Methamorphic Rocks


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Methamorphic Rocks

  1. 1. Methamorphic Rocks Unit-III
  2. 2. Syllabus • Metamorphic Rocks Introduction, Metamorphism, Common Structures and Textures of Metamorphic Rocks, Classification of Metamorphic Rocks, Descriptive Study of Common Metamorphic Rocks (Quartzite, Marble, Slate, Gneiss, Schist).
  3. 3. Metamorphism • Methamorphic rocks are geologically classified as one of the major group of rocks which have been formed out of meta-morphism of preexisting igneous and sedimentary rocks. The term ortho-metamorphic and parametamorphic rocks are applied to those rocks which have originated from igneous and sedimentary rocks, respectively. Sometimes metamorphic rocks may also undergo metamorphism again. When a rock undergoes metamorphism more than once, the process is called poly-meta-morphism.
  4. 4. Metamorphism
  5. 5. Metamorphism • The word metamorphism means change of form (meta= change; morph= form). In petrology, it indicates the effect of temperature, pressure and chemically active solutions over the texture, minerals and composition of parent rocks. Igneous and sedimentary rocks which serve as parent rock as formed under a certain physiochemical environment, i.e.. at the time of their formation, they were in equilibrium with the surrounding in terms of temperature, pressure and chemically active fluids.
  6. 6. Metamorphism
  7. 7. Metamorphism • This means the constituent minerals, texture and composition of parent rocks by metamorphism change over to new minerals or new texture or new composition which are more stable and suitable under new condition. For ex, As a result of metamorphism (i) Granite changes to gneiss, sandstone changes to quartzite, limestone changes to marble
  8. 8. Metamorphism Gneiss Quartzite Marble
  9. 9. Metamorphism • The range of temperature and pressure which occurs in nature is very wide. The normal surface temperature and pressure affect the rock by causing weathering. As accepted by many, the term metamorphism does not include either weathering of rocks or magma formation. The extreme state of metamorphism are represented by paligenesis or ultramorphism. In these the intensity of temperature and pressure will be so high that it involves partial melting and mixing of rocks. Since the identity of the altered rock is not totally destroyed, in these states, they come under metamorphism
  10. 10. Metamorphism
  11. 11. Metamorphic Agents • The process of metamorphism occur in rocks due to effect of high temperature, pressure and chemically active fluids. These three are known as metamorphic agents. Generally, all these three act together and cause metamorphism. But sometimes, any one or two of them dominate and play an active role. The following are a few relevant details about metamorphic agents.
  12. 12. Metamorphic Agents
  13. 13. Metamorphic Agents Temperature • The source of temperature which is responsible for metamorphism is either due to depth or due to the contact with magma. The metamorphic changes mainly take place in the temperature range of 350 – 850 0 C. The temperature rise also increases the chemical activity in rocks and facilitates reaction during metamorphism.
  14. 14. Metamorphic Agents Pressure • The pressure which causes metamorphism is of two different kind namely, uniform pressure and direct pressure. • Uniform pressure increases with depth. It acts vertically downwards and affects the volume of both liquids and solids. Naturally, its effect is significant only at great depths. But not at or near the surface. This also means that high temperature will also be associate with high temperature will also be associated with high uniform pressure. So, both of them act together and bring about metamorphism.
  15. 15. Metamorphic Agents • The direct pressure, which is also called stress, is due to tectonic forces. Such pressure act in any direction, i.e. upward or downward or sideward's It acts only on solids and affect the shape of rocks or minerals. It is effective in the upper layers of the crust and increases with depth to some extent. With further increase in depth, of high temperature and pressure the rocks will not be rigid and become plastic. • In this way the regions of influence of uniform pressure and direct pressure are mutually opposite. The application of stress gives rise to shearing movements in the rock and produces new minerals which have platy habit and arranges approximately parallel to each other.
  16. 16. Metamorphic Agents Chemically Active Fluids • Chemically active fluids play a key role in different ways in causing metamorphism. First Since metamorphism of any type cannot take place for the solid minerals in a perfect dry state, the presence of a liquid medium of some kind is indispensable. Thus the liquids act as a carrier of chemical components that actually take part in chemical reaction. In other words, liquids provide the necessary medium and facilitates the reaction to take place. The most common liquid which play such role is water. • Second The huge quantity of volatiles that are associated with magmatic bodies ultimately permeate through the surrounding rocks by means of diffusion and cause compositional changes even in rocks. Far from magma. • Third: the magma or the hot juvenile hydrothermal solutions may react directly with those rocks with which they come in contact.
