1.1 Introduction To Subject | unit 1 general geology mineralogy and petrology

E GEO

Unit - 1

General Geology, Mineralogy and Petrology

1.1 Introduction To Subject

Geology is the study of the earth (geo -means earth, and ology- means study of).Geologists study some of society most important problems, such as energy, water, and mineral resources; the environment; climate change; and natural hazards like landslides, volcanoes, earthquakes, and floods.

Further geology also includes the study of various physical, dynamic and physiochemical process operating on or within the earth and of the agents and force involved and evolved in such process. Geology is rightly considered as one of the fundamental basic sciences like physics, chemistry and biology.

1.2 Scope and Sub Divisions

Various Sub-Division of Geology

1. Physical geology:-The origin, development and ultimate fate of various surface features of earth and also with its internal structures is comes under physical geology. The deposition of rock bodies, water-bodies, and huge moving deposit of ice on the surface and their structure also comes under the subject of physical geology.

2. Geomorphoogy:-It also comes under physical geology. It deals with study of surface features of earth. It give detailed investigation regarding development and disposition of mountains, valleys, basin and plateaus and other various other landforms associated with them.

3. Mineralogy:-Mineralogy is the branch of geology which deals with formation, occurrence, aggregation, and use of minerals. Crystallography is a branch of mineralogy that deals with internal and external manifestations of minerals occurring in crystallized form in the natural process or manmade synthetic processes.

4. Petrology:-This is the important subdivision of geology and is further subdivide into three distinct types:

A] Igneous petrology

B] Sedimentary Petrology and

C] Metamorphic Petrology.

How the rocks formed, their types of occurrence, composition, textures and structure comes under this topic.

5. Historical Geology:-It deals with the study of rocks with references to their past. History of rocks gives their formation, constitution, structural disposition and composition etc. Paleogeography. Paleontology and sttigraphy are there subdivision of Historical Geology.

6. Economic Geology:-It deals with the study of minerals and rocks occurring on and in the earth that can be exploited for the benefit of man. These materials include precious and base metals, non-metallic minerals, construction-grade stone, petroleum, natural gas, coal, and water.

Scope of Geology:-

1. Knowledge of ground water Geology is necessary in connection with excavation works, water supply, irrigation and other purposes.

2. Wetland and habitat restoration programs.

3. It investigate geological phenomena such as earthquakes and volcanoes.

4. Geology is useful to know the method of mining of rocks and mineral deposit on earth surface and subsurface

5. Coastal engineering, sand replenishment, bluff or esa cliff stability, harbour, pier etc.

6. Planning, Designing, and construction of civil related works.

7. Governmental and military installation.

1.3 Earth as a Planet

Earth is one of nine planets orbiting almost a 4.5 billion year old medium-sized star we call the Sun. It takes around 365.26 days to make 1 rotation, while spinning on its axis once every 23.93 hours.

Earth’s orbit lies about 149.59 million kilometres from the Sun in between those of Venus and Mars. All Together Mercury, closest to the Sun, Venus, Earth, and Mars make up a group called the “terrestrial planets.” The Earth is the largest, densest, most massive of these four rocky planets, the only world where water exists not only as a surface liquid, but also as a solid and gas. The outer planets, Jupiter, Saturn, Uranus, and Neptune are made up from hydrogen (although Uranus and Neptune contain a large proportion of ice) and are known as the “gas giants.”

The Origin of Earth

The solar system is formed around almost 4.5 billion years ago from a huge cold cloud of gas and dust called the “solar nebula.” With 90 % of the mass at centre, the cloud contract and was heated by gravitational compression, forming the Sun.

The accumulation of matter around the central mass began to form individual composing of material called “proto-planets” before the solar wind drove outward the lighter elements making the distribution of masses, densities we see in the solar system today.

Whether the early Earth started out hot or cold, as this depends on whether it accumulated slowly (100 million to 1 billion years) or quickly (100 000 to 10 million years). Scientists now lean towards a fast accumulation theory, which mean that the Earth remained molten throughout much of its early past.

The Earth would been a cluttered mixture of materials, growing in size as it composed up smaller objects in its path. During this time the heavier materials fall towards the centre, making the core, and the lighter materials rose to the surface, making the mantle and the crust. The process of separation is known as differentiation, and is played a key role in the formation of terrestrial planets.

1.4 Interior and General Composition of the Earth's

The Earth is unique among the planets. It has an abundance of water. Other worlds — involving a few moons — have atmospheres, ice, and oceans, but only Earth has the right combination to support life.

Oceans cover almost 70 percent of the planet's surface with a depth of 2.5 miles (4kilometers). Fresh water make up in liquid form in lakes and rivers and as water vapour in the atmosphere, it causes much of Earth's weather.

