Chapter: Civil : Engineering Geology : Petrology

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

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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Civil : Engineering Geology : Petrology : Metamorphic Rocks |