  17. 17. Types of Metamorphism Thermal Metamorphism • All kinds of metamorphism in which heat plays a predominant role are given the common name “ thermal metamorphism”. The surrounding region of magma chamber or magma intrusion in which the heat effect is perceptible is called metamorphic aureole. Heat becomes an important factor at great depth and also in the vicinity of magma. In the latter, the country rocks are sometimes soaked in fluids emanating from magma. This results in mineral transformation.
  18. 18. Types of Metamorphism
  19. 19. Types of Metamorphism Dynamic Metamorphism (Direct Pressure Predominant) • The type of metamorphism that occur mainly due to direct pressure or stress, is called Cataclastic metamorphism, or dynamic metamorphism. With the little heat, when directed pressure acts, rocks are forced to move past one another resulting in their crushing and granulation. This kind of effect is called catalysis. Cataclasis is mere mechanical breakdown of rocks without any new mineral formation. • Dynamic metamorphism is also sometimes called dislocation metamorphism. This takes place along great fracture belts of the earth‟s crust.
  20. 20. Types of Metamorphism Geothermal Metamorphism (Uniform Pressure Predominant) • The type of metamorphism in which uniform pressure and heat are predominant is called static or load or geothermal metamorphism. • Usually this will not bring out any change in silicate rocks. But oceanic salt deposits, known for the variety of their minerals, have suffered considerable changes in this way.
  21. 21. Types of Metamorphism Metasomatic Metamorphism (Chemically active Fluids predominant) • The type of metamorphism in which significant compositional changes occur in the parent rock due to the predominant role played by chemically active fluids is called metasomatic metamorphism. This metamorphism alters the composition of rocks significantly. Due to this, similar rock may be formed out of different parent rock or different rocks may be formed out of the same parent rock. • The country rocks which are in contact with magma chamber or those rocks which come in contact with it during its upward journey as an intrusive body may react and change over to rocks of different composition.
  22. 22. Types of Metamorphism • Dynamothermal Metamorphism (Direct Pressure and Heat Predominant) • The type of metamorphism in which direct pressure and heat plays a dominant role is called dynamothermal metamorphism. Among the different kinds of metamorphism, this is the most common type. Gneisses and schists which are the most common metamorphic rocks are produced by dynamothermal metamorphism.
  23. 23. Types of Metamorphism • Platonic Metamorphism (Uniform pressure and heat predominant) • The type of metamorphism in which uniform pressure and heat dominate is called plutonic metamorphism. As this metamorphism is related to depth and overburden, it occurs at great depth which are also associated with high temperature.
  24. 24. • Plutonic metamorphism has the tendency to produce minerals which can accommodate more matter in less volume; for example transformation of limestone to marble. • Generally, this type of pressure –temperature effect will be insufficient to induce notable metamorphic effects in silicate rocks.
  25. 25. The Mechanism of Metamorphism • The change in minerals composition that occurs in any rock during its metamorphism are exclusively due to the chemical reaction under high temperature. The mechanism of such reaction is as follow: • The solid mineral constituents of the rock body cannot react as such in a dry condition. Therefore, they should either melt or undergo solution first, so that reaction take place and produce new minerals. In a view of very high melting point and latent heat of fusion of common rock forming minerals, it is natural that their fusion becomes improbable during metamorphism. In this way, reactions take place from point to point within the rock and slowly the whole rock formation is metamorphosed into new one, exhibiting a new mineral assemblage,
  26. 26. Visible Changes Produced by Metamorphism in Rocks • The following are some of the significant visible change that are produced as a result of metamorphism. • Crystallization of calcareous sedimentary rocks (i.e.. limestone) and re crystallization of some igneous and other sedimentary rocks. • Formation of new minerals which are diagnostic of metamorphism rocks. • Development of foliation with or without segregation of minerals of parent rocks • Formation of drags folds, joints, etc during the rock failure under pressure. • Formation of slaty cleavage in argillaceous rocks. This factor of course, represents a type of foliation itself that has developed excellent in a very fine grained rock.
  27. 27. Nature of Metamorphism with example • In nature “rocks under the influence of metamorphic agents incorporating such changes in themselves, so as to survive under new physicochemical condition”. The actual changes that occur in parent rocks under the influence of metamorphic agents and how such changes help the rock in its survival under new conditions can be appreciated by considering a few typical metamorphic rocks like granite, gneiss, marble, quartzite and slate.
