The document provides a comprehensive overview of petrology, focusing on the classification, composition, and characteristics of igneous, sedimentary, and metamorphic rocks. It details their formation processes, including the rock cycle, and distinguishes various types of rocks based on their origin and mineral content. Additionally, the text discusses the physical features, structures, and textures of rocks that are essential for understanding their properties and geological significance.

1. PETROLOGY
Y.S.G.GOVIND BABU

2. SYLLABUS
Definition of rocks-geological classification of
rocks -dykes and sills-common structure and
textures of igneous, sedimentary and
metamorphic rocks and their distinguishing
features-megascopic and microscopic study of
granite, dolerite, basalt, pegmatite, laterite,
conglomerate, sandstone, shale, limestone,
gneiss, schist, quartzite, marble and slate –
rock excavation –stone aggregates

3. Petrology
 PETROLOGY literally means STUDY OF ROCKS
 It comprises of ORIGIN, ASSOCIATION, OCCURRENCE,
MINERAL COMPOSITION, CHEMICAL COMPOSITION,
TEXTURE, STRUCTURE and PHYSICAL PROPERTIES of
rocks
 PETROLOGY can be categorized into two
I. Petrography: it deals with descriptive part of rocks
II. Petro genesis: it deals with mode of formation of rocks

4. Definition of a Rock
 A rock may be simply defined as “an aggregate of minerals”.
 Since the crust of earth is composed of rocks, a rock may also be
defined as “a unit of the earths crust”.
 Due to occurrence of different minerals or different proportions of
same mineral combinations can give rise to different rock types.
 Close observation reveals that in nature only some rock types are
common and among large variety of mineral species only very
few take part in formation of different rocks
 A rock can also include non-minerals, such as the organic matter
within a coal bed, or within some shale's

5. Rock cycle
 The materials that make up the rocks of the crust are slowly but constantly being
changed from one form to another. The inter-relationships between rock types can
be summarized on what is known as the rock cycle diagram
 Magma can either cool slowly (over centuries to millions of years) within the crust
or onto the surface—forming intrusive or extrusive igneous rock.
 All types of rocks are uplifted and exposed at surface and they are weathered
leading to transportation and sedimentation of deposits. The sediments are buried
and compressed and become hardened and cemented into sedimentary rock.
 Again through various means, largely resulting from plate tectonic forces, different
kinds of rocks are buried deep within the crust where they are heated up,
squeezed and chemically changed into metamorphic rock.

7. Geological classification of rocks
 Geological classification of rocks is based on their mode
of origin
 It is most proper because grouping of rocks is more
logical, less ambiguous, orderly and comprehensive.
 As per geological classification, rocks are classified as
I. IGNEOUS ROCKS
II. SEDIMENTARY ROCKS
III.METAMORPHIC ROCKS

8. Igneous rocks
 Igneous rock (derived from the Latin word igneus meaning of
fire, from ignis meaning fire) is one of the three main rock
types.
 Igneous rock is formed through the cooling and solidification of
magma or lava.
 Igneous rock may form with or without crystallization, either
below the surface as intrusive (plutonic) rocks or on the surface
as extrusive (volcanic) rocks.
 Igneous rocks are most abundant rocks in earths crust.

9. INTRUSIVE IGNEOUS ROCKS:
 Intrusive rocks are igneous rocks that cool below the surface.
Liquid rock below the surface is called magma.
 Rocks that crystallize below the surface are well-insulated and
cool slowly.
 Slow cooling gives the crystals time to grow large. The crystals,
which are large enough to see with the naked eye, are mostly
angular or irregular in shape.
 Based on associated country rocks, intrusive rocks form into
Concordant and Discordant
If igneous body is parallel to planes of country rock , its called
Concordant.
If the body of rock cuts cuts across the planes of surrounding
rocks , its called Discordant

10. FORMS OF INTRUSIVE IGNEOUS ROCKS:
Most common forms of intrusive igneous rocks are: dykes, sills,
laccolith, lopolith, bysmalith, phacolith, batholiths
DYKES:
 Dykes are discordant, sheet like, vertical or steeply inclined, intrusive igneous rock
bodies
 They occur cutting across the bedding planes of country rocks in which they are
found.
 Dykes may be horizontal or vertical . Steeply inclined or vertical dykes extending to
great depth are more common.
 They may be very long (50-60km) and thick ( upto30m) or they may be very short
and thin also.
 A number of dykes occur parallel or radially and are called swarm or system of
dykes.