Earth has multiple layers. The ocean basins and the continents makes the crust, the outermost layer. Earth's crust is between three and 46 miles (five and 75 km) depth. The thickest parts are beneath the continents and the thinnest parts are under the oceans.

Crust

According to "Essentials of Geology" by Frederick K. Lutgens and Edward J. Tarbuck, Earth's crust is composed up of several elements: oxygen 46.6 percent by weight; silicon 27.7 %; aluminium 8.1 percent; iron 5 percent; calcium, 3.6 percent; sodium 2.8 percent, potassium 2.6 percent, and magnesium, 2.1 percent.

Crust is subdivided into huge plates that float on the mantle. The plates are constantly in motion, they move at about the same rate as fingernails grow, according to NASA. Earthquakes occur when these plates crush against each other. Mountains form when the plates hit and deep trenches form when one plate skid under another plate.

Mantle

Mantle under the crust is almost 1,800 miles deep (2,890 km). It is made up mostly of silicate rocks rich in magnesium and iron. Intense heat makes the rocks to rise. Then cool and sink back down to the core. This move — with the consistency of caramel — is thought to be what causes the tectonic plates to move. When mantle pushes through the crust, volcanoes erupt.

Core

The centre of the Earth is the core .It has two parts. The solid, inner core of iron has a radius of 760 miles (about 1,220 km), according to NASA. It is adjacent by a liquid, outer core composed of a nickel-iron alloy. The outer core is 1,355 miles (2,180 km) thick. The inner core spins at a speed than the rest of the planet. It is thought to cause Earth's magnetic field.

When charged particles from the solar wind hit with air molecules above Earth's magnetic poles, it makes the air molecules to glow, causing the auroras — the northern and southern lights

1.5 Rock Cycle

There are three types of rock signeous rocks, sedimentary rocks and metamorphic rocks. Sedimentary rocks form by breaking down other kinds of rocks into small particles and washing or blowing them away; metamorphic rocks form from other rocks and igneous rocks form by melting other rocks. Thus rocks are always changing form and are redistributed as part and form as cycle. This cycle is called the Rock Cycle.

1.6 Introduction to mineralogy:-

Physical Properties of Minerals

The various physical properties of minerals are as follows:-

1. Color

2. Luster

3. Streak

4. Hardness

5. Cleavage

6. Parting

7. Fracture

8. Tenacity

9. Structure

10. Form

11. Specific Gravity

Classification of Minerals

Color:- Color is a light dependent property. Easy to see but is least reliable property. Many different minerals are same color (calcite, quartz, Dolomite). Same mineral can come in many colors (quartz can be pink, white, clear, purple, yellow)

A mineral may belong to any one of three types:-

A] Idiochromatic- fairly constant colour related primarily to the composition of minerals.

Example are as follows malachite, peridot, and almandine.

B] Allochromate- variable colour, examples are as follows like quartz, calcite, Fluorite etc. Basically this color impart due to impurity.

C] Pseudochromatic- Those color are due to light diffraction and the colour is false colour.

2. Luster:-it is called as shine of mineral or reflection of light from surface of minerals. It is important to know that whether a mineral is metallic or not for the properties of luster which are given as:-

1. Metallic -looks like a piece of broken or polished metal. Examples are pyrite and galena.

2. Submetallic - has a high luster that is transitional between that of broken metal and that of broken glass. An example is black sphalerite.

3. Non-metallic-does not look like metal.

Properties of Luster:-

A] Reflective surfaces

B] Capacity of mineral to absorb light

C] Refractive Index (Since refractive index is a fundamental physical property of a substance, it is often used to identify a particular substance, confirm its purity, or measure its concentration.)

3. Streak:-The color of a mineral when it is powdered is called the streak of the mineral. Crushing and powdering a mineral eliminates some of the effects of impurities and structural flaws, and is therefore more diagnostic for some minerals than their color.

4. Hardness:-The resistance of mineral which offer outer changes example as scratching, abrasion, rubbing, etc. are called as Hardness of minerals or the hardness of a mineral is its ability to resist scratching, the mineral hardness scale of Mohs is based on the ability of one natural mineral sample to visibly scratch another mineral. All different minerals are the samples of matter used by Mohs. Minerals are naturally found pure substances. Rocks consist of one or more minerals. Diamonds are at the top of the scale as the hardest known naturally occurring substance when designing the scale

The mohs scale of hardness comprises ten mineral arranged in order of ascending hardness, the softest is assigned a value of 1 and hardest as 10.