  28. 28. Granite Gneiss • Granite, on metamorphism, produce granite gneiss. The difference or change observed in these two rocks is textural change, i.e. Mutual arrangement of constituent minerals. In granite, the different minerals (feldspar, quartz, biotite, and hornblende) are uniformly or randomly distributed throughout the rock. But in granite gneiss, these minerals get aligned i.e. Oriented in a direction normal to the stress of metamorphism. Granite Gneiss will be more strong and competent to withstand the pressure effect in a direction perpendicular to it.
  29. 29. Granite Gneiss
  30. 30. Marble • Limestone, on metamorphism, becomes marble, since limestone is amorphous it has a random arrangement of molecules. This lessens the strength of the rock. But during metamorphism, under the influence of pressure and temperature, the amorphous matter is formed to reduce in volume by crystallization, i.e. It making it possible for more matter to occupy less volume. This compactness and denseness accompanying the crystallization of limestone enables the rock to withstand greater load • Thus, the crystallization character introduce a suitable change in the parent limestone is advantageous for its survival by increasing its strength.
  31. 31. Marble
  32. 32. Quartzite • Quartzite is formed from sandstone under the influence of thermal or dynamic metamorphism. During metamorphism, the visible change that occurs is the disappearance of cementing material. This recrystallization of constituent material of the parent rock also causes the disappearances of bedding planes and reduction of porosity and permeability. Such a physical change not only makes the rock denser and stronger but also eliminates the inherent weakness of the parent rock caused by its cementing material and bedding planes. This newely acquired additional strength enables the rock to withstand the pressure effect that would have prevailed during metamorphism.
  33. 33. Quartzite
  34. 34. Slate • Slate is the product of metamorphism of shale. Shale is a sedimentary rock made up of random arranged clay particles. Under the influences of different metamorphic agents, an infinite number of very minute mica flakes not only originate but also align within the rock as innumerable parallel planes perpendicular to the direction of pressure or stress that prevailed during the metamorphism of shale. Thus, the change introduced in the parent rock during metamorphism is mineralogical and textural. These changes increase relatively the competence of the rock in the direction of pressure and enable it to withstand the stress effect during metamorphism.
  35. 35. Slate
  36. 36. Common Structures and Textures of Metamorphic Rocks • The process of metamorphism, which is always accompanied by pressure, induces alignment of constituent minerals in a rock. The alignment or orientation of minerals takes place perpendicular to the direction of the great stress. When platy, lamellar, blady or flaky minerals occurs in rocks, they orient themselves parallel to one another and of course perpendicular to the direction of the (greatest) pressure of metamorphism, such a texture is called foliation. When prismatic, columnar or rod like minerals occur in rocks they orient themselves parallel to one another and perpendicular to the direction of pressure of metamorphism. This is called lineation. In a section perpendicular to lineation, such a rock shows evenly granular texture
  37. 37. Foliation
  38. 38. Common Structures and Textures of Metamorphic Rocks • In other direction, it will appear linear. Talc schists, mica schists, chlorite schists, etc. shows good foliation, whereas hornblende schist, tremolite schists, staurolite schiists etc, shows good foliation, whereas hornblende schists, tremolite schists, staurolite schists, etc, shows good lineation. • Since a majority of natural rocks have minerals with shapes of the a fore mentioned kind, the alignment of minerals provides a readily recognizable clue to identify metamorphic rocks in hands specimens. Gneisses and schists which represents the bulk of metamorphic rocks exhibit this phenomenon.
  39. 39. Textures • In metamorphic rocks, the distinction between texture and structure is very vague; hence they may be considered synonymous. In some metamorphic rocks, textures of parent rocks are retained as relief features. In some other rocks, textures formed due to recrystallization occur in still others, both may be present together.
  40. 40. Textures Crystalloblastic and Palimpsest Textures • The textures which have developed newely during the process of metamorphism are called crystalloblastic textures. The other textures which belong to parent rocks but still retained in metamorphism rocks are called palimpset textures.
  41. 41. Crystalloblastic and Palimpsest Textures
  42. 42. Textures Xenoblastic and Idioblastic Textures • The crystalloblastic textures are of two kinds, namely xenoblastic and idioblastic. In the xenoblastic textures, the constituent minerals of the rock have no well-developed crystal faces. If the minerals have well developed crystal faces and forms, the texture is known as idioblastic.
  43. 43. Xenoblastic and Idioblastic Textures
  44. 44. Textures • In the description of metamorphic rocks, the crystalloblastic textures are named with the term “blast” as a suffix. Similar, the palimipset textures are named with the term “blasto” as a prefix. On the other hand, if such a texture were present in the parent rock and continuous to remain so in it ever after, metamorphism it is called blastoporphyritic.
