Download What are Igneous rocks?

PREPARED BY:
MR: ALEX CHRISTOPHER
ASSISTANT PROFESSOR
DEP.OF CIVIL ENGG
VELAMMAL ENGG. CLG.
CHENNAI-66
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UNIT – III
PETROLOGY
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Classification of rocks
Distinction between Igneous, Sedimentatary and
Metamorphic rocks. Description occurrence,
engineering properties and distribution of following
rocks-Igneous rocks - Granite, Syenite, Diorite, Gabbro,
Pegmatite, Dolerite and Basalt
Sedimentary rocks sandstone, Limestone, Shale Conglo,
Conglomerate and breccia.
Metamorphic rocks, Quartzite, Marble, Slate, Thyllite,
Gniess and Schist.
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Petrology:
The branch of geology dealing the various aspects of rocks such as
their formation, classification and occurrence is called
petrology. Based on the formation, origin and chemical
composition the rock can be classified into three classes.
• Igneous rock
• Sedimentary rock
• Metamoraphic Rock
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IGNEOUS PETROLOGY
What are Igneous rocks?
The rocks that have been formed from an original hot, molten
materials through the processes of cooling and crystallization, may
be defined as igneous rocks.
Formation of igneous rock
The hot molten material occurring beneath the crust of the earth it is
called magma.(when it comes out from the earth interior to the surface
of the earth is called lava).
During their journy it comes out on the earth surface and solid to from
rock, this is called extrusive igneous rock some part of the magma may
be arrested within the crust during their upward journey and solidifying to
from rock it is called intrusive igneous rocks. The intrusive igneous rocks,
which formed at great depth is called plutonic rock. The intrusive igneous
rocks which formed at shallow depths is called hyp-aby sal rocks. So
based on the modes of formation the igneous rock can be classified into
three 1) Plutonic 2) Hyp – abyssal
and
3) Volcanic rock.
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Average Chemical Composition of igneous rocks
Sl No
Oxide - %
Sl No
Oxide - %
Sl No
Oxide - %
1
SiO2 = 59.14
4
Fe2O3 = 3.08
7
Na2 = 3.84
2
Al2 = 15.35
5
CaO = 5.08
8
K2O = 3.13
3
FeO = 3.80
6
MgO = 3.49
9
H2O = 1.15
10
Others = 1.34
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Average Mineralogical Composition Of Igneous Rocks
Feldspars
Biotite
Pyroxenes and Amphiboles
Titanium
Quartz
Apatite
Accessory Minerals
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59.54
3.82
16.85
1.53
12.06
0.67
5.8
FORMS OF IGNEOUS ROCKS
Based on the mode of forms igneous rocks can be classified into different forms as
follows
Discordant Igneous Body
If an intrusive mass happens to cut across the structure of the pre – existing rocks
of the country , it is said to be a discordant body.
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Concordant igneous body:
It an intrusive mass run parallel to the structure of the country
rocks is called concordant.
Dyke:
Vertical formation of discordant intrusive igneous rocks are
called dyke.
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Sills: Horizontal formation of intrusive igneous rocks are
called sills.
Radiating dykes: Dykes commonly occur in groups and
such groups may exhibits radiating or arcuate.
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Ring dyke: An arctuate dyke occurring more or less in the
form of a complete or nearly complete circular called as ring
dyke.
Cone sheet: Some time sheets of igneous materials are found
to occur in the form of coaxial cones with thin layer of country
rocks is called cone – sheet
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LACCOLITHS: Are discordant igneous bodies with their lower
surface flat and upper surface arched in the form of a dome
LOPOLITH: Are discordant igneous bodies with their shape
more or less similar to that of a sancer.
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Pacolet:
Concordant igneous bodies more are
less crescentic in shape occurring along
the crests and troughs of the fold of
country rocks.
Batholiths:
It is an discordant igneous body
of extremely large dimensions so much
so that their exposed upon the surface.
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Sheet structure:
The development of joints within the igneous
rocks has often been found to bring about the
formation of certain types of characteristic
structure. Development of one set of well
defined joints always brings about a slicing
effect on the massive igneous rocks body if all
such slices are horizontal, formed due to
development of one set of closely spaced
horizontal joints plants the structure is define as
sheet structure
Platy structure:
Some times one of the different sets of joints
developed in an igneous rock body may be so
conspicuous that on striking the rock mass, only
thin plates of rocks are broken off from the mass
and this feature may be defined
as platy
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structure.
