Igneous rocks

Igneous Rocks
• Igneous Rocks - Forms of Igneous Rocks- Plutonic &
Hypabasal, Common Igneous Rocks and relation of
their Constituent Minerals, Classification of Igneous
Rocks, Structure and Textures, Suitability of Igneous
Rocks for Building and Foundation, Megascopic
Description of Common Igneous Rock Types.

Introduction
• Among the different types of rocks, igneous
rocks are most abundant. They are also known
as primary rocks.

Forms of Igneous Rocks
• Igneous rocks are formed out of very hot lava or
magma. The extruded (or thrown out) lava on
solidification over the earth’s surface give rise to
extrusive rocks. The magma on solidification
below the earth’s surface give rise to intrusive
rocks. The extrusive rocks occur as simple lava
flow. The intrusive rocks, on the other hand,
assume different forms (or shapes) which in
turn are influenced by geological structures
occurring in country rocks and fluidity (i.e..
viscosity) of magma.

• Based on the relation of an igneous body with
the attitude of associated country rocks, their
forms are called concordant or discordant. If
an igneous body is parallel to or occur along
bedding planes of country rocks. Its forms is
called concordant. On the other hand. If it
cuts across the bedding planes of
surrounding rocks, it is called discordant.

Influencing factor
• Magma, after its formation under the influence of
pressure exerted by a huge overburden, attempts
to move upwards, forcefully. In its upward
journey, when magma reaches sedimentary
strata, their bedding planes offer convenient
and suitable sites for magmatic intrusion or
injection. Thus, the bedding planes play an
important role in the formation of igneous bodies.

• Another influencing factor in the types of form
of an intrunsive igneous body is the nature of
fluidity of magma. The fluidity or viscosity of
magma is dependent on its silica content. Through
magma has no fixed composition, it is usually
either silica-rich or silica poor. Viscous magmas
do not spread easily but have a tendency to pile
up at a place. The basic magmas being less
viscous easily spread over larger areas. The three
typical forms of igneous intrusions, namely, sill,
laccoliths and bysmalith, can be explained by
this property of the viscous nature of magma.

• The most common forms of intrusive igneous
bodies as observed in the field are dyke, sill,
laccoliths, lopolith, bysmalith, phacolith,
chonolith, volcanic neck or plug, batholith,
etc, lava flows and pyroclasts are forms of
extrusive igneous bodies.

Dykes
• Dykes and sills are the most common forms of
igneous rocks. Dykes are discordant, sheet-like,
vertical or steeply inclined, intrusive igneous
bodies, Since they are sheet-like, they are
narrow in width and have nearly uniform
thickness, i.e.. with parallel sides. They occur
cutting across the bedding planes of the
country rocks in which they are found.

• During the forceful upward journey, magma
intrudes through the discordant fractures,
cracks, crevices, joints, shear zones or any
other weak planes or zones. Subsequent
solidification of this gives rise to dykes. Some
dykes may serve as feeding channels or passages
for other igneous bodies such as laccoliths,
lopolith and sill.
• Dykes may be horizontal or inclined or
vertical. Steeply inclined or vertical dykes
extending to greater depths are common in nature.

• Dykes are important from civil engineering
point of view for the following reasons.
• They are undesirable at the sites of
foundations of dams because they introduce
heterogeneity in the region and also their
sides turn out to be weak planes.

• Dykes are like walls and act as barriers for the
flow of underground water. Thus, like quartz veins,
they interrupt the ground water movement which in
good or bad potential of ground water in a region.
• Dykes may give rise to new spring or seal old
springs
• Dykes may cause oil accumulation and thereby
contribute to the occurrence of oil and gas deposits.
• As dykes are hard, durable black and fine grained,
they are used in making statues, sculptures, etc

Sills
• Sills are similar to dykes in being sheet-like, intrusive
bodies but, unlike dykes, these are concordant. Sills
are formed due to the penetration of magma into
bedding planes of country rocks and their spreading
capacity depends on the viscosity of magma, its
temperature and the weight of the overlying rocks.
• Basics magmas being more fluid and more hot,
usually occur as sills. During sill formation, the
ascending magma, under great pressure, pushes the
overlying rocks upwards and accommodates itself by
intruding and spreading along weak bedding planes.