Mohs hardness	Mineral	Chemical formula
1	Talc	Mg3Si4O10(OH)2
2	Gypsum	CaSO4·2H2O
3	Calcite	CaCO3
4	Fluorite	CaF2
5	Apatite	Ca5(PO4)3(OH−, Cl−, F−)
6	Orthoclase Feldspar	KAlSi3O8
7	Quartz	SiO2
8	Topaz	Al2SiO4(OH−,F−)2
9	Corundum	Al2O3
10	Diamond	C

5. Cleavage:-It is defined as the mineral to break along certain definite direction yielding more or less smooth, plane surfaces. Cleavage is the result of weaker bond strengths or greater lattice spacing across the plane in question than in other directions within the crystal. Greater lattice spacing tends to accompany weaker bond strength across a plane, because such bonds are unable to maintain a close interatomic spacing

6. Fractures:-Fracture is the tendency of a mineral to break along curved surfaces without a definite shape. These minerals do not have planes of weakness and break irregularly.

Common type of fractures are as follows:-

1] Even: example Chert

2] Uneven: ex Fluorite

3] Conchoidal: ex Quartz

4] Hackly: ex Native copper

5] Earthy: ex Chalk

6] Splintry: ex Kyanite.

Silicate & Non silicate minerals

Different kinds of minerals exist. It is divided into two broad classes, the silicate and non-silicate minerals. The silicates are richest, although, non-silicates are very common as well.

The two exhibit differences not in their composition but also in their structure. The structure of silicates tends to be more web, while the structure of non-silicates features a deal of variability.

Silicate Minerals

Silicate minerals composed of silicon and oxygen -- the two richest elements in the Earth's crust. Silicates are by far the richest among the two groups of minerals, comprising some 75 % of all known minerals and 40 % of the most common minerals.

Igneous rocks are made from silicate minerals; mostly metamorphic and sedimentary rocks are made from silicates as well. It can be subdivided into smaller groups on the basis of their structure.

Composition of Silicates

Silicates are subdivided into different groups based on their structure. The first of these is the neo-silicates, which is made from atoms arranged in four-sided shapes called tetrahedral, with four oxygen on each unit that can join with atoms arranged in other shapes composing of positively charged ions (cations) like aluminium or potassium. Soro silicates have units of two tetrahedral which share one oxygen atom, while cyclo silicates have rings of tetrahedral, each tetrahedron have two oxygen atoms with its neighbours.

Cations can become confined in the centre of these rings. Ino silicates has continuous chains of tetrahedral units, each atom of which shares two oxygen. Phyllo silicates have sheets of tetrahedral, each shares three oxygen with immediate neighbours; the sheets are divided by other groups and arrangements, and cations can be trapped in the spaces in between tetrahedral. Tecto silicates have a continuous framework of tetrahedral, each shares all four oxygen atoms with its neighbours.

Non-Silicates

Non-silicates are minerals that do not composed of silicon-oxygen unit characteristic of silicates. It contain oxygen, but not in fused with silicon. Its structure tends to be more variable &less complex than that of the silicates, it further divided into different classes based on their formation.

Sulphates include the sulphate anion, SO4 with a minus 2 charge, oxides include oxygen in partnership with a metal like aluminium. Many of the non-silicates are economically important, especially that include valuable metals.

Examples

Examples of silicate minerals include quartz, olivine and garnet minerals. Quartz is especially common; sand, for example, is made up of primarily of quartz. One abundant non-silicate mineral is pyrite, or "fool's gold," a compound of iron and sulphur well known for its e metallic luster.

Rock-Forming Minerals

Rocks are made up of minerals. A mineral is a naturally occurring substance is usually solid, crystalline, stable at room temperature and inorganic.

There are about 5000 known mineral species, but vast majority of rocks are formed from composition of a few common minerals, known to as “rock-forming minerals. For example: feldspars, quartz, amphiboles, micas, olivine, garnet, calcite, pyroxenes.

Minerals occurring in a rock in small form are referred to as “accessory minerals”. Accessory minerals are present in only small form, it provide valuable insight into the geological past of a rock, and used to ascertain the age of a rock. Common accessory minerals are: zircon, monazite, apatite, titanite, tourmaline, pyrite and other opaque.

The richness and diversity of minerals depend on the richness in the Earth’s crust of the elements of which they are made. Eight elements make up 98% of the Earth’s crust: oxygen, silicon, aluminium, iron, magnesium, calcium, sodium and potassium. The formation of minerals formed by igneous processes is controlled by the chemistry of the parent body. For example, a magmais rich in iron and magnesium will form minerals such as olivine and pyroxene (as found in basalt). .

What Are Rock-Forming Minerals?

Feldspars

Feldspars (KAlSi3O8–NaAlSi3O8–CaAl2Si2O8) is a collection of rock-forming tectosilicate minerals which make up by weight about 41% of the mainland surface of the Earth. In both intrusive and extrusive igneous rocks, feldspars crystallize from magma as veins and also available in many kinds of metamorphic rock. It is known as an orthosite rock made almost of calcium plagioclase feldspar.