  45. 45. Structures • The most common structures found in metamorphic rocks are gneissose, schistose and granulose types, though these are called structures, they are in fact, textures only because (i) they represent mutual arrangement of minerals in the rock, and also (ii) these are small-scale features which can be studied even in small hand specimen of rocks.
  46. 46. Gneissose Structure • If the rock consists of equidimensional minerals along with other (i.e.. platy and prismatic minerals) first segregation of the minerals occur and alternating bands are formed, then foliation and lineation of platy and prismatic minerals take place. Such a texture or arrangement of minerals is called gneissose structure. So, in gneissose structure, both equidimensional and other minerals occur in considerable proportions and they appear in alternating bands along with the alignment of prismatic and platy minerals.
  47. 47. Gneissose Structure
  48. 48. Schistose Structure • If the rock consists of only prismatic or platy minerals, then no segregation takes place but only foliation and / or lineation such a texture is called a schistose structure. So, in a schistose structure, platy or prismatic minerals will be dominating and they occur with orientation (alignment). If a few equidimensional minerals are present, they appear in between the layers of other minerals as discontinuous patches or lenses or bands or streaks.
  49. 49. Schistose Structure
  50. 50. Granulose Structure • If the rock is composed predominantly of equidimensional minerals, then neither segregation nor foliation takes place. Such a texture is called granulose structure. So, in granulose structure-bearing rocks, prismatic or platy minerals will be negligible or absent; only equidimensional minerals will be present. • The preceding relations are seen in rock types as granite gneiss, mica or chlorite schists and marble or quartzite.
  51. 51. Granulose Structure
  52. 52. Cataclastic Structure • In addition to the aforementioned structures, another structure called Cataclastic structure also occurs in metamorphic rocks. As the name indicates, it is produced under the influence of direct pressure in the upper zones of earth‟s crust. • Due to this, soft rocks like shales develop cleavage and hard rocks are shattered to produce crushed breccias. In some rocks more resistant minerals may remain unaffected, while the softer minerals are powdered to fine material. This result in an appearance similar to porphyritic texture and is called porphyroclastic structure.
  53. 53. Cataclastic Structure
  54. 54. Classification of Metamorphic Rocks • Metamorphic rocks have been grouped as orthometamorphic or parametamorphic rocks based on whether they have been formed out of igneous or sedimentary rocks. In a different way, metamorphic rocks can be continently classified based on their physical appearance , i.e.. as massive or foliated rocks.
  55. 55. Classification of Metamorphic Rocks Massive or Non-foliated Rocks • Three important rock types coming under this category are quartzite, marble and hornfels. Of these, quartzite and marble are formed out of contact, dynamic or dynamothermal metamorphism, while hornfels is always the product of contact metamorphism. Quartzite and marble have been described later. Hornfels is dense, fine grained and, for practical purpose, can be called impure quartzite.
  56. 56. Massive or Non-foliated Rocks
  57. 57. Classification of Metamorphic Rocks Foliated Rocks • Metamorphic foliation, are already explained, refers to the parallelism of in equidimensional minerals, which develops under the influence of pressure. More or less synonymous with foliation are the terms flow cleavage, schistosity and slaty cleavage. The common foliated rocks in the order of increasing grain size are; slaty, phyllite, schist and gneiss.
  58. 58. Foliated Rocks
  59. 59. Descriptive Study of Common Metamorphic Rocks Gneiss • Among different metamorphic rocks, gneiss is more widespread and abundant than others. • Gneiss is a general name given to any metamorphic rock which shows a gneissose structure. In many cases, gneisses are derived from granites. Hence, the minerals composition, grain size, color, etc, will be more or less similar both in granite and gneisses. For this reason, it is commonly referred to as granite gneiss.
  60. 60. Gneiss
  61. 61. Descriptive Study of Common Metamorphic Rocks • A few details of its physical description are as follows • Diagnostic character: Foliation Present • Color: different shades of gray and pink, but generally pale colored. • Grain size: Medium to coarse grained • Texture and structure: Generally equigranular, but sometimes porphyroblastic. Foliation i.e.. alignment of minerals is characteristically seen. • Minerals Present: Feldspar and quartz usually make up the bulk of a gneiss.