• Sills being concordant, on consolidation,
looks like beds themselves. The sills, which
spread over larger areas, are generally thin and
show uniform thickness.
• Lava flows may resemble sill closely,
because both are relatively thin, horizontal
sheet-like igneous bodies spreading over
larger areas. But they can be distinguished
from one another easily as follows

• Sills produce baking effects on rocks on both sides,
whereas in the case of lava flow, the baking effect is
seen only on the lower side.
• Lava flows show an irregular surface, whereas sills
have more or less two flat and parallel sides.
• Lava flows show vesicular character on the upper
surface, whereas sills have more or less two flat and
parallel sides.

• Lava flows undergo quick cooling
producing very fine grained igneous rocks,
whereas sills cool down rather slowly and
therefore produce medium and coarse
grained igneous rocks.
• Sills give out minor intrusions into the
overlying rock masses, whereas lava flow do
not.

Laccolith
• Like a sill, laccoliths is also a concordant
igneous body. It has a nearly flat bottom but it is
convex upwards, i.e. it is domed shaped. When
viscous magma is injected along a bedding plane,
as it cannot spread easily, it pushes up the
overlying rocks and piles up more at the place
where it reaches the bedding plane and thins out
away from it. This gives the shape of an
inverted bowl to the igneous body.

• Laccolith may also be formed when the
supply of magma from beneath is more than
that can be accommodated by lateral
spreading. Laccolith is relatively a small
igneous body. In the ground plan it is either
circular or elliptical, depending upon weather
the magma supply is from a cylindrical vent
or an elongated fissure. Laccolith cause the
overlying rocks folded upwards to take a
dome-like shape.

Bysmalith
• When the magma happens to be highly
viscous, the lateral spread along the bedding
plane will be very less and the intruding
magma acquires a somewhat cylindrical
shaped body. It is called “bysmalith”.

• During the formation of bysmalith, since the
overlying rocks are pushed up more or less
vertically, they cannot bend unduly, but rupture
along sides and number of actuate faults occur
around bysmalith. The net result is that this
igneous body appears as if it has punctured
through the surrounding country rocks.
Such a form appears partly concordant and
partly discordant.

Phacolith
• When thick sedimentary are folded, along
the crests and troughs, some empty spaces
occur. These spaces are readily occupied if
magmatic intrusion get assess to them. On
solidification these appear as crescentic or
lens-shaped across the axial plane. These
igneous bodies are called “phacoliths” These
are concordant igneous rocks.

Lopolith
• This is a basin or saucer-shaped concordant
intrusive body of enormous size. Its top is
nearly flat and the bottom is convex
downwards. In the way, it is just the reverse
of laccolithh in shape. Lopolith is a very huge
igneous
body
with
a
thickness
approximately 1/10 to 1/ 20 of the diameter.

Volcanic Necks
• These are the igneous bodies which seal the
vents and conduits of ancient volcanoes. These
are somewhat cylindrical in shape and vary in
diameter from a few hundred metres to a
kilometer or more. Volcanic necks are circular or
elliptical or irregular in plan and vary in diameter.
These are filled with crystalline rock or
fragmentary material or both. Volcanic necks
are also called volcanic plugs.

Batholiths
• These are the largest known intrusive igneous bodies
characteristically occurring in mountain regions
Batholiths have sides sloping away from each other
which makes them larger and larger downwards and
then extended to very great depths.
• They occur as core bodies of folded mountains, and
appear elongated along mountain ranges. By virtue
of their position they appear as mountain roots. Since
they are deep seated, only very prolonged erosion
exposes them on the surface.

Chonolith
• This is general term applicable to all other
forms of igneous rocks. It includes any
irregular form of igneous rocks and hence it
does not have any specific shape.

Forms of Extrusive Igneous Rocks
Lava flow
• On eruption of a volcano, lava simply flows on
the surface and on consolidation gives rise to lava
flows. These closely resemble sills in shape.
Based on surface appearance, lava flow are
described as block lava and ropy lava. Block
lava is less mobile and has a rough and irregular
surface. The ropy lava is more mobile and has
wrinkled but smooth and shining surface.

Pyroclasts
• The rock fragments thrown out at the time
of volcanic eruption are called pyroclasts.
These are described variously, based on size
and shape.

