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Metamorphic Rocks

METAMORHISM is the term used to express the process responsible for all the changes that take place in an original rock under the influence of changes in the surrounding conditions of temperature, pressure and chemically active fluids.

METAMORPHIC ROCKS

 

METAMORHISM

 

METAMORHISM is the term used to express the process responsible for all the changes that take place in an original rock under the influence of changes in the surrounding conditions of temperature, pressure and chemically active fluids.

 

 

METAMORPHIC ROCKS

 

Definition

 

Metamorphic rocks are defined as those rocks which have formed through the operation of

 

Stress Minerals

 

various types of metamorphic processes on the pre-existing igneous and sedimentary rocks involving changes in textures, structures and mineralogical compositions.

 

     The direction of change depends upon the type of the original rock and the type of metamorphic process that operates on the rock.

 

     Heat,  pressure  and  chemically  active  fluids  are  the  main  agents  involved  in

 

metamorphic processes.

 

Plastic deformation, recrystallisation of mineral constituents and development of parallel orientation are typical characters of metamorphic rocks.

 

MINERALOGICAL COMPOSITION

 

Metamorphic rocks exhibit a great variation in their mineralogical composition that depends in most cases on

 

(i) the composition of the parent rock;

 

(ii) the type and degree of metamorphism undergone by the rock.

Two broad groups of minerals formed during metamorphism are:

v        Stress minerals and

v  Anti-stress minerals

 

Stress minerals

 

The minerals, which are produced in the metamorphic rocks chiefly under the stress factor, are known as stress minerals.

 

They are characterised by flaky, platy, lamellar, flattened and elongated forms.

 

kyanite, staurolite, muscovite, chlorite and some amphiboles.

 

Anti-Stress Minerals

 

     These are metamorphic minerals produced primarily under the influence of temperature factor.

 

     Such minerals are generally of a regular equidimensional outline. Examples: sillimanite, olivine, cordierite and many pyroxenes

 

Textures of Metamorphic Rocks

 

These can be broadly grouped under two headings:

 

(a) Crystalloblastic

 

       Textures which include all those textures that have been newly imposed upon the

 

rock during the process of metamorphism and are, therefore, essentially the product of metamorphism.

 

(b) Palimpsest (Relict)

 

     Textures that include textures which were present in the parent rock and have been retained by the rock despite metamorphic changes in other aspects.

 

     Among the crystalloblastic textures, Porphyroblastic and Granoblastic types are most common. outlines) of stronger minerals.

         In the granoblastic texture, the rock is made of equidimensional recrystallised

 

minerals without there being any fine grained ground mass.

  Palimpsest  textures  are  similar  in  essential  details  as  in  the  parent  rock  with

 

little  or  no modifications taking place during metamorphism.

 

  These  are described by using the term blasto  as  a prefix  to  the  name of the

 

original texture retained by the rock.

 

1. CLASSIFICATION OF METAMORPHIC ROCKS

 

     Metamorphic rocks have been variously classified on the basis of texture and structure, degree of metamorphism, mineralogical composition and mode of origin etc.

 

      A very general two-fold classification based on the presence or absence of layered structure or

 

FOLIATION as defined above is as follows:

 

(a) Foliated Rocks

 

     All metamorphic rocks showing development of conspicuous parallelism in their mineralogical and structural constitution falling under the general term foliation are grouped together as foliated rocks.

 

      The parallelism indicating features include slaty cleavage, schistosity and gneissose structures

 

         Typical rocks included in this group are slates, phyllites, schists and gneisses of great variety.

 

(b)Non-Foliated Rocks

 

Included in this group are all those metamorphic rocks characterised with total or nearly total absence of foliation or parallelism of mineralogicaJ constituents.

 

Typical examples of non-foliated rocks are quartzites, hornfels, marbles, amphibolites and soapstone etc.

 

2.IMPORTANT METAMORPHIC ROCKS

 

Definition

 

     Slate is an extremely fine-grained metamorphic rock characterized by a slate cleavage by virtue of which it can be readily split into thin sheets having parallel smooth surfaces.

 

       The slaty cleavage is due to parallel arrangement of platy and flaky minerals of

 

the slate under the dominant stresses operating during the process of metamorphism.

 

Composition

 

Mineralogically, slate is made up of very fine flakes of mica, chlorite and microscopic grains of quartz, felspar, oxides of iron and many other minerals, all of which cannot be easily identified even under microscope because of their fine grain size.

 

Origin.

 

       Slate is a product of low-grad regional metamorphism of argillaceous rock: like clays and shales.

 

     When state is subjected to further action of dynamothermal metamorphism, recrystallisation leads to the development in number and size of some minerals, especially micas.

 

         Such metamorphic rocks with conspicuous micaceous constituents and  general

 

slaty appearance are termed PHYLLITES. Uses.

 

Slate is used locally (where available) for construction purpose as a roofing and paving material only.

 

Schists:

 

Schists are megascopically crystalline foliated metamorphic rocks characterised by a typical schistose structure.

 

The constituent flaky and platy minerals are mostly arranged in parallel or sub parallel layers or bands.

 

Texture and Structure

 

          Most varieties are coarsely crystalline in texture and exhibit a typical schistose structure.

 

     Quite a few types show lineation and porphyroblastic fabric.

 

Composition

     Platy and rod-like acicular minerals form the bulk of most of the schists.