  62. 62. Gneiss
  63. 63. Descriptive Study of Common Metamorphic Rocks Origin • Gneisses are usually formed out of dynamothermal metamorphism of granite, sandstones and conglomerates, etc. Among these, granite gneiss is far more abundant than all others. Occurrence • Due to close mineralogical and other resemblance, granite gneisses may be treated as varieties of granites themselves.
  64. 64. Properties and Uses of Civil Engineering Importance • As gneiss is a silica-rich rock, it is durable • By virtue of mineralogical similarity to granite, gneisses also have pleasing color • Gneiss is reasonably non-porous and impermeable, contributing to its strength. • The gneissose structure with its alternating bands of contrasting colors on polishing, produces a very good appearance. • The foliation, to some extent, improves the workability of gneiss. • The occurrence of gneiss in plenty, in many places, makes it an important building stone or material
  65. 65. Gneiss
  66. 66. Descripative Study of Common Metamorphic Rocks Schists • Like gneiss schists is also a very common metamorphic rock and it is a general name given to all metamorphic rocks bearing a particular structure. Further, like gneiss it is also a characteristically foliated rock.
  67. 67. Schists
  68. 68. Descriptive Study of Common Metamorphic Rocks • Some of the important megascopic description of schists are as follows: • Color: Different schists show different color. For example mics schists is dazzling silvery white in color; biotite is jet black in color; chlorite schists is dark green in color and so on. • Grain Size: This is variable; some schists are fine grained, while others are medium grained or coarse grained. • Texture and structure: Constituent minerals occur with perfect alignment. Good lineation or foliation occurs, depending on when prismatic or platy minerals occur predominantly. As the name indicates, the characteristic structure shown by schist is always schistose.
  69. 69. Schists
  70. 70. Descriptive Study of Common Metamorphic Rocks Minerals present • Schists are normally composed of prismatic or platy minerals which contribute to the development of schistose structure. The prismatic minerals which occur commonly in schists are hornblende, sillimanite, tourmaline etc. Chlorite, muscovite, talc, kyanite etc. are commonly platy minerals occurring in schists.
  71. 71. Descriptive Study of Common Metamorphic Rocks • Origin: Schists are formed under the influence of dynamothermal metamorphism of different kinds of igneous and sedimentary rocks. • Mica schists is formed out of shale. • Mica-quartz schists is formed out of feldspatic sandstone. • Occurance • Schists occur in many places and frequently too because of the derivation from a large variety of rocks, especially out of shales which are very common sedimentary rocks.
  72. 72. Schists
  73. 73. Descriptive Study of Common Metamorphic Rocks • Schists are in general, considered weak, incompetent, harmful, and undesirable rocks from the civil engineering point of view Biotite, hornblende etc which occur as the usual dominant minerals make the rocks unpleasantly colored. • The mineral of schists such as talc, chlorite, biotite are relatively very soft. Hence, these rocks are not hard, strong and durable. Thus, the various properties of schists make it unsuitable for any important civil engineering purpose such as : • (i) Site rock for foundation • (ii) as a building stone • (iii) as an aggregate for concrete making • (iv) as road metal • (v) as railway ballast.
  74. 74. Descriptive Study of Common Metamorphic Rocks
  75. 75. Quartzite • Quartzite is a typical example of a parametamorphic rock. It is siliceous in composition and is formed out of dynamic or thermal metamorphism of sandstone. The sand grains which make up the bulk of sandstone are mainly quartz minerals or, in some cases feldspar. • A quartzite has following properties • Color : Uniform color throughout the rock. Generally white or pale colored; but red, brown, grey , green and other colors also may occur. • Grain size: Variable, some rocks are fine grained, while other may be coarse grained.
  76. 76. Quartzite
  77. 77. Quartzite • Fractures: In fine grained rocks the broken surface is concoidal or subconchoidal • Hardness: Very Hard, not scratched by glass or penknife • Appearance: Looks fresh and shining • Texture and structure: Granulose structure, it is crystalline dense and compact. • Minerals present • Quartz is essential or predominating constituent. Some quartzite are more or less exclusive composed of quartz. The other minerals which may occur in small quantities are mica, feldspar, kyanite, magnetite. Etc.
  78. 78. Quartzite
  79. 79. Properties and Uses of Civil Engineering Importance • Because of the predominance of quartz, the color of quartzite is generally white or pale colored and pleasing. The silica composition of the rock makes it highly durable and resistant to weathering. The predominance of quartz makes the rock very hard. The recrystallization process which produces these rocks makes them denser and stronger. • Thus the quartzite rock is strong, hard, durable and has a pleasing color. But by virtue of its very high hardness, the workability or dressing becomes very difficult. Quartzite rocks are highly suitable as road metal, railway ballasts, concrete aggregate, paving blocks.