• Bigger and angular fragments are called
volcanic blocks. If they are somewhat rounded
they are known as volcanic bombs. Smaller
fragments are called lapilli or cinders.
Fragments still smaller in size are called
volcanic sand, volcanic dust and volcanic
ash.

Volcanic blocks &Volcanic bombs

• A rock formed out of volcanic dust and ash is
called volcanic sand, volcanic dust and
volcanic ash. A rock formed out of volcanic
dust and ash is called tuff.
Volcanic
agglomerate or volcanic breccias is the
name
given
to
the
consolidation
heterogeneous mass of pyroclastic material.

Miscellaneous
Primary and secondary minerals
• Primary minerals are those which have been
formed directly from the solidification of magma
or lava. These are also called pyrogenetic
minerals.
• Feldspar, pyroxenes, etc. are typical examples
of this kind. Secondary minerals which have
been formed due to weathering or metamorphism
or from precipitation or evaporation or circulating
of natural solutions. Bauxite, limonite, talc,
chlorite, calcite, and opal are typical examples of
secondary minerals.

Primary and secondary minerals

Miscellaneous
Essential and Accessory Minerals
• Both these are primary minerals. Essential minerals
are defined as those which are necessary for naming
or identification of rocks. These minerals will
appear as major constituents and their characteristic
occurrence is helpful in naming a rock. For example
Quartz and feldspar. It means that both of these
must occur predominantly in order to call a rock
granite. If any one of these is either less or absent,
the rock cannot be called granite.

Miscellaneous
• Accessory minerals are those which usually
occur in a rock, but their presence or absence
will not be considered in naming the rock.
These minerals usually occur in small quantity
in rock. Eg. hornblende and biotitic are the
common accessory mineral of granite.

Common Igneous rocks and relation of
their constituents minerals
• Generally, igneous rocks are named on the basis of
the minerals present in them and their relative
proportions. But minerals present in the rock do not
bear any definite ratio depending on the composition
of the parent magma, the same rock may show
changes in proportions of essential minerals and
types of accessory minerals. When essential minerals
change considerably a rock is named differently.
• The intercepts on any vertical gives the proportion of
the constituents minerals for the rock concerned.

Common Igneous rocks and relation
of their constituents minerals

Classification of Igneous Rocks
• There are different types of classification of
igneous rocks, Simple classification based on
silica percent, silica saturation and depth of
formation

Classification Based on Silica Percent
• The chemical composition of a rock is generally
expressed in terms of different oxides like SiO2, Al2O3,
Fe2O3, FeO, MgO, and CaO. Among different oxides
silicon dioxide is always predominant in igneous rocks.
• Since Silica percent is also responsible for the
formation of different minerals and their association. It
serves as a suitable basis for the classification of
igneous rocks. When silica content exceeds 66 % the
igneous rocks are called acidic, when it is 52- 66 %, the
rock are called intermediate, The basic rock have 45 –
52 %.

Classification Based on Silica Percent

Acidic Igneous Rocks
• These rocks are compositionally rich in silica,
alumina and alkalis, but are poor in calcium,
magnesium and iron.
• They are composed of quartz, alkali feldspar
and muscovite mica- representing the late stage
of crystallization of magma.
• They are leucocratic because these are rich in pale
colored minerals and poor in dark magic minerals.
Mafic minerals occur only as accessory
minerals, in small quantities.

• They have, characteristically, free primary
quartz and are always devoid of unsaturated
minerals.
• They are relatively lighter rocks and have high
melting points.
• Granites and many pegmatites are typical
examples of this group. Granites may have 70
% or more of silica content.

Intermediate Igneous Rocks
• These rocks may be lacking in free quartz
completely or may be having very little of it.
• These are mainly composed of alkali feldspar
with a few accessory minerals.
• They are leucocratic or mesocratic.

Basic Igneous Rocks
• In these rocks, magic minerals occur as essential minerals
i.e.. they occur as important constituents.
• Quartz and olivine are generally absent, or any one of them
may occur in small quantities.
• Feldspar are of the plagioclase type.
• Silica and alkalis are less and calcium and magnesium and
ferrous iron are more in content when compared with acidic
rocks.
• The dominant occurrence of magic minerals makes these
rocks to have a slightly higher specific gravity.
• Gabbros, norite and basalt are examples of this group.