11. Figure showing dykes, sills, laccolith, lopolith

12. Importance of dykes from civil engineering point of view:
 They are undesirable at sites of foundation of dam because they
introduce heterogeneity in the region and also their sides turn out be
weak planes.
 Dykes are like walls and act as barriers for the flow of underground
water which may result in good or bad potential of ground water in that
region
 Dykes may give rise to new springs or seal old springs
 Dykes may cause oil accumulation and there by contribute to
occurrence of oil and gas deposits under favorable conditions.
 As dykes are hard, durable , black and fine grained, they are used in
making statues and sculptures etc.,

13. SILLS:
 Sills are similar to dykes but unlike dykes these are concordant
 Sills are formed due to penetration of magma into bedding planes of
country rocks .
 The sills which were spread over large areas are generally thin and
show uniform thickness.
 Sills may occur as horizontal, inclined or vertical bodies. But often
they are horizontal or gently inclined.
 Sills produce baking effect on rocks on both sides. Sills cools down
rather slowly resulting in coarse grained igneous rocks.

14. LACCOLITH:
Laccoliths are somewhat large intrusions that result in uplift
and folding of the pre existing rocks above the intrusion.
They are also concordant types of intrusions.
PLUTONS:
Plutons are generally much larger intrusive bodies that have
intruded much deeper in the crust.
They may show sharp contacts with the surrounding rocks
into which they intruded, at deeper levels in the crust the
contacts are often gradational.

15. LOPOLITH:
Lopolith are relatively small plutons that usually show a concave
downward upper surface.
This shape may have resulted from the reduction in volume that
occurs when magmas crystallize, with the weight of the overlying
rocks causing collapse of into the space once occupied by the
magma when it had a larger volume as a liquid.

16. BATHOLITHS:
 Batholiths are very large intrusive bodies, usually so large that there
bottoms are rarely exposed. Sometimes they are composed of several
smaller intrusions.
 They occur as core of fold mountains and appear elongated along
mountain ranges.
BYSMALITH:
 when magma happens to be highly viscous, a lateral spreading along
the bedding plane will be very less and the intruding magma acquires a
cylindrical shaped body.
 Its appearance seems to be as if it has punctured through surrounding
country rocks.

18. EXTRUSIVE VOLCANIC ROCKS:
 Extrusive rocks are igneous rocks that cool on the surface.
 Liquid rock that is erupted onto the surface is called lava.
 Rapid cooling does not allow crystals in the lava to grow very large
and can be glassy, aphanitic, porphyritic or pyroclastic.
 the lava on the surface is no longer under great pressure resulting in
escape of gases out of lava.
 The gas bubbles escape leads to a vesicular structure. The most
common type of volcanic rock is basalt

19. PRIMARY AND SECONDARY MINERALS:
 primary minerals are those which have formed directly from the
solidification of magma or lava. These are also called pyrogenetic
minerals.
 Feldspars and pyroxenes are typical examples of this kind.
 Secondary minerals are those which have formed due to weathering or
metamorphism or from precipitation or evaporation of natural
solutions.
 Bauxite, limonite (products of weathering), talc and chlorite (products
of metamorphism) calcite, zeolite and opal( formed from natural
solutions)

20. 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
 Eg: Quartz and Feldspar are described as essential minerals of rock GRANITE. If any
one of these is either less or absent , the rock cannot be called granite.
 Accessory minerals are those which usually occur in rock. But their
presence or absence will not be considered in naming the rock. These
minerals usually occur in small quantities in rock.
 Eg: Hornblende and biotite are referred as accessory minerals of granite. These make
up 10-15% of granite. A rock will be said granite with or without these accessory
minerals but possess essential minerals.

