PETROLOGY

Petrology is the department of science which is concerned with mineralogical & chemical compositions of rocks with their classification.

Rocks are classified mainly three groups:

·         Sedimentary Rocks

·         Metamorphic Rocks

·         Igneous Rocks

SEDIMENTARY ROCKS:

Sedimentary rocks are formed from overburden pressure as particles of sediments are deposited out of air, ice or water flows carrying the particles in suspension. As the sediment deposition builds up, the overburden pressure (Lithostatic pressure) squeezes the sediments into layered solids. This rock formation process is known as Lithification (rock formation). 

These rocks are further classified in three categories. This classification is based on three main ways of formation of sedimentary rocks:

(1) Clastic / Terrigenous / Detrital / fragmental sedimentary rocks: Clastic means broken or fragmented. Clastic rocks are composed of broken up pieces of pre-existing rocks and minerals. We can further identify these rocks based on the size of the fragments that they are composed.

(2) Biological / Bioclastic: sedimentary rocks: These sedimentary rocks are formed by the deposition of result of some biogenic activities. Coals , Cherts and shell limestone are examples of biological sedimentary rocks.

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(3) Chemical sedimentary rocks : These rocks are formed by deposition of chemicals, which are generally precipitated from solutions. These rocks however, form from a water solution, and not from molten rock. Various salts (rock salt and gypsum), Dolostone, limestones are few examples of chemical sedimentary rocks. Chemical sedimentation involves water and therefore the depositional environments will be most often found near or under water.

Composition of sedimentary rocks :

Sedimentary rocks are composed of mainly silica (quartz), feldspars, amphiboles and clay material.

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METAMORPHIC ROCKS:

In a simple definition we can say that metamorphic rocks are formed from transformation of one type of natural stones to another type of natural stones through mix of heat, pressure and minerals.

These changes can be either: -

Crystalline formation or/and Texture change or/and colour change

Before understanding the metamorphic rocks, it is necessary to understand METAMORPHISM.

METAMORPHISM:

The term metamorphic is derived from the Latin term meaning, "change of form”. It can be defined as mineralogical, chemical & crystallographic changes in a solid-state rock, in response to new conditions of pressure & or temperature & or introduction of fluids, causing the existing rocks minerals to recrystalize. They may even become different minerals all together. It is important to realize that the rock remains in its solid state. Once rock minerals are melted, and then recrystallize, the new material will be igneous. Temperature change can range from 100 c to 900 c.

METAMORPHISM can be of four types:

·         Regional metamorphism:

This type of metamorphism occurs in broad areas of earth’s crust. Regional metamorphism deforms the rocks strongly. This type of metamorphism is further classified in two categories Barrovian or Buchan type metamorphism depending upon temperature and pressure gradients.

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·         Impact metamorphism :

This type pf metamorphism occurs in ultrahigh pressures & low temperature which are the result of collision of a meteorite or extraterrestrial object or during an extremely violent volcanic eruption.

·         Contact(thermal) metamorphism :

This occurs typically around igneous intrusive rocks. As a result of the temperature increase caused by igneous body itself. Magma fluids coming from the intrusive rock may also take part in contact metamorphism. Contact metamorphic rocks are known as hornfels and usually are fine grained. 

·         Hydrothermal metamorphism :

Hydrothermal metamorphism is the result of the interaction of a rock with high temperature fluid of distinct composition. This metamorphism results from reaction between protolith (original rock before transformation) & fluid. This kind of metamorphism is responsible for many metal deposits on earth. 

We have studied metamorphism now we will be able to define metamorphism rocks more systematically.

Metamorphic Rocks results from mineralogical and structural adjustments of solid rocks to physical and chemical conditions differing from those under which the rocks originally formed. Temperature and pressure are important factor in the formation of metamorphic rocks.

Classification of Metamorphic Rocks:

Metamorphic rocks can be subdivided into foliated and non-foliated types. This is determined by the presence of minerals that are aligned parallel to each other. This results in a layered appearance. A non-foliated metamorphic rock generally consists of equal dimensional grains.

A rock can also be identified by its metamorphic grade. This is simply a description of the overall intensity of metamorphism the rock was subjected to. What this implies is that a low-grade metamorphic rock shows textural or mineralogic evidence of having been subjected to low pressures and/or temperatures.