Quartz

It is a mineral composed of carbon and water particles in a constant frame of SiO4 silicon-oxygen tetrahedral, share each carbon between two tetrahedral, giving SiO2 a general chemical formula. Quartz is Earth’s second most common mineral, behind feldspar, in the continental crust.

Two forms of quartz, the normal α-quartz and the β-quartz high-temperature, both chiral. There is a huge transformation from α-quartz to β-quartz at 573 ° C (846 K). Since the transition is followed by a quantity shift, ceramics or rocks that pass through this temperature limit can readily be induced to fracture.

Amphibole

Amphibole is a significant cluster of inosilicate minerals which form prisms or needle-like crystals, made up of SiO4 tetrahedral double chain, connected at the vertices and usually carrying ions of iron and/or magnesium in their constructions. It can be green, black, white, yellow, blue, or brown. It is presently classified by the International Mineralogical Association as a mineral super group, in which there are two categories and several subgroups.

Mica

Mica group of sheet silicate (phyllosilicate) minerals consist of several near-perfect basal cleavage associated products. They are all monoclinic, with a tendency to pseudo hexagonal crystals, and in chemical composition are close.

The term mica comes from the Latin term mica, it means a crumb to glitter, and is likely affected by micare.

Olivine

Mineral olivine is a formula (Mg2 +, Fe2+)2SiO4 zinc iron silicate. It is a kind ofnesosilicate or orthosilicate.

Olivine consist of only small quantities of non-oxygen, silicon, magnesium and iron components. The extra components found in the greatest levels are manganese and nickel.

Garnet

Garnets are a set of minerals of silicate which is used as gemstones and abrasives since the Bronze Age.

All garnet species have special physical and crystal shapes, but vary in chemical composition. The various species are pyrope, almandine, spessartine, gross, uvarovite and and andradite.

Two solid solution series are made up of garnets: pyrope-almandine-spessartine and uvarovite-grossular-andradite.

Calcite is a mineral carbonate and stable calcium oil polymorph (CaCO3). The mineral hardness scale of Mohs, based on the contrast of scratch hardness, describes value as “calcite”.

Pyroxenes

Pyroxenes (frequently shortened to Px) are a set of significant minerals found in many igneous and metamorphic rocks which form rock inosilicates. Pyroxenes have overall formula XY(Si, Al)2O6 where X depicts calcium, sodium, iron (II) or potassium and commonly zinc, manganese or lithium, and Y includes ions of lower magnitude such as chromium, aluminium, iron (III), magnesium, cobalt, manganese, scandium, titanium, vanadium or even metal (II).

In silicates such as feldspars and amphiboles, aluminium widely replaces silicon, the replacement happens in most pyroxenes only to extent. It share a framework made up of silica tetrahedral single chains.

1.7 Igneous Petrology: Plutonic, Hypabyssal And Volcanic Rocks, Structure, Texture And Classification Of Igneous Rocks.

Igneous Petrology:-

Igneous rocks are one of three main types of rocks (along with sedimentary and metamorphic), and they include both intrusive and extrusive rocks. A very high temp and molten state are 2 very important conditions for the original material from which the igneous rocks are believed to have been formed.

Approximately 90% rocks are igneous rocks in earth surface. These rocks are formed in 2 types that is by Extrusive and Intrusive igneous rocks which are as follows:-

Extrusive igneous rocks can be in the form of:-

lava flows

ash

Intrusive rocks can be in the form of:-

batholiths

dykes

sills

laccoliths

Classification of Igneous Rocks:-

A] Volcanic Rocks:-

These rocks are formed on the surfaces of the earth by cooling and crystallization of lava erupted from volcanoes. Volcanic rocks are rocks formed when lava cools and solidifies on the earth's surface. Volcanic rocks are also known as 'extrusive igneous rocks' because they form from the 'extrusion,' or eruption, of lava from a volcano. The basalts of Columbia Plateau and Prana plateau of Brazil are examples of volcanic rocks.

B] Plutonic Rocks:-

These rocks are formed at depth of 7-11Km below earth surface. Pluton is a body of intrusive igneous rock (called a plutonic rock) that is crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, stocks, dikes, sills, laccoliths, lopoliths.

Examples are as follows gabbros, syanites and Granites etc.

C] Hypabyssal Rocks:-

These rocks are formed at the depth of 2Km below the surface of earth and give the properties of both volcanic and plutonic rocks. These are the intermediate in texture between coarse-grained intrusive rocks and fine-grained extrusive rock.

Various Structure of Igneous Rocks

Primary Structures developed in the igneous rock during the time of cooling, crystallization and solidification of magma or lava. Since, there are two main classifications of igneous rocks: intrusive igneous rocks and extrusive igneous rocks, therefore, Primary structures in the igneous rock may also be classified into two main divisions:

1. Intrusive or Plutonic Igneous Rock Structures

2. Extrusive Igneous Rock Structures.

Intrusive Igneous Rock Structures

Concordant Intrusion: Concordant Intrusion are those which are parallel to the bedding plane, i.e. they don’t disturb the individual layers of country rocks but however they deposited with the bed or layer.

