzite; and b) sketch of the hand specimen. 9.6 LABORATORY EXERCISES

Study megascopic characters of the rock samples such as slate, phyllite, schist, gneiss, marble, quartzite given to you by your counselor. Follow the instruction given below: 1. Get a hand lens, knife blade, streak plate, coin, hardness box, broken glass piece and rock specimen from your academic counsellor. 2. Study the megascopic characters of the rock by taking specimen in hand and observe its properties with the help of the devices provided by your academic counsellor.

181 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... 3. Write down the megascopic characters of the specimen that you have observed in the laboratory file. 4. Examine the rock with the help of a magnifying lens to determine its texture and composition. 5. Identify whether the rock is a foliated or non-foliated metamorphic rock. Identify its colour, grain size, texture/structure, mineral composition and other characters as given in Table 9.1. 6. Name the given rock on the basis of texture and mineral composition as given in the Unit 4 Classification of metamorphic rocks of BGYCT-135. 7. Write down the facies, name of parent rock, uses and Indian occurrences from the knowledge you have gained while studying theory and practical courses BGYCT-135 and BGYCL-136. 8. Finally, draw the sketch of the hand specimen observed in the laboratory given by your instructor. 9. If you find any problem during the study of the rock specimens do not hesitate to ask your counsellor. 10. Handle rock specimens and all the devices provided to you with great care and do not damage them.

Exercise 1: Identify the above discussed rock specimens by studying its megascopic characters and write in your laboratory file.

Study all the rock hand specimens one by one as identified above and write the megascopic characters in the manner as listed below in the Table 9.8.

Table 9.8: 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 :

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Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. 10. Indian Occurrences :

Exercise 2: Discuss uses and Indian occurrences of gneiss and schist.

Exercise 3: Differentiate foliated and non-foliated metamorphic rocks.

Exercise 4: Discuss uses and Indian occurrences of marble and quartzite.

9.7 REFERENCES

• Alexander, Pramod, O. (2009) Minerals, Crystals, Rocks and Ores. New India Publishing Agency, 675p. • Bard, J.P (1980) Microtextures of Igneous and Metamorphic Rocks (Petrology and Structural Geology), Springer, 264p. • Singh, Parbin (2008) Engineering and General Geology. S.K. Kataria and Sons, 600p. • Tyrell, G. W. (1973) The principles of Petrology. John Wiley & Sons, 349p.

9.8 LEARNING RESOURCES

• Classifying metamorphic rock samples Link: https://www.youtube.com/watch?v=SYg1Wxq5M_o • Rocks & Minerals: Identifying Types of Rocks Link: https://www.youtube.com/watch?v=tQUe9C40NEE • Metamorphic rocks Link: http://opengeology.org/textbook/6-metamorphic-rocks/ • Rock and Mineral Identification Link: https://www.youtube.com/watch?v=7MvXv66b5h4

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