Metamorphisms produced with increasing pressure and or temperature is known as PROGRADE METAMORPHISM. Conversely decreasing temperature and or pressure characterize RETROGRADE METAMORPHISM.

Slates, marbles & Quartzite are example of metamorphic rocks.

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IGNEOUS ROCKS:

Igneous rocks are formed from solidification of molten magma erupted through the mantle or crust of earth. As this magma in earth is less dense then surrounding solid rocks, it rises up towards surface. This lava has two options depending upon it’s surrounding conditions either it may erupt at the surface from the volcano’s mouth or it may settle within the earth’s crust. This classifies the types of igneous rocks.

1.      Intrusive or plutonic igneous rocks are the rocks, which results due to solidification of lava deep within earth.

2.      Extrusive or volcanic rocksare the rocks, which results due to solidification or crystallization of lava on the outer surface of earth.

These two types of igneous rocks can be identified by visualizing them Since slow cooking of “intrusive rocks” promote the growth of mineral beds which can be identified without microscope. While in extrusive igneous rocks these minerals cannot be seen by naked eye. If molten magma cools at extremely high rates (as in extrusive igneous stones) disallowing crystallization, the result is a volcanic glass called obsidian.

Igneous rock textures .

Phaneritic

The size, shape and arrangement of the mineral grains of any rock is called its texture . Generally, if magma cools slowly, its mineral crystals will have more time to grow and the resulting rock will demonstrate a course or phaneritic texture. This texture will be more characteristic of intrusive rocks and you will be able to see the mineral grains .

Aphanitic

On the other hand, if the magma or lava cools quickly, the matrix of tiny crystals that are formed give the rock a fine or aphanitic texture. Fine textured rocks are usually volcanic or shallow intrusive. These rocks cool so quickly that you will usually not see the individual mineral grains with an unaided eye.

Porphyrhitic

A third type of texture occurs when a molten mass begins to cool slowly, and some crystals of one mineral begin to form. Sometimes this material is then moved (or erupted) and the remaining material cools quickly. The resulting rock will have large crystals that are in a finegrained groundmass. This is called a porphyrhitic texture, and the rock is porphyry.

Igneous rocks are also classified by the amount of silica they contain .

As a rule, acidic and intermediate rocks form by the solidification of molten crystal material. Basic rocks more often form from molten material brought up from the mantle.

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BY COMBINING THE TEXTURE AND THE CHEMICAL COMPOSITION OF A ROCK WE CAN DEFINE THE MOST COMMON IGNEOUS ROCKS.

A melt goes through many stages as it solidifies in a process called fractionation. As the melt cools the minerals that generally crystallize first are those low in silica such as olivine and pyroxene. The remaining melt becomes more silica rich. This may then be erupted and form acidic rocks.

Magma Formation (molten materials below the surface):

Elevated temperatures within the Earth (possibly caused by the decay of nearby radioisotopes) cause the rocks ions to vibrate in their lattice position. Increased heating may eventually collapse the lattice, forming liquid or molten material.

Another cause of temperature elevation is the natural increase in temperature as depth increases. This is known as the geothermal gradient.

As pressure increases, the melting temperature also increases (provided the rocks are dry). Conversely the presence of water or water vapor in the rocks will lower the rock’s melting point. A wet rock will melt at a lower temperature than a dry rock of the same composition.

2. As pressure rises, so do the effects of water, therefore as the pressure on a wet rock increases the temperature at which it melts decreases. This is the exact apposite of dry rock.

At a depth of 35km to 40 km the geothermal gradient is just enough to start melting average crustal rock (in the presence of water) forming magma.

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General information about stones

LIME STONE:

This is a type of chemical sedimentary rocks. Limestone is principally composed of calcium carbonate (Caco3) (calcite) or calcium magnesium carbonate (Dolomite). Marine organisms mainly form calcite deposition settles out of the water column and get deposited on ocean floors. . Limestone is originally white but it may have different colors due to impurities like iron oxide and carbon. In India limestone is quarried in Kota (northern India) and Cuddupha (south India). Its elegant light colors are specially adored by Europeans & Americans.