Laccoliths

Lopoliths

Sills

Discordant Intrusion: Discordant Intrusion are those which are Perpendicular to the bedding plane, i.e. they cuts across the bedding layers of country rocks.

Dike

Stock

Batholith

Extrusive igneous rocks can be in the form of:-

lava flows

ash

Intrusive rocks can be in the form of:-

batholiths

dykes

sills

laccoliths

Various Textures of Igneous Rocks

Textures defined the size, shape and arrangement of these constituents within the body of rocks. The grain size of an igneous rocks depends on the rate of cooling of magma, for this 4 points are considered:-

1. Degree of crystallization:-

A] Holocrystalline Texture:-Rock is made up of full crystal is defined as Holo crystalline

B] Merocrystalline texture:-Rock is made up partly by glass and partly by crystal is defined as merocrystalline.

C] Holohyaline texture:-Rock is made up of full glassy material is defined as Holohyaline.

Size of grains:-

In rapid cooling the mineral grains crystallize quickly as a mass of tiny crystals which are generally less than 1 mm in size. Some common grain shapes are:

Tabular - a term used to describe grains with rectangular tablet shapes.

Equant - a term used to describe grains that have all of their boundaries of approximately equal length.

Fibrous - a term used to describe grains that occur as long fibres.

Acicular - a term used to describe grains that occur as long, slender crystals.

Prismatic - a term used to describe grains that show an abundance of prism faces.

The texture of phaneric igneous rocks can be further subdivide into as:-

1. Coarse grained (more than 5mm in diameter)

2. Medium Grained (1mm to 5mm in diameter)

3. Fine grained (less than 1mm in diameter)

4. Microcrystalline (can be seen by microscope)

5. Cryptocrystalline (texture are very small i.e. not visible)

3. Shape of Crystal:-

They are as follows:-

1. Euhedral (crystal faces)

2. Subhedral (partly crystal faces)

3. Anhedral (crystal faces are absent)

4. Mutual Relations of Grains:-

These are classified as four major groups which are as follows:-

A] Equi-granular texture

B] In Equi-granular texture

C] Directive texture

D] Inter growth Texture

Ophitic texture - laths of plagioclase in a coarse grained matrix of pyroxene crystals, wherein the plagioclase is totally surrounded by pyroxene grains. This texture is common in diabases and gabbros.

Subophitic texture - similar to ophitic texture wherein the plagioclase grains are not completely enclosed in a matrix of pyroxene grains.

Poikilitic texture - smaller grains of one mineral are completely enclosed in large, optically continuous grains of another mineral.

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Inter granular texture - a texture in which the angular interstices between plagioclase grains are occupied by grains of ferromagnesium minerals such as olivine, pyroxene, or iron titanium oxides

Hyalopilitic texture - a texture wherein microlites of plagioclase are more abundant than groundmass, and the groundmass consists of glass which occupies the tiny interstices between plagioclase grains.

Trachytic texture - a texture wherein plagioclase grains show a preferred orientation due to flowage, and the interstices between plagioclase grains are occupied by glass or cryptocrystalline material.

1.8 Study Of Common Rock Types Prescribed In Practical Work And Their Engineering Applications.

Igneous rocks as engineering materials:-

Examples like granites, dolerites are characterized by very high crushing strength and hence can be used for construction works. Pumice is light in weight it can be used for decorative purposes. Crystalline rocks are used in jewellery and other similar things. Granite looks smooth and shiny when polished and is the most commonly used. The rough surface of pumice makes it a good abrasive for cleaning and polishing. The most common types of igneous rocks are:

Andesite.

Basalt.

Dacite.

Dolerite (also called diabase)

Gabbro.

Diorite.

Peridotite.

Nepheline

1.9 Sedimentary Petrology: Sedimentary Petrology: Rock Weathering, Sedimentary Structures, lithification and digenesis Process, Genetic Classification of Secondary Rocks and Grain Size Classification and Textures.

Sedimentary petrology and rock weathering

When rocks (igneous, sedimentary, or metamorphic) are at or near the surface of the earth they are exposed to the processes of weathering.

In mechanical weathering rocks are broken up into smaller pieces by frost-wedging ,tree and other plant roots growing into cracks, and abrasion caused by for example, sand-blasting of a cliff face by blowing sands in the dessert, or the scouring of water transported sand, gravel, and boulders on the bedrock of a mountain stream. Mechanical weathering breaks rocks into smaller and smaller pieces.