21. Classification of Igneous rocks
 The composition of an igneous rock is described on the basis of the
minerals present.
 The broad compositional classes are FELSIC and MAFIC
 felsic rocks are dominated by non-ferromagnesian minerals and
mafic rocks that are dominated by ferromagnesian minerals.
 Rocks with compositions between mafic and felsic are termed
intermediate, while those with an overwhelming predominance of
ferromagnesian minerals are termed ultramafic.
 Felsic and intermediate rocks are also known as SIALIC - which refers
to the predominance of silica and alumina.
 mafic and ultramafic rocks are known as SIMATIC - referring to the
predominance of magnesium and iron.

23. LEUCOCRATIC, MELANOCRATIC AND MESOCRATIC
ROCKS
 Rocks rich in felsic minerals are generally light in color and are of low
density and such rocks are called leucocratic rocks
Eg: Granites and Pegmatite
 Rocks rich in mafic minerals are generally dark in color and are
relatively heavier. Such rocks are called melanocratic rocks
Eg: Dolerite and Gabbro
 If an igneous rock is neither dark colored nor light colored it is called
mesocratic rock
Eg: Syenite a reddish brown colored igneous rock

24. A. CLASSIFICATION OF IGNEOUS ROCKS BASED, ON
THE OCCURRENCES AND DISTRIBUTION;
Volcanic rocks (also called extrusive igneous rocks) include all
the products resulting from eruptions of lava (flows and
fragmented debris called pyroclasts). Solidified under low
pressure and higher temperature conditions. Ex: basalt, rhyolite,
andesine etc.
Hypabyssal rocks, these rocks are intermediate solidified rocks
in between volcanic and plutonic rocks i.e., at shallow depths
from ground, under medium pressure and temperature conditions.
Ex: pegmatite, dolerite and kimberlites
Plutonic rocks (also called intrusive igneous rocks) are those that
have solidified below ground; plutonic comes from Pluto, the
Greek god of the underworld. Solidified under higher pressure
and temperature conditions. Ex: granite, pyroxinite, dunite and
gabbro.

25. B. MAJOR CHEMICAL ELEMENTS FORMING IGNEOUS ROCKS BASED ON
SILICA (SIO2) PERCENTAGE
 Volcanic and plutonic rocks are divided further on the basis of chemistry and
mineral composition.

27. C. CLASSIFICATION BASED ON SILICA SATURATION
Based on the silica saturation in minerals of rocks classify into four
types; those are
Over saturated rocks
When the parent magma is very rich in silica, only saturated minerals are
formed and the surplus quantity of silica crystallizes as free quartz and
the resulting rock will always have quartz and saturated minerals like
feldspar. Unsaturated minerals like olivine , nepheline never occurs in
them.
Saturated rocks
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 mineral. such rocks will have saturated minerals like
feldspar. syenite is an example of saturated rocks.

28. Under saturated rocks
When the parent magma has silica slightly less than required or deficient
in silica content, then saturated minerals will be formed to the possible
extent or only unsaturated minerals will be occurring in resulting rocks.
Among the rocks free quartz will be absent and these rocks are referred
as under saturated rocks.

29. Structure and Textures of Rocks
These are physical properties associated with rocks. They occur along the
formation of rocks. They are important because
1. They contribute to strength or weakness of rock
2. They serve as distinguishing feature of rocks
3. They reveal the mode of origin of rocks concerned
STRUCTURE refers to large scale features or field charecters of rocks
as bedding, columnar structure, pillow structure etc.,
TEXTURE refers to small scale features like porphyritic txture and
interlocking texture(which can be studied in hand specimens)

30. Importance of structure and texture of rocks
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.
They contribute to the strength or weakness of rocks.
They serve as distinguished features of rock groups, and also,
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.