Limestones are further divided into three categories depending upon its density.

·         Low density :- lime stones having density ranging from 110 lb\ft3 to 135 lb\f3(i.e. 1760 kg\m3 to 2160 kg\m3)

·         Medium density :- limestones having density between 135 to 160 lb\f3(i.e 2160 to 2560 kg\m3)

·         High density :- limestones having density greater than 160 lb\f3(25560 kg\m3)

Lime stones textures are associated by a number of distinguishable natural characteristics including open texture streak, honey comb for iron spots, travertine like formatting. Limestones show resistance in freezing and thawing conditions.

SANDSTONES :

Sandstones are clastic type of sedimentary rock. It is formed by deposition of remains of other rocks. The chemical composition of sandstone is just like sand; the rock is composed essentially of Quartz. Feldspur is other main constituent of coarse-grained (crystals have diameter of greater than ½ inches.)

These grains of sands are cemented/ binded together by silica, CaCO3 or iron oxide. It is this cementing element, which determines its colour.

Strength of any sandstone is determined by percentage of Quartz & clay matrix (Binding Agent). More percentage of Quartz will decrease percentage of clay matrix in stone. Hence increase in strength & vice versa. Depending upon percentage of clay matrix, sandstones are mainly divided into three groups

1.) Arenites : Arenites are the sandstones which contains less than 10% of clay matrix in total composition. Arenite is the word taken from Spanish word “Arena” which means sand in English. Arenite is subdivided in two categories.

§         Arkose : A feldspar rich sandstone is known as Arkoses ( Feldspar 25% ).

§         Litherenites : litherinites are sandstones which are lithic rich.

      2.) wake:- wacke are the sandstone which contain 10 to 50% clay matrix.

§         quartz wacke: These types of wackes have predominantly quartz   surrounded by a mud or clay structure.

§         feldspaaar wacke: feldspar is surround by mud structure.

§         Lithic wacke: These are also known as grey wacke. It is predominantely lithic surrounded by clay structure.

3.) sandy mudstones:- rocks with more than 50%. Clay matrixes are known as sandy mudstones.

Sandstones are quite hard and have great compressive strength. It is available in many colors and shades and streaks. In India we have white mint, red, pink, beige, gray, brown, rainbow(multicolor), teak like stone, multishades of green etc. these all have low water absorbing prosperity. So because of its qualities sand stones can be used almost every where from hospital’s floor to wall interiors as well as exteriors.

In India, Gwaliar, Shivpuri, Lalitpur, Dholpur, Kota, Kishangarh are known for sandstone minning.

Conglomerate: - It is another type of sedimentary rocks which is formed from cementing of rounded coble and pebble sized rock fragment (clastic sedimentation)_. River movements or ocean wave action forms conglomerate.

Breccia: - It is another clastic sedimentary rock which is formed in the in a similar fashion to conglomerates. The difference between two rocks is that breccias rocks fragments are very sharp and angular the reason for these sharp and angular fragments is that the fragments of rocks are not transported by wind, water or glaciers long enough to be rounded and smoothed like in conglomerate rocks. The cementing agents (silica, calcite etc) both rocks are same.

Now we will discuss some of metamorphic rocks:-

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SLATES: - Natural slates are compact fine grained metamorphic rocks which are found extremely low weight. Skate is produced by low grade metamorphism which is caused by relatively low temperature and pressures. These are extremely useful as roofing, wall cladding or paving stones. Slates are primarily composed of silica and aluminum. Slates have distinguished lllusture and shinning which are found very attractive and hypnotizing. Slates are relatively less dense and can be splitted in thin layers by using chisels. These are an architect’s choice because of its shinning property, fireproof property and non-slipry properties. It can be used in interiors as well as exteriors. India exports some of finest slates in the world.

Other names of slate stones:-

Grey slates, flags, flagstones, thick stones, slabs, flat stone, stone tiles and tile stones. In India slate stones are moved in Kund, Ddevli and Markapur mainly.