Ultrabasic Rocks
• If the acidic rocks represent one extreme, the
ultrabasic rocks represents the other extreme. The
important characters of this group are:
• Free quartz is always absent
• Unsaturated minerals and/ or magic minerals occur as
essential minerals
• These have the highest density among the different rock
types.
• Compositionally, these are the poorest in silica and the
richest in magnesium.

Classification Based on Silica
Saturation
• The Silica content of the Parent magma is
responsible for the occurrence of saturated minerals
or unsaturated minerals. In igneous rocks and quartz
will never co-exist with unsaturated minerals.
• When the parent magma is very rich in silica, only
saturated minerals are formed and the surplus
quantity of silica crystallizes as free quartz. This is
the resulting rock will always have quartz and
saturated minerals like feldspar. Unsaturated
minerals like olivine will never occur in them

Saturation
• When the parent magma has just enough silica for
the formation of saturated minerals, the resulting
rock will possess neither free quartz nor any
unsaturated minerals.. Such rocks are characterized
by the presence of saturated minerals like feldspar.
These rocks are appropriately called saturated rocks.
• When the parent magma has silica slightly less than
what is required for the formation of all saturated
minerals. That is, such rocks are composed of both
saturated and unsaturated minerals. It is obvious
that free quartz will always be absent in them.

Saturation
• When the parent magma is highly deficient
in silica, saturated minerals may not be formed
at all and only unsaturated minerals will be
occurring in the resulting rocks. This group
also represents under saturated rocks.
• The preceding grouping of rocks into
oversaturated and under saturated categories
depends on minerals associated which in tern
is related to the silica content of magma.

Classification based on depth of
formation
• According to this classification, igneous rocks are
grouped into plutonic rocks, hyperbyssal rocks and
volcanic rocks. Those igneous rocks which have
formed under high pressure at great depths in the
earth’s crust are called plutonic rocks. The igneous
rocks formed on the surface are called volcanic
rocks. The other igneous rocks which have formed
at shallow depths are called hypabyssal rocks.

formation
• The plutonic rocks are formed (i) under great
pressure, (ii) at high temperature and (iii)in the
presence of huge quantity of volatiles. Great pressure
ensure total crystallization of minerals formed and the
hot surroundings slow down the process of
solidification. This provides a lot of time for the
mineral molecules to crystallize. The presence of
volatiles considerably reduces the viscosity of magma.
Which facilitates easy movement of mineral molecules
in the body of magma. The net result of all these
processes is the development of a characteristic
coarse texture for plutonic rocks.

formation

formation
• In the case of volcanic rocks, the physical conditions that
prevail are
• (i) Solidification under low pressure (only under
atmospheric pressure) and in the absence of volatiles.
• (ii) the underlying surface rocks are relatively cold
overlying atmospheric gases circulate continuously. On the
lava surface, cold atmospheric gases keep on circulating
because the air above the lava flow is occupied by the dense
cold air.
Further, under these circumstances the
associated volatiles also escape into the atmosphere
which renders lava devoid of them; absence of volatiles
results in high viscosity and low pressure resulting in the
formation of fine grained or glassy matter. The net result
is that the volcanic rock are fine grained.

formation
• The hyperbyssal rocks are formed at
shallow
depths
under
moderate
temperature and pressure. Surrounding by
neither cold nor very hot rocks, resulting in a
medium rate of cooling. Naturally, such are
medium grained, that is these are more
coarse grained than volcanic rocks but more
fine than plutonic rocks. Dolerite is the
typical example.

Structures and Textures
• Structures and textures are physical features
associated with the rocks. They are generally primary
in nature. i.e.. they occur along with the formation of
rocks and are important because.
• (i) the contribute to the strength or weakness of rocks.
• (ii) they serve as distinguished features of rock
groups, and also,
• (iii) They reveal the mode of origin of rock
concerned.
• The term structure used in this context is different from
secondary geological structure like folds and faults
which also occur in rocks.

Structures and Textures
• The term structure as commonly used refer
to large-scale features or field characters of
rocks such as bedding or columnar
structure or pillow structure.
• Textures on the other hand refers to smallscale features like porphyritic texture and
interlocking texture which can be studied in
hand specimen.