Columnar structure:
Most of the extrusive igneous bodies the cooling of the mass brings about
the development of a few sets of vertical joints. Such set of vertical joints
of ten intersect and thus divide the rock mass into a number of vertical
columns which may be square, rectangular, rhombic, trigonal etc., when
an igneous rock mass is divide in this fashion into a number of vertical
columns the structure is described as columnar structure.
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Block lava:
Acidic lavas do not flow, due to their remarkable
viscosity, to great distances and during solidification are
found to often a very rough surface. Such lava flow is
known as block lava.
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Roby lava:
If the lava is fluid and during
consolidation, the flow offers a
smooth surface. it is described
ropy lava.
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SEDIMENTARY ROCKS
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Natural agents like blowing wind, running water, peculating
underground water, and glaciers in motion etc., are in
ceaseless operation, causing continuous wear and tear
of the rocks exposed to their fury.
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The products of such decay are subjected, under favorable
condition to sedimentation and sequent compaction.
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The resulting rock masses thus formed under ordinary
pressure temperature conditions are known as the
sedimentary or secondary rock.
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• Compaction and cementation transform
sediments into sedimentary rock.
sedimentary
rock
• Most transported sediment settles out of a fluid
and hence ends up being deposited in rivers, lakes
and most of all in the oceans.
• However, sediment is found almost
everywhere.Geologists estimate that sedimentary
rocks account for only about 5% (by volume) of
Earth’s outer 16 kilometers.However, sedimentary
rock is very important.The vast majority of rocks
exposed at Earth’s surface are sedimentary.
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Classification of Sedimentary Rocks
RESIDUAL SEDIMENTARY ROCKS: These rocks are formed due to
accumulation and consolidation of those materials which were left as
residue during the operation of the processes of weathering and
transportion, common example are laterite, bauxite, etc.,
MECHANICAL SEDIMENTARY ROCK : These are formed due to deposition,
under suitable conditions, of the pebbles, sand, silts etc., which were carried
is suspension with the surface runoff. Common examples of rocks belonging
to group are sandstone, conglomerate, grit, shale, mudstone, etc.,
CHEMICAL SEDIMENTARY: These are formed due to precipitation and
consequent accumulation of the soluble constituents traveling along with
the surface run-off. The precipitation may take place under a suitable
chemical environmental or due to activity of organisms.
ORGANIC SEDIMENTS:
These are the products of accumulation of organic
matter and are preserved under suitable conditions.(Example : coal,
fossiliferous limestone, etc.,)
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Structures of Sedimentary Rocks
Lamination : Layering, bedding or stratification is a very characteristic
features of the sedimentary rocks. If the individual layers be extremely
thin, the structure is known as lamination and the layers are described
as laminae.
Bedding plane : It is sedimentary sequence the surfaces separating the
different beds are known as bedding plane.
Fissility : Sometimes flaky minerals like mica lie parallel to the bedding
planes of sedimentary layers and due to this reason, the rock may be
easily fractured along the direction of bedding plane is known as
fissility.
Current bedding :The sedimentary beds are generally
parallel to one another. Sometimes, however, within a
sedimentary sequence, some of the beds may be
Found to lie slightly oblique to the major bedding
planes. This structure is known as current bedding.
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Torrential bedding : When deposition takes place in such a manner that thin
horizontal beds, made up of fine materials, are found to lie in alternation with coarse
and current bedded deposits, the resulting structure is known as torrential bedding.
Delta structure :The deltas or river are formed due
to deposition of sediments and show a characteristic
structure of this own there are generally three sets
of beds of which the upper and lower ones show a
very gentle slope towards the sea, in conformity with
the slope of the floor which they settle down. These
are known as top set and bottom set beds .in
between two sets lies the third set which exhibits a
considerable amount of slope towards the sea.
These are known as the forest beds.
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Ripple marks :These are the undulation surface of loose sediments due either
to action of wide of waves in a shallow water – body.
Mud cracks : In huge masses of fine – grained sediments, drying under sub –
aerial conditions due to this processes the creaks are developed. This is known
as mud – cracks these are also known as sun cracks since they are formed due
to the effect of solar heat.
Rain-Prints :These are formed on the top – surface of loose sediments many
cause the development of markings, impressions or foot – prints, etc, which are
known as tracks and trails.
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Sedimentary Rocks are Classified
Sedimentary rock have been classified on the basis of their
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mineralogical composition
environmental of deposition
mode of formation
texture and structural feature
Clastic rock
Gravel
Sand
Silt
Clay
Rudites
Arenites
Lutites
Non clastic rocks
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Clastic rock
• These rocks are also called as mechanically formed sedimentary rocks .
• These kind of rocks are formed due to weathering, transportation and
deposition of rock fragment by the natural agents like wind, water and ice
etc.,
• Further the clastic rocks are classified based on the average grain size.