  80. 80. Quartzite
  81. 81. Marble • The term marble is derived from marmore “a shining stone”. Like quartzite, marble is also a parametamorphic rock. It is a calcareous metamorphic rock formed out of the thermal metamorphism of limestone. Though it is not very hard or strong it is the most valuable rock occurring in nature. Its value is due to its pleasant color, good appearance, easy workability, charming translucency and the availability to take brilliant polish. It is also valuable because it is not abundantly available in nature.
  82. 82. Marble
  83. 83. Marble • Marble shows the following properties • Color: Some marbles show uniform color throughout the rock. Pure marble is milky white in color. Pleasant shades of green, yellow, brown, blue or grey color may also occur. • Grain Size: Fine medium or coarse grained, but the rock is equigranular. • Texture and structure: Marble shows a typical granulose structure. This is because calcite, which is neither platy nor prismatic, is the most predominant mineral of the rock.
  84. 84. Marble
  85. 85. Marble • Hardness: Marble is relatively soft and is easily scratched with a glass piece or penknife. For this reason, marble can be conveniently cut mechanically. • Appearance: When a hand specimen of polished marble is turned around, a twinkling appearance is noticed because the surface of the rock passes through cleavage planes, it is reflected giving a twinkling appearance. • Translucency: Pure white marble are reasonably translucent, which is an additional virtue of marble. • Mineral Present: Just as quartz is the most predominant mineral constituent of quartzite, so is calcite in marble.
  86. 86. Marble
  87. 87. Properties and Uses of Civil Engineering Importance • Occurance: As a geological formation, marble is found in the form of thick or thin beds in association with other metamorphic rocks like gneiss, schists and slate. • „as already mentioned, marbles are well known for their beautiful colors, pleasing appearance and easy workability. Being denser and less porous, it is of course, stronger than limestone. Marbles occurring in nature are quarried, dressed and used as building stones for temples or other construction. Marbles provide aesthetic beauty and a pleasing appearance to the construction. They are used in innumerable ornamental, decorative and monumental works where carving, engraving and polishing etc are involved.
  88. 88. Marble
  89. 89. Slate • Slate is a dense, fine grained, argillaceous, parametamorphic rock. It has the unique character of slaty cleavage. It is formed out of dynamic or regional metamorphism of shale. By virtue of its cleavage character. It is formed out of dynamic or regional metamorphism of shale. By virtue of its cleavage character, it splits easily into very thin sheets or slabs of considerable size. Extreme fine grained size, absence of reaction with acid, slaty cleavage and shining on surfaces are diagnostic character of slate.
  90. 90. Slate
  91. 91. Slate • Physical properties of slate are as follows • Color: Slate usually exhibit uniform color. Generally, they are black or dark grayish black. But other colors like brown, red, green and grey and yellow occurs. • Grain Size: Slate is very dense looking and extremely fine grained. Individual grains are too fine to be seen from the naked eye. • Texture: Foliation is clearly visible through constituent minerals are fine and unrecognizable. The layers of shale which are differently colored appear as ribbons or bands in slaty formation in fields.
  92. 92. Slate
  93. 93. Slate • Bedding and fossil content: Some slate retain their bedding or lamination character of shale as a relict feature. Fossil may occur rarely but such fossil are often distorted and squeezed out of their original shape. • Hardness: Slates are relatively soft when compared with other metamorphic rocks. • Minerals Present: Slates are mainly made up of secondary mica and quartz other minerals which may occur are biotite, talc, feldspar etc.
  94. 94. Slate
  95. 95. Slate Origin • The majority of slates are the result of dynamic metamorphism of argillaceous sediments. A few slates may also be formed from altered basic igneous rocks.
  96. 96. Properties and Uses of Civil Engineering Importance • Slates are dense, fine grained, impermeable and relatively resistant to decay. However, since slates are soft and incompetent, they cannot withstand great loads. So they are not suitable for foundation purposes. Due to cleavage character and softness, they split easily. Hence they cannot be used as building stone. But since slates are impermeable and can be split into thin but big slab of uniform thickness, they can be conveniently used for roofing, flooring, mantle and shingles etc. they can also be used as shelves.
  97. 97. Slate
  98. 98. References • Engineering and General Geology :By Parbin Singh • Textbook of Engineering Geology :N.Chenna Kesavullu
  99. 99. Thanks !