Common Structures of Igneous
Rocks
• In igneous rocks the common structures are:
• Vesicular
Structure,
amygdaloidal
structure, columnar structure, sheet
structure, flow structure and pillow
structure.

Rocks
Vesicular Structure
• This structure is due to the porous nature, commonly
observed in volcanic rocks and is attributed to the
following reasons. Magma is an intimate mixture of rock
melt and volatiles (i.e.. gases). Eruption of any volcano is
accompanied by flow of such melt and volatiles (i.e.. gases).
Eruption of any volcano is accompanied by the flow of such
melt on the surface.
• Then the gases being lighter move upward and as they
escape into the atmosphere create empty cavities of
various sizes and shapes near the surface of lava flow.
This cavities are called vesicles. Obviously, vesicles will be
more nearer the top of lava flow and less at the base.

Rocks
• The degree of porosity in volcanic rocks due to
the presence of vesicles varies widely. When
the volcanic rock is highly porous and
spongy in appearance, it is called scoria. The
extreme cases of porosity produce a very light
rock called pumice. It is usually pale grey in
color and looks like a solidified form of foam
or froth of lava. Pumice is so light that it
floats on water.

Rocks
• The importance of this structure from the civil
engineering point of view is as follows:
• The vesicles, if plenty, make the rock hollow and less
strong. Therefore, highly vesicular rocks are
undesirable at foundation sites of huge civil
construction like dams.
• (ii) If the vesicles are interconnected with fractures,
the rock become permeable too and behaves as an
aquifer. Such a condition is good in terms of
increasing ground water potential but the same is
undesirable in tunneling as it may cause ground
water problems.

Rocks
Amygdaloidal Structure
• The vesicles which are empty to start with in
the vesicular structure, are subsequent
filled up by the deposits of surface waters or
underground waters or hydrothermal solutions.
Such infillings are called “amygdales”.
When empty cavities are filled with
amygdales, the vesicular structure is called
an amygdaloidal structure

Rocks
Columnar Structure
• In this structure, the volcanic igneous rock appears to be
made up of numerous parallel polygonal prismatic
columns bundled together. This is the result of the
contracting of lava during cooling.
• Suppose , while flowing, lava occupies a depression on the
earth’s surface. Then cooling of such lava commences
from the surface downwards and the development of
centres of contraction takes place on the cooling surface.
The line joining these centres are the direction of great
tensions. Therefore vertical cracks appear perpendicular to
these lines. These cracks intersect each other and
produce parallel polygonal prismatic columns. Ideal
condition produces hexagonal columns.

Rocks
Sheet Structures
• In this structure, the rocks appear to be made up of
number of sheets, because of the development of
nearly horizontal cracks. This is the effect of erosion
over rocks formed at a depth.
• Plutonic rocks are formed under great depths, which
means under great pressure (due to overburden). When
erosion takes place, the overlying strata gradually
disappears, ultimately exposing plutonic rocks on
the surface.

Rocks
• In this process, the earlier pressure no longer
remain and this release or disappearance of
pressure results in the development of joints and
cracks, roughly parallel to the surface. These
are sheet joints. As it may be inferred from the
cause of the origin of this structure, these joints
occur more prominently and closely spaced
nearer the surface. With increasing depth,
gradually, they become less significant and get
more and more widely spaced. Ultimately, they
ceases at considerable depths.

Rocks

Rocks
Pillow Structure
• In this structure, the volcanic igneous body appears
as a pile of numerous overlapping pillow or sacks. It
occurs particularly in soda-rich basaltic rocks known
as spillities.
• The pillows are generally interconnected and have
glassy top. As the lava flows, its upper surface gets
solidified, while the interior remains hot and fluid.
Such a situation may result in the rupturing of the
earlier formed thin crust and the draining out of
unsolidified lava. This processes, when repeated,
produces a pillow structure usually under
submarine conditions.

Common Texture of Igneous Rocks
• The term texture mainly refers to the mutual relationships
of the constituents minerals in a rock in addition to
crystalline, granualrity, and shapes of minerals in a
rock.
• Texture Based on the degree of Crystallinity
• Igneous rocks are formed due to cooling and
solidification of magma or lava. Crystallization of
different minerals takes place when respective
molecules in magma move to their centres of
crystallization and arrange themselves in a definite
pattern. If reasonable time is available, crystallization of
different minerals take place from the melt giving rise to a
rock.