Gravel: All the sediments and fragments or rocks irrespective of their
composition and shape which are bigger than 2mm it is further divided into
Boulders (256 mm)
Cobbles (256mm –16mm)
Pebbles (16mm 2 mm)
Sand: The sand grain size between 2mm and 1/16 mm . it is further subdivide
into
Coarse sand (2mm to ½ mm)
Medium Sand (1/2 mm to ¼ mm)
Fine sand (1/4mm to 1/16 mm)
Silt: These are very fine size particles in the size between (1/16mm –
1/256mm)
Clay: All sedimentary particle fines in size than 1/256mm.Based on the
predominance of sediments of a particular grade the clastic rocks are sub divide
into following three major classes.
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1. Rudites: These are also called as rudaceous rocks, it include all
coarse grain clastic rocks having the grain size above 2mm clay is the
common cementing materials (breccias and conglomerates)
2. Arenites: These are also called arenaceous rocks. These are mode up
sediments of sand grade(2mm-1/16 mm)
i.e. sand stone, Grey wackes and Arkose.
3. Lutites: These rocks are also called as argillaceous rocks. These rocks
are made-up of very fine particles like silt and clay.(i.e. Shale’s, Clays
and Mudstone)
Non clastic rocks: This groups includes all those sedimentary rocks that have
been formed by
Operation of simple chemical processes such as evaporation, precipitation and
crystallization of ordinary temperature and pressure.
Accumulation of hard parts of organisms following by the compaction and
consolidation.
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METAMORPHIC ROCKS
The physical and chemical alteration of solid rocks that transform their
texture, structures and mineral composition in response to change in
their environment conditions within the earth.
These are pre – existing rocks of igneous and sedimentary rocks that
have been changed in texture , structure and mineral composition by
physical and chemical processes, the physical changes include
• Crushing of grain
• Recrystallization
• Interlocking of grain
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CAUSES OF METAMORPHISM
Pressure :
The pressure is either vertical rocks load, causes by the weight of overlying rock.
Hydrostatic pressure of inter granular fluids. Both types of pressure increase when
increase the depths .
The dynamic or unbalanced tangent pressure associated with tectonic movement .
Heat : The heat may be earth heat, associated with the thermal gradient that is a
heat increase of 20 C per KM of depth.
Fluids :The active fluids includes not gases, especially stream and carbon dioxide
and hot solutions conditions the ions of common elements such as sodium and
calcium.
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CLASSIFICATION OF METAMORPHISM
Thermal metamorphism: Heating of rocks buried to great depths or adjacent to
igneous intrusions and extrusions includes recrystallization. Conversion of
limestone into marble take place due to thermal met amorism this is called thermal
metamorphism.
Contact metamorphism : Metamorphism suffered by the country rock, in the
neighborhood and due to the heat of an igneous intrusive body, is called as contact
metamorphism and the region around the igneous body within which such
metamorphism is in operation, is known as the contact aureole.
Pyrometamophism:
Due to access of chemically reactive substance from the
igneous body and their reaction with the country rocks , the processes of contact
metamorphism passes on to which is known as pyrometamorphism.
Optalic metamorphism: Burning of the country rocks along the fringing of minor
igneous intrusive is defined as Optalic metamorphism .
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• Hydrothermal metamorphism :This type of metamorphism is produced
by hot magmatic water that have been heated mobilized and chemically
changed by igneous intrusions.
• Plutonic metamorphism: The uniform pressure and temperature can
change in mineral composition and texture of rocks and the nearly
formed minerals are such that the resulting mass commonly suffers a
reduction in volume .
• Cataclastic metamorphism : Direct pressure or stress can dominate at
or near the surface where lateral forces, leading to mountain – building
are in plug due to this process the texture of the rock changed , this kind
of metamorphism is known as cataclastic metamorphism.
• Dynamo thermal metamorphism: It is also called as regional
metamorphism. This involves development of large scale changes in the
structural and chemical constitution of the pre – existing rocks under the
combined action of pressure , temperature and fluids. Such conditions
are available during the mountain building activity in the history of the
earth.
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Igneous Rocks
Sedimentary Rocks
These are the products of consolidation of
magmas
Formed due to eruption and solidification or
cooling of magma.
Based on the formation, it can be classified into
two type intrusive igneous rock and extrusive
igneous rocks.
Internal forces operate the formation of
igneous rocks. (ie. Volcanos, arthquake and
tectonic movement)
Occurrence of mineral grains igneous
rocks can be classified into polycrystalline and
hemi crystalline .
Granite, diorite, syenite, gabbro and
dolerite are the common igneous rocks.