 

     Micas (both muscovite and biotite), chlorite, hornblende, tremolite, actinolite 'and kyanite are quite common constituents of most of the schists

     Quartz and felspars are comparatively rare but not altogether absent.

 

Porphyroblasts of granular minerals like staurolite, garnet and andalucite make their appearance in many schists.

Varieties

 

     Specific names are given to different types of schists on the basis of predominance of anyone or more minerals.

 

     Thus some commonly found schists are: muscovite schists, biotite schists, sericite- schist, tourmaline- schist etc.

 

     Sometimes schists are grouped into two categories on the basis of degree of metamorphism as indicated by the presence of index minerals:

a) Low-grade schists

     Formed under conditions of regional metamorphism at low temperature.

 

     These are rich in minerals like albite, muscovite and chlorite that are unstable at high temperature.

 

     Examples Mica-schist, chlorite-schist and talc-schist are a few from this group.

 

b) High-grade schists

 

     These are formed under conditions of regional metamorphism and are rich in minerals that are stable at high temperatures such as andalusite, cordierite, gamet, staurolite and sillimanite etc.

     Gamet-schists, cordierite-schists and sta1'rolite-schists are common examples.

 

Origin

 

     Slates and Schists are generally the product of dynamothennal metamorphism of argillaceous sedimentary rocks like clays and shales.

 

These indicate the final and stable stage in the metamorphism of shales through the intervening stages of slates and phyllites.

GNEISS

Definition                               

       A       gneiss          is  a  megascopically  crystalline  foliated  metamophorphic  rock           characterised  by segregation  of       constituent minerals  into        layers  or    bands  of           contrasting  colour,  texture  and composition.            

       A       typical        gneiss   will  show          bands   of  micaceous     minerals          alternating with   bands          of equidimensional minerals like felspars, quartz and garnet etc.

Composition

 

     Gneisses are generally rich in the minerals of parent rocks that are simply recrystallised during the process of metamorphism.

     Felspar and quartz are more common in gneisses than in schists.

 

Dark minerals of pyroxene and amphibole groups are also common, as are the typical metamorphic minerals like staurolite, sillimanite, gamet, kyanite and epidote etc.

 

Texture and Structure

 

     Gneisses show a variety of textures and structures, the most common being coarsely crystalline texture and the gneissose structure.

 

      Augen-gneisses show a typical cataclastic structure in which the hard minerals are

 

flattened and elongated.

 

Varieties

Important types are:

 

Orthogneiss formed as a result of metamorphism of granites and other igneous rocks.

       Paragneiss   these   are   formed   from   the   metamorphism   of    sedimentary  rocks  like sandstones;

       Banded gneiss typical gneiss in which the tabular and flaky minerals are segregated in very conspicuous pands of alternating dark and light colours.

 

     Gneisses of all varieties are generally the result of advanced stages of metamorphism of a variety of parent rocks such as sandstones, conglomerates, granites and rhyolites etc.

 

      There  is  difference  of  opinion  on  the  original  of  the  granitic  gneisses;  their

 

mineralogical composition is close to granites but in structure they appear more metamorphic.

 

Uses

 

Compact, dense and massive varieties of gneisses find applications as road stones and in some cases as building stones.

QUARTZITE

Definition

 

       Quartzites are granular metamorphic rocks composed chiefly of inter sutured grains of quartz.

 

       The name Orthoquartzite is used for a sedimentary rock of similar composition

 

but having a different (sedimentary) origin, in which quartz grains are cemented together by siliceous cement.

 

Composition

 

Besides quartz, the rock generally contains subordinate amounts of micas, felspars, garnets and some amphiboles which result from the recrystallisation of some impurities of the original sandstone during the process of metamorphism.

 

Origin

 

Metamorphic quartzites result from the recrystallisation of rather pure sandstones under the influence of contact and dynamic metamorphism.

Uses

     The rock is generally very hard, strong, dense and uniformly grained.

 

     It finds extensive use in building and road construction.

 

MARBLE

 

Definition

 

               Marble is essentially a granular metamorphic rock composed chiefly of recrystallised limestone

 

(made of mineral calcite).

 

     It is characterized by a granulose texture but the grain size shows considerable variation in different varieties;

 

It varies from finely sachhroidal to highly coarse grained. Marbles often show a banded structure also; coarse varieties may exhibit a variety of structures.

Composition       

   Small    amounts  of many  other          granular  minerals  like  olivine,  serpentine, garnet         and   some  amphiboles  are    also  present  in  many  varieties,  which  are derived from the impurities present in the original limestone during the process of metamorphic recrystallisation.

 

Varieties

 

      Various types of marble are distinguished on the basis of their colour, composition and structure.

 

White marble, pink marble and black marble are known on the basis of their colours, which is

 

basically due to fine dispersion of some impurity.

 

       Dolomitic marble is a variety distinguished on the basis of composition; it may

 

show slightly schistose structure.

 

Origin

 

Marble is formed from contact metamorphism of carbonate group of sedimentary rocks: pure white marble results from pure limestone; coloured marbles from those limestones that have some impurities and dolomitic marbles from magnesian limestones.

 

Uses

 

     Marble is commonly used in the construction of palatial and monumental buildings in the form of blocks, slabs, arches and in the crushed form as chips for flooring.

 

Because of its restricted occurrence and transport costs, it is mostly used as ornamental stone in costly construction.

 

 

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