In chemical weathering minerals are changed into new minerals and mineral by products. Some minerals like halite and calcite may dissolve completely. Others, especially silicate minerals, are altered by a chemical process called hydrolysis. Hydrolysis is the reaction of minerals in weakly acidic waters. Most natural surface waters are slightly acidic because carbon dioxide from the air dissolves in the water. Some of the dissolved CO2 reacts with the water forming the chemical compound carbonic acid

Genetic Classification of Secondary Rocks and Grain Size Classification

Sedimentary rocks may be divided into three basic categories:-

1. Clastic Sedimentary Rocks

These are broken fragment of parent rocks which are already existing sedimentary. Clastic sedimentary rocks may first be classified according to their grain size. Clay-sized particles are too small to be seen with a microscope.

Castic sediments are classified as:

i. Rudaceous Rocks-gravel, pebbles and boulders are example of it. "Gravel"-sized grains range from > 2 mm granules to very large boulders. Rock containing these large size particles are called conglomerate and are typically very poorly sorted (e.g., they may contain, sand, gravel, and boulders all in one rock). If the gravel particles are little weathered and are still angular (un-rounded) the rock is called breccia.

ii. Arenaceous Rocks-Sand stone and grit are the example.

iii. Argillaceous Rocks-very fine grained sediments such Rock formed from clay-size particles are called shale. Silt-sized particles are visible with a microscope.

Rock formed from these are called siltstone. Sand-sized grains are visible to the naked eye and range from 1/16 mm to 2 mm.

Sand is further subdivided into very fine, fine, medium, coarse, and very coarse. Rock formed from these are called sandstone.

2. Biogenic Sedimentary Rocks

Carbonate Rocks (based on CO3). While some carbonate rocks form as simple chemical precipitates most carbonate rocks are the product of marine organisms such as molluscs and corals. They precipitate calcite (calcium carbonate, CaCO3) or other similar carbonate minerals directly from the dissolved chemicals in the water to create their shells. Limestone is the product. At some later time (e.g. after burial) calcite may be transformed into dolomite, CaMg(CO3)2. Calcite will react vigorously with dilute hydrochloric acid (HCl). Powdered dolomite will react sluggishly with HCl.

Coal is also formed by biological activity but in this case the material is organic matter from decaying plants that may accumulate if plant growth is faster than the rate of decay. The organic matter will be buried and compacted by layer upon layer of partially decayed plants, eventually becoming coal.

3. Chemically formed Rocks

These are formed when mineral matter in solution is precipitated from water where the dissolved ions encounter supersaturated conditions they come out of solution and combine together forming an orderly arrangement of atoms .They are said to precipitate - go from the liquid, dissolved state to the solid crystal state. These are classified as:-

1. Carbonate rocks-ex limestone and dolomites. These are formed with precipitation of calcium carbonate from sea

2. Salt rocks-ex Gypsum and anhydrite. These are formed when evaporation is major role played by atmosphere in deposition of chemical precipitation. The main minerals of these deposits are chlorides and sulphate of Na, K, Mg and Ca.

3. Freuginous rocks-ex Hematite, Pyrite and Siderite. These are formed by precipitation of iron oxide.

Rocks formed in this way include halite, gypsum, anhydrite, and some limestone. Layers of precipitated rocks are called Evaporate deposits because they typically form where evaporation is high in arid regions like the desert southwest and in the eastern Mediterranean

Sedimentary structure:-

Stratification. -Most sedimentary rocks contain internal layering called bedding. This may be developed very prominently and can be seen from distance of miles or in other cases may have to be ascertained after close examination of the rocks. Stratification may range from a bed thickness of many meters down to fine millimetre size laminations. Bedding is generally horizontal or nearly so.

.Planes of weakness –the bedding planes separate the bed from each other. Large scale cross-bedding in sandstone, within horizontal layers a few to many feet thick, indicates deposition in sand dunes.

2. Ripple marks-these are commonly seen in shallow water environment. They are defined by asymmetric and symmetric ripples. Ripple marks indicate deposition in a current. Asymmetric ripples (one side steeper than the other) indicates a consistent current direction as in streams. Symmetric ripples indicate oscillating (waves) or weak currents.

3. Mud cracks- These are the common structural features of many fine grained sedimentary rocks. Mud cracksare produced by drying of wet muds. Once these cracks are covered under further layer of mud, they get preserved in the body of deposit. Raindrop impression may also be preserved in sediments. They indicate deposition in a terrestrial setting.

4. Fossiliferrous structure is are very important indicators of depositional environment. Sedimentary rocks are known as the only source of fossils. Fossils include preserved skeletal fragments, plant roots, etc., and also trace fossils such as burrows, footprints, leaf impressions, etc. Coral and many shell fossils indicate marine deposition.

5. Rain print- It’s is also like mud cracks. They are formed irregular small crater shaped depression seen on fine grained dried sediments.

6. Cross Bedding-Internal stratification within a larger bed may be parallel or there may be cross-stratification caused by ripples, sand bars, and dune structures.