This kind of rocks are also called Primary Rocks
Sedimentary rocks are the products of
weathered igneous Rocks.
Formed due to deposition and cementing of
weathered rock material on the surface of
the earth due to pressure.
Based on the formation it can be classified
into 1. residual sedimentary rocks,
2.mechanical sedimentary rock
3.chemically formed sedimentary rock and
organic sedimentary rocks
External agents such as running water,
glacier, wind, sea water, etc.,are the force
which operate the formed of sedimentary
rocks.
Based on the grained size it can be
classified into rudaceous, arenaceous and
argillaceous rocks.
Breccias, Conglomerates, Sandstone,
Shale’s, Limestone, Bolomite and Coal are
the common sedimentary rocks.
This kind or rocks are also called as secondary
rocks
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Metamorphic rock
Metamorphic rocks are the products of the igneous and
sedimentary rocks
Formed due to change of texture, structure and mineral
composition by physical and chemical processes.
Based on the formation these rock can be classified into thermal
metamorphism, dynamic metamorphism and dynamo thermal
metamorphism.
Physical and chemical agents such as 1.temperature 2.pressure
3.stree and 4.chemical reactive substances operate the
formation of metamorphic rocks.
Based on the texture, structure and mineral composition,
metamorphic rocks can be classified into foliated and non –
foliated rocks
Slate, gneiss, schist, quartzite’s, morble, phyllite and hornfels are
the common metamorphic rocks.
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IMPORTANT SEDIMENTARY ROCKS
• Breccias
• Conglomerate
• Stand stone
• Shale
• Lime Stone
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Breccias
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It is a mechanically formed sedimentary rocks.
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It consists of angular rock fragments embedded in a fine matrix of clayey
material.
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The fragments are generally greater than 2mm in average diameter.
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Breccias are further classified in to basal breccias , fault breccias and
agglomerate breccias.
Basal Breccias
This is formed by the sea water advancing over a coastal region covered
with fragments of rocks.
The advancing waters supply the fine mud, which spread over the rock
fragment and act as a binding material.
Once the sea water retreats the rock fragment get cemented together to
form breccias.
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Fault Breccias or Crush Breccias
The angular and clay like rock materials are produced during the
processes of faulting.
Subsequently get embedded in clay and form a cemented rocks.
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Agglomerate Breccias or Volcanic Breccias
•This kind of sedimentary rocks formed by the volcanic eruptions.
•The angular and fused rock materials cemented to form rocks.
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Conglomerate
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These are sedimentary rocks of elastic nature and belonging to
rudaceous group.
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They consist of rounded fragments of various sizes but generally
above 2mm cemented together in a clayey matrix.
Based on Grain Size the Conglomerate can be classified in to:
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Boulder (gravel > 256mm)
Cobble (gravel 64-256 mm)
Pebble (gravel 2 – 64 mm)
Based on Source of Gravel
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Basal conglomerates
Glacial conglomerates
Volcanic conglomerates
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Stand stone
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It mechanically formed sedimentary rocks of arenaceous group.
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These are mostly composed of sand grade partials.
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The grain size between 1/162 – 2mm .
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Si O2 is the dominate mineral in sandstone.
Texture : Sand stone are medium to fine – grained in texture.
Color
: Stand stone show a variety of colors ,red , brawn, gray and
white being the common.
Types
: Siliceous, calcareous, argillaceous and ferruginous sandstones
are the common types of sandstone.
Uses
: Sandstone of hard, massive and compact character are most
commonly
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used as building
stones.
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IMPORTANT METAMORPHIC ROCKS
• Quartzite
• Marble
• Slate
• Phyllite
• Gneiss
• Schist
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QUARTZITE : It is a granular metamorphic rocks composed
chiefly of quartz grains. It is a granular rock characterized by a
tendency of fracturing through the grains rather than along the
grains under heavy load
Composition : it is a mixture of mica feldspars, granet and
some amphiboles.
Origin : Metamorphic quartzite’s result from the recrystallisation
of pure sandstones under the influence of contact and dynamic
metamorphism.
Uses : The rock is generally very hard strong, dense and
uniformly grained. it using in building and roads construction
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QUARTZITE
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SLATE : Slate is an extremely fine – grained metamorphic rock
characterized by a slaty cleavage by virtue of which it can be split into
thin sheets with parallel smooth surfaces.
Composition: Mineralogically, slate is composed of very fine grains of
micas, chlorite, quartz, felspers, 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 the product of low – grade regional metamorphism on
argillaceous rocks like shales. When slate is subjected to further
action of dynamo thermal metamorphism, recrystallisation may lead to
the development in number and size of stone minerals, especially
micas.