• But if sudden chilling of lava occurs, then there may
not be any time for crystallization to take place.
Under such conditions, lava solidifies as completely
amorphous or glassy matter without any minerals.
On the other hand, if cooling time is intermediate,
then the resulting rock will be composed partly of
glassy matter and partly of minerals.

• Thus, depending on the nature of cooling the
resulting igneous rocks are
• (i) Completely crystalline
• (ii) Completely glassy
• (iii) partly crystalline and partly glassy Thus the
preceding three different types of crystallization
give rise to three textures of igneous rocks
namely;
• Holocrystalline,holohyaline, and
merocrystalline.

Texture Based on Granularity
• Depending on the physical condition that had
prevailed during the crystallization of magma,
minerals grain occur in different sizes. The presence
of volatiles, low viscosity, slow cooling and great
pressure help to grow large minerals.
• The following textures have been recognized based on
the granularity of minerals. If minerals in the rock are
big enough to be seen by the naked eye, the texture is
described as phaneric texture. On the other hand, if
minerals are too fine to be seen separately by the naked
eye, the texture is described as aphanitic texture.

Texture based on shape of crystal
• These textures are of two different kinds; The first term of
development of crystal faces or boundary outlines and the
other with reference to the nature of the growth of the
minerals.
• When the mineral is completely bounded by crystal faces
it is called “euhedral” ; when crystal faces are absent, it is
called “anhedral” and when only a part of the mineral is
bounded by crystal faces it is called “subhedral”.
Euhedral minerals are rather rare in nature. The c/s of
euhedral minerals as seen in their thin sections appear with
polygonal boundary outline, whereas in the case of anhedral
minerals, they appear rounded or surrounded in thin
sections.

Suitability of Igneous rocks for building
and foundation
• From the civil engineering point of view, the very
purpose of studying petrology is to get a concept
about what makes some rocks very competent and
other less competent. Among the various types of
rocks. Igneous rocks are inherently very competent
and desirable for different civil engineering
purposes.

and foundation

and foundation
• The requirement of good building stones are;
strength,
durability,
color,
appearance,
workability and availability. These properties of
rocks except the last one are, in turn a function of
their minerals, texture, structure, grain size,
porosity and permeability.

and foundation
• The igneous rocks are composed mainly of silica
minerals. Among various minerals, silicate minerals
are the most durable. Further, rocks rich in silica
content are pleasingly light colored as in the case of
granite.
• Since igneous rocks are formed out of solidification
of a melt, they are necessarily dense, compact and
massive. In other words, these rocks do not have any
internal opening or hollow nature. This contribute to
the strength and heaviness of these rocks. Further
igneous rocks do not have an inhererent weakness
due to the occurrence of bedding plane or mineral
alignment.

Suitability of Igneous rocks for
building and foundation
• Igneous rocks by virtue of their texture and
minerals present in them have the ability to
take very good polish and thus are become
increasingly popular for face work .

Megascopic Description of relatively
common igneous rock types
Granite
• Granite is a plutonic igneous rock because it is
formed due to solidification of magma at
greater depth. It is holocrystalline and leucocratic
rock because it is complete crystalline and light
colored rock.
Minerals present in granite
• Granite is composed of only primary minerals.
Among these, feldspar and quartz occur as
essential minerals and common accessory
minerals are such as hornblende, biotitic.

Structure
• Granite is compact, dense, massive and hard
rock. But mural joints occur in some, dividing the
rock into number of rectangular blocks,
thereby facilitating the quarrying processes.
Texture
• Granites typically exhibit an interlocking,
coarse grained texture. Granites are usually
equigranular. But some shows inequigranular,
textures and are called granite porphyries.

Appearance of Granite
• Granite is generally medium to coarse grained and
grayish or pinkish in color.
Mode of Occurrence and relative abundance
• Granite rock occur in the form of very large igneous
bodies such as batholiths, stocks, often they occur
as cores of mountains ranges and are thus related
to mountain-building activity.

Physical properties of Granite
• Granite is massive, unstratified and dense,
therefore it is very strong and competent
• Granite has an interlocking texture, which keeps
minerals firmly held and this cohesion contributes
greater strength.
• Granite is either equigranular or has porphyritic
texture.
• Since granite is massive and formed from melt it
is neither porous nor permeable. So no
saturation or percolation by water is possible.