7. Lamination-layered structure similar to bedding as found in sedimentary rocks. The individual layer are called as lamina and are distinguished commonly on the basis of difference in colour.

Sedimentary texture:-

Sedimentary rock show considerable variation in their texture which depend on a no.of factor:

1. Origin of grains: Clastic or non-clastic textures

2. Size of grains-

a) Coares grained ->5mm

b) Medium grained-5mm -1mm

c) Fine grained->1mm

3. Shape of grains-> Brecciase and conglomerates

4. Packing of grains-open packed or porous

5. Fabric of grains

6. Crystallization trend-crystalline granular, texture or amorphous

Lithification process

The processes by which loose sediment is hardened to rock are collectively called LITHIFICATION. After this, continued erosion and re-transport of the sediment become much more difficult. There are three basic ways that lithification is accomplished.

First way to harden sediment is simple COMPACTION. Coarser sediment particles usually do not lithify in this way. Clay minerals are different. These tiny particles are very much like miniscule, plate-shaped mica grains. The electrical charges tend to make them stick to each other. As with Van der Waals bonds, the attraction is not terribly strong.

A second natural method of lithifying sediment is by RECRYSTALLIZATION. Limestone and other chemical sedimentary rocks are most permitting to this process because their minerals are easy to adapt in surface environments. CaCO3 in the Earth's ocean is not calcite but a different mineral called "aragonite". It is not really stable in fresh water, so once rainwater comes into contact with it, it changes into calcite.

The common method of lithifying coarse grained sediments is by CEMENTATION. Sedimentary particles are deposited in touch with each other, but there is also a certain amount of void space in a pile of sediment. As water carrying dissolved ions fills in that empty space, the ions may crystallize new minerals between the grains

Diagnesis process

It is the physical and chemical processes that affect sedimentary materials after deposition and before metamorphism and between deposition and weathering. The effects of diagenetic processes on rock properties such as porosity and the degree of lithification are progressive.

1.10 Study Of Common Rock Types Prescribed In Practical Work And Their Engineering Applications.

Lime stone can be used for variety of purposes including highway and building construction. Rock gypsum is used to make plaster. Sandstone are used for building stone.

Sedimentary rocks are also important sources of natural resources like coal, fossil fuels, drinking water or ores. Sand and gravel for construction come from sediment

1.11 Metamorphic Petrology: Agents, Types of Metamorphism, Texture and Structures

Metamorphic Rocks

Metamorphic rocks are defined as those rocks which are formed through the pre-existing igneous and sedimentary rocks involving chances in textures, structures and mineralogy. Metamorphic rocks have been changed over time by extreme pressure and heat. Metamorphic rocks can be formed by pressure deep under the Earth's surface, from the extreme heat caused by magma or by the intense collisions and friction of tectonic plates. Uplift and erosion help bring metamorphic rock to the Earth's surface. Examples of metamorphic rocks include anthracite, quartzite, marble, slate, granulite, etc.

1. Coal with a high carbon count give Anthracite

2. Basalt to Granulite

3. Mudstone to Slate

4. Sandstone to Quartzite

5. Limestone to Marble

Various agents of Metamorphic Rocks:-

1] Heat- .it contributes to the process in two ways. First, atoms may combine differently at different temperatures. This means that a mineral stable at one temperature might become unstable at a higher (or lower) temperature and be converted to a different mineral with a more stable atomic structure. This may or may not involve changing the exact elemental composition. Second, heat makes practically all chemical reactions go faster, meaning that mineral transformations are much easier at higher temperature.

2]Pressure-Any given rocks at some depth below the surface is subject to pressure from 2 sources-first load of overlying burden and second crystal movement during the convergence of tectonic plates. Some minerals may be converted to minerals with similar composition but different atomic packing simply because pressure is increased. The exact nature of the pressure is not important in this case, only the amount.

Thus pressure created by deep burial of rocks under sediment may have this effect as as direct pressure by converging plates. The second effect of pressure is to reorient minerals with linear or platy structure or to create a preferred orientation of them as they form. Thus elongate minerals such as amphiboles, or platy minerals such as clays or micas tend to align themselves parallel to each other when under pressure.

3] Fluids-.With the rise in temp. The pore fluids undergo expansion and becomes very active in disturbing or even breaking the original crystal boundaries of the involved minerals serve only to speed up other metamorphic processes, or even allow them to happen at all. Chemical reactions require water, and most proceed much faster as the amount of water goes up. Dissolved ions in the fluid also make those mineral transformations that require chemical changes in the minerals to occur, whether by supplying needed ions or flushing away excess ones.