Uses : Slate is locally used as a roofing material. It cannot be as a
building stone or road stone because of its low crushing strength and
slaty cleavage.
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SCHIST
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SCHIST : These are megascopically crystalline metamorphic rocks
characterized by a typical schistose structure.
Texture and structure : Most varieties are typically schistone in structure and
megascopically crystalline in texture.
Composition : Micas, chloride, talk, hornblende, tremolite and kyanite are quite
common constituents of many schists.
Varieties : muscovite schist ,biotite schist and tourmaline schist etc.
Origin : Schists are generally the produced of dynamo thermal metamorphism.
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SCHIST
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GNEISS: A gneiss is a megascopically crystalline metamorphic rock.
A typical gneiss will show bands of mica and other play minerals alternating
with bands of equidmensional minerals like felspars, quartz, garnet etc.,
Composition : Felspars and quartz are more common in gneisses then in
schists. Dark minerals of amphibole and pyroxene groups are also common
as are the typically metamorphic minerals like staurolite, sillimanite,
garnet,kyanite and epidiote etc.,
Structures : Gneissose structure and Cataclastic structure.
Origin : Gnesses are generally the product of advanced stages of
metamorphism of a variety of parent rocks such as sandstone, conglomerates
and granites.
Uses : Compact, dense, massive varieties of gneisses are used as building
and road stones.
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GNEISS
MARBLE : Marble is essentially a granular metamorphic rock
composed chiefly of recrystallized calcite
Compostion : CaCO3 Small amount of many other granular minerals
like olivine, serpentine, garnet and some amphiboles are also present
in many varieties.
Varieties : Various types of marbles are distinguished on the basis of
their color, composition and structure.
Origin : marbles are formed as a result of carbonate rocks chiefly
limestone and sometimes dolomites – under conditions of contact
metamorphism.
Uses : Marbles are commonly used in building construction in the
form of blocks, slabs, arches and as chips for flooring. In most cases
the stone is used for decorative and ornamental purpose.
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MARBLE
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PHYLLITE : It is a medium to fine grained metamorphic rock of
complex silicate composition.
Composition : Phyllites consist chiefly of chloride, muscovite and
quartz grains all of which are in fine state, but can be seen with a hand
lense. In fact this is the main point of difference between slates and
phyllites.
Origin : Phyllites are formed in nature as a result of dynamo- thermal
metamorphism of clay bearing rocks like shale.
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PHYLLITE
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IMPORTANT IGNEOUS ROCKS
Granite : Granite is plutonic light colored igneous rocks and
equigranular texture.
Composition: Ouarts, orthoclase feldspar, albite feldspar, oligoclase
and microcline.
Associated minerals like mica, bioteie, hornblend, augite and
hypersthene.
Texture: It is coarse grained, holocrystalline and equigranular rocks
Granular, graphic, porphyritc and intergrowth texture are common.
Types: Muscovite-granite, biotite-granite, Muscovite biotite-granite,
hornblend – granite, augite-granite and tourmaline – granite.
Occurrence : It is most commonely distributed igneous rocks in the
crust. It occur chiefly as deep-sheeted intrusive body likesills, dyke,
bosses, stocks and batholiths.
Use: Architexturaland massive
construction
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Diorite: It is intermediate type of plutonic igneous rocks with high silick
percentage.
Composition: It is typically rich in feldspar plagioclase of sodic group, hornblend,
biotite and pyraxene are associated minerals.
Texture: Course to medium grain and holocrystalline texture.
Occurrence: Commonely occur as intrusive bodies of dyke, sill, stocks and other
intrusives.
Use: Architexturaland massive construction
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Syenites: It is plutonic, even grained igneous rocks. Alkaline feldspar are the
chief constituent minerals.
Composition: Feldspar is the common mineral
Texture: medium to course grained , holocrystalline nature.
Gabbro: It is a coarse-grained, dark colored, plutonic intrusive igneous rock
Composition: It contains feldspar, augite and sometimes olivine
Texture: Coarse to medium grain and reaction rim.
Diorite: is a grey to dark grey intermediate intrusive igneous rock
Composition: composed principally of plagioclase feldspar, biotite,
hornblende, and pyroxene. It may contain small amounts of quartz,
microcline and olivine. Zircon and apatite.
Texture: Course to medium grain and holocrystalline
Occurrence: Occur as small VELAMMAL
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Diorite
Gabbro
Dolerite: These are hypabyssal igneous rocks formed as shallow sills
and dykes.
Composition: calcic plagioclase, augite, olivine, iron etc.
Texture: Medium to fine grained rocks
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