• Granite is very rich in silica; therefore it is very
much resistant to decay
• Presence of mural joint permit easy quarrying
• Presence of rift and grain permit easy dressing.
• Granites have the ability to take superb polish, and
hence are becoming increasingly popular for face
works of construction.
• Granites offer reasonable fire and frost resistance,
because minerals are not many and these rocks are
free from fractures.

Pegmatites
• These are holocrystalline, phaneric-coares
grained igneous rocks with an interlocking
texture.
• Many
pegmatites
are
acidic
and
oversaturated. They resembles granites in
mineralogy and hence are described as
granite pegmatites.

Pegmatites
Minerals present in pegmatites
• Granites pegmatites are mainly composed of
alkali feldspar and quartz but may also be rich
in muscovite and biotitic micas. Tourmaline,
beryl, topaz, apatite and lepidolite minerals which
are rich in rare volatiles also occur often. In
addition to these, rare minerals of tin, arsenic etc.
also occur in pegmatites. Thus pegmatites are
storehouse of rare and valuable minerals.

Mode of formation and occurrence of pegmatites
• The peculiar grain size and mineral composition
suggest that pegmatites are formed as product of
solidification of final magmatic residues which
are specially rich in volatile constituents.
Structure and Texture
• Pegmatites are rather less in occurrence and do
not possess any specific structure diagnostic to
them. But texturally, the minerals a large in size
and interlocked Some of the constituent minerals
develop very beautiful crystal outlines.

Pegmatites
Physical properties
• Like granites, these rocks also have similar
mineral content and interlocking texture .
But from the civil engineering point of view,
these rocks are not very useful because the
extreme large minerals considerably
influences the physical properties locally
and hence the rock mass cannot behave
uniformly throughout.

Dolerite
• The term dolerite was coined by Hauy to refer a dark,
heavy, fine grained igneous rock. This was the most
commonly found hyperbyssal rock. It is intermediate in
composition and characteristically melonocratic. Miner
logically and chemically it is similar to basalt.
Mineral present in Dolerite
• Dolerite is a rock, normally composed of feldspar and
pyroxene as essential minerals. Iron oxides, hyperstene
and biotitic occur as common accessory minerals. In
general accessory minerals occur in very small quantities.

Mode of Occurrence
• Very often, dolerite occur in nature as an intrusive rock i.e.
as dykes (and less commonly as sills) in granites. These
dark colored rocks are prominently noticed in the field by
virtue of color contrast with surrounding granites which are
light colored.

Structure and Texture
• Dolerite is very dense, massive and compact rock. It is
neither porous nor permeable. It is relatively heavier than
granite as it is richer in mafic minerals. The texture is
generally equigranular, phaneric fine grained.

Physical Properties and Uses
• Dolerite has all the merits and virtues possessed by
granite, except its color. Since dolerites are more fine
grained, they are stronger and more competent they
are stronger and more competent than granites. They
are suitable as railway ballast, concrete or bitumen
aggregate, etc. As road metal, though they do not have
good cementing values like limestone, they can be
used if locally available.

Basalt
• The term basalt was first used by pliny to refer a black,
ferruginous rock. Now the term is applied to very fine grained,
black volcanic rock in which plagioclase feldspar and mafic
minerals occur approximately in equal amounts.
Minerals
• Basalt is a simple mixture of Labradorite, augite(essential) and
iron oxide. It is similar to dolerite in mineral content. Biotitic
and hornblendes are the other rare accessory minerals.
Structure and texture
• Vesicular and amygdal structures are the most common in
basalt. Columnar structures and flow structure are also
observed in some cases.

Availability and mode of Occurrence
• Basalts are the most abundant among volcanic rocks. Their
quantity is five times greater than all other volcanic rocks put
together. They occur as flat or slightly inclined lava flow.
Properties and Uses
Massive basalts are highly durable and the strongest. This is
because not only they are compact, hard and tough but also
more fine grained than dolerite. For this reason basalts are
extensively used as building stones.
As a road metal, the basalts are excellent for macadam and
bitumen roads. They are hard, tough and wear-resisting and
have good binding properties.

Igneous rocks

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