Types of Metamorphism:-

The various types of metamorphism are as follows:-

1. Cataclastic metamorphism

2. Dynamic metamorphism

3. Contact metamorphism

4. Plutonic metamorphism

5. Regional metamorphism

6. Metasomatism and

7. Retrogressive metamorphism.

1. Cataclastic metamorphism:-

When directed pressure or lateral stress play dominant role in metamorphism then the process is called as Cataclastic metamorphism. Cataclastic metamorphism occurs as a result of mechanical deformation, like when two bodies of rock slide past one another along a fault zone. Heat is generated by the friction of sliding along such a shear zone, and the rocks tend to be mechanically deformed, being crushed and pulverized, due to the shearing. Examples are mylonites, etc.

2. Dynamic metamorphism:-

When there is some increase in temp with high pressure then process is called as dynamic metamorphism. These ultrahigh pressures can produce minerals that are only stable at very high pressure, such as the SiO2 polymorphs coesite and stishovite. In addition they can produce textures known as shock lamellae in mineral grains, and such textures as shatter cones in the impacted rock.

2. Contact metamorphism:-

The other name is given as THERMAL METAMORPHISM. Contact metamorphism occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion.

Since only a small area surrounding the intrusion is heated by the magma, metamorphism is restricted to the zone surrounding the intrusion, called a metamorphic or contact aureole. Outside of the contact aureole, the rocks are not affected by the intrusive event.

3. Plutonic metamorphism:-

In the high depth of earth high temp and static load works together the metamorphism caused by these factors is called the plutonic metamorphism, examples are Granulite.

4. Regional metamorphism:-

When pressure and heat act together in the hydrothermal fluids the rocks are metamorphosed over the wider area this is the logic behind the Regional Metamorphism. Regional metamorphism occurs when rocks are buried deep in the crust. This is commonly associated with convergent plate boundaries and the formation of mountain ranges. Because burial to 10 km to 20 km is required, the areas affected tend to be large, examples are Foliate rocks splits easily into flaky sheets.

6. Retrogressive metamorphism:-

When high temperature metamorphic mineral assemblages are change to low temperature mineral assemblages the process is called as Retrogressive metamorphism. In general, the changes in mineral assemblage and mineral composition that occur during burial and heating are referred to as prograde metamorphism, whereas those that occur during uplift and cooling of a rock represent retrograde metamorphism

Various Textures of Metamorphic Rocks:-

The 2 general types of textures being recognized in metamorphic rocks are as follows:-

1. Crystalloblastic Textures:-the new formed textures during process of metamorphism are called as Crystalloblastic Textures, example: Mortar(deformational texture resulting from reduction in grain size in matrix surrounding larger original grains),poikiloblastic, porphyroblastic(large crystal mineral like garnet or and alustic grown in metamorphic rocks which is surrounded by smaller grain of other mineral, similar to phenocryst of igneous rocks) etc.

2. Relict or Palimpsest:-Textures inherited from the original rock type and which have survived metamorphism, example: porphyritic, oolitic, Inter granular etc. These textures are generally described by adding the word blasto as prefix to the original texture that has been retained by the rocks. Thus an igneous rocks with porphyritic texture undergoing metamorphism and retaining the same texture will be classified as metamorphic rock with blasto porphyritic texture.

Structures of metamorphic rocks (macrotextures)

(i) Slatycleavage: A pervasive, parallel foliation (layering) of fine-grained platy minerals (chlorite) in a direction perpendicular to the direction of maximum stress. It produces the rocks slate and phyllite.

(ii) Schistose: A schist has a lepidoblastic foliation if this foliation is defined by oriented micas, and a nematoblastic foliation if such a foliation is defined by the orientation of prismatic minerals as amphiboles and pyroxenes.

(iii) Gneissic: A complex banded texture made of schistose layers or bands alternating with bands commonly characterized by a granoblastic texture

(iv) Granoblastic: granular, interlocking equi dimensional grains of sub equal size; no preferred orientation or cleavage.

(v) Hornfelsic: Fine-grained, granular interlocking grains, possibly of variable shapes and sizes. No preferred orientation.

1.12 Study Of Common Rock Types Prescribed In Practical Work And Their Engineering Applications.

These include slate, quartz, marble, granite, etc. Gemstone are used for jewellery and in industry as cutting tools. Decorative rocks such as marble can be used in homes for fireplaces and kitchen work surfaces. Quartzite and marble are commonly used for building materials and artwork. Metamorphic rocks are very hard and can be used in the construction industry. Marble is a beautiful rock that is commonly used for building.

Reference:

1. Physical Geology by P. K. Mukharjee, World Press, 2013.

2. Physical Geology by Arthur Holmes, ELBS Publication.

3. Principles of Engineering Geology and Geo techniques by D. P. Krynine & W. R. Judd. CBS Publishers, New Delhi.

4. Engineering Geology by F. G. H Blyth and De Frietus, 2006, Reed Elsevier India Ltd.

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