Schist: - it is a medium grade metamorphic rock i.e. these have been subjected to more pressure and heat than slate stones. Schists are usually named based on minerals which may be present:

o        mica schist

o        garnet schist

o        chlorite schist

o        kyanite schist

o        talc schist

Geneiss: -

These are high grade metamorphic rock which means geneiss has been subjected to more temperature and heat that schist. Gneiss is coarser that schist and has distort banding. This banding has alternating layers that are composed of different minerals. Feldspar, mica and quartz form the geneiss. Geneiss the metamorphic rock formed by alteration in sedimentary rocks properties by heat and pressure but geneiss can be result metamorphism of igneous rock granite.

Above three types of stones were layered or foliated metamorphic stones. Now we will see non foliated metamorphic rocks. A banded or striped rock with alternating layers of dark and light minerals. The dark layers commonly contain biotite, and the light layers commonly contain quartz and feldspar.

Marbles: - The word “marble” is derived from Greek word “madrmoros” which means shinning stone. Marble is a metamorphic lime stone composed of very pure calcium cadrbonate, caco3. The softness and elegance of marble is very desirable for sculpture and building stone marble can be divided into four groups:

Group a: Marble width most uniform and favo9raable working qualities with no flows or voids.

Group b: May have natural faults with less favorable working qualities.

Group c: Marbles with some variations in working qualities, geological flaws, voids, veins and line of separation are common. It is standard to repair these variations by one or more of several methods i.e. whizzing, sticking filling and cementing. Liners and other forms of reinforcement are used when necessary.

Group d:-Similar to preceding group But containing larger purporting of natural faults, maximum variations in working qualities and requiring more of same methods of finishing.

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Quartzite: - Quartzite common and widely distributed rock composed mainly or entirely of quartz. The compact granular rock is a form of metamorphosed sandstones in which silica, or quartz has been deposited between the grains of quartz of which sandstone is essentially composed.

Unlike sands stone, quartzite breaks through not around the quartz grains, producing a smooth surface instead of a rough and granular one. Quartzites are white pink or gray. They yield a thin and very barren soil and because they weather slowly, tend to project as hill or mountain masses.

Most quartzites contain more than 90% or more quartiz, but some contain 99% and are largest and purest concentration of silica in the earth’s crust. Sand stones turn into quartzite stone in turn different ways. In one method, under the high pressures and temperatures of deep burial, the mineral grains recrdystallize and all traces of original sediments are erased. The result is a true metamorphic rock called metaquartzite. This boulder is probably a metaquartzite.

Quartzite is very hardstone which is difficult to mine and cutting. So quartize is a bit expensive stone.

Granite: - The word “granite” comes from latin word granum, which means grain, in reference to the grained structure of such a crystalline rock. Granite is a common and widely occurring igeneous rock of intrusive group. These rocks are formed at great depths and pressure under continents. Granite consists of orthoclase, plagioclase, quartz, hornblende, biotite, muscovite and other monor accessories such as magnetite, garnet, zircon and apatite minerals. Rarely a pyroxene is present. Generally granites carries small amount of plagioclase feldspar causes granite to pass into granodiorite. A rock consisting of equal proportions of orthoclase and plagiclase plus quartz can be considered a quartz monozonite. Granite containing both muscovite and biotite micas is called binary granites. Specific gravity of granite ranges from 2.63 to 2.75. Its crushing strength is from 1050 to 14000 kg per sq cm (15000 to 20000 lb per sq inches).

o        Fine grain: - fine grained granites are those in which feldspar crystals average about 1/16” to 1/8” in diameter.

o        Medium grain: - medium grain granites are those in which feldspar crystals average about ¼” in diameter.

o        Coarse grain: - coarse grain granites are those in which feldspar crystals average 1\2” or greater in diameter

Note: - more the size of grain, lesser is the density of granite.

Gabbros: - It is a dark colored, coarse-grained intrusive rock. It is mostly composed of mineral plagioclase feldspar with similar amount of pyroxene and olivine.

Basalts: - These are also dark colored but fine grained extrusive rocks. The grains are so fine that they are impossible to distinguish with naked eye.

Dacite: - Dacite is an extrusive igneous rock. The principle mineral, which makes the dacite, are plagioclase quartz, pyroxene or hornblende.

Rhyolite: - similar to granite but much finer crystals. As in another extrusive rock the grains cannot be seen by naked eyes. The minerals that make up rhyolite are quartz, feldspar, and mica hornblende.

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