31. COMMON STRUCURES OF IGNEOUS ROCKS
Vesicular structure:
 This term refers to vesicles (cavities) within the igneous
rock. Vesicles are the result of gas expansion (bubbles),
which often occurs during volcanic eruptions. Pumice and
scoria are common types of vesicular rocks.
Amygdular structure:
 if vesicles have been filled with material (usually calcite
and quartz, then the term amygdular should be added to the
textural description of the rock. An amygdule is defined as
a refilled vesicle.

34. Columnar structure:
In this volcanic rock appears to be made up of numerous
parallel polygonal prismatic columns bundled together.
This is the result of contraction of lava during cooling.
Sheet structure:
In this the rock appears to be made up of numerous sheets
because of development of nearly horizontal cracks.
This is the effect of erosion over rocks formed at depth.

37. Flow structure:
This structure refers to nearly parallel features occurring in
volcanic rocks which develop as a consequence of flow of lava.
Pillow structure:
In this, the volcanic igneous body appears to be as a pile of
numerous overlapping pillows or sacks. It occurs particularly in
soda-rich basaltic rocks known as spillites.

40. TEXURES OF IGNEOUS ROCKS
A. BASED ON DEGREE OF CRYSTALLINITY
 1. Holohyaline
Rock composed entirely of glass; no crystals visible even with
magnification. Indicates cooling so rapid that no crystal growth
could occur, i.e. quenching.
 2. Holocrystalline
Rock composed entirely of crystals (which may or may not be visible
without magnification). Indicates cooling that was sufficiently slow
to allow complete crystallization to occur.

41.  3. Hypocrystalline (or Hyalocrystalline)
Rock composed of both crystals and glass. Indicates a period
of relatively slow cooling (sufficiently slow to allow
crystallization) followed by quenching of the remaining
magma.

42. B. BASED ON CRYSTAL SIZE
1. Equigranular.
All of the crystals are approximately the same size. Indicates that the
entire rock crystallized under a single set of P-T (depth) conditions.
a. Aphanitic.
Uniformly fine-grained texture in which the individual crystals are too
small to be seen easily without magnification. Indicates rapid cooling
(but not quenching), i.e. volcanic extrusion.
Microcrystalline. Individual crystals large enough to be seen easily
with a petrographic microscope.
Cryptocrystalline. Individual crystals to small to be seen easily
even with a petrographic microscope; but sufficiently crystalline to
give a strong x-ray diffraction pattern.

43. Rock with Apanitic structure

44. b. Phaneritic.
Uniformly coarse-grained texture in which all the individual crystals are
easily visible without magnification. Indicates slow cooling, i.e.
intrusion.
Fine phaneritic. Average crystal size < 1 mm. Indicates shallow
intrusive cooling, i.e. hypabyssal intrusion (dikes and sills).
Medium phaneritic. Average crystal size 1-5 mm. The most
common texture for common plutonic rocks (i.e. granites).
Coarse phaneritic. Average crystal size 5-10 mm. Indicates deep
intrusive cooling, i.e. plutonic intrusion (batholiths).
Pegmatitic. Average crystal size > 10 mm. Indicates intrusive
cooling of an abnormally gas-rich, silicic magma.

46. 2. Porphyritic.
Texture consisting of crystals of two distinctly different sizes. Indicates two
distinctly different modes (episodes) of cooling. The larger crystals (usually of
at most two or three different minerals) are called phenocrysts and are usually
nearly euhedral. The material surrounding the phenocrysts is called the matrix
or groundmass.
 Porphyritic-hyaline (or Vitrophyric).
Phenocrysts surrounded by glassy groundmass. Indicates an initial period of
slow (intrusive) crystallization followed by quenching of the remaining magma.
 Porphyritic-aphanitic.
Phenocrysts surrounded by aphanitic groundmass. Indicates an initial period of
slow (intrusive) crystallization followed by rapid (extrusive) crystallization of
the remaining magma. The most common texture for common volcanic rocks
(i.e. basalts).
 Porphyritic-phaneritic
Phenocrysts surrounded by phaneritic groundmass. Indicates two stages of slow

48. Megascopic and Microscopic study of Rocks
1. 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, biotitie.
 The quartz grains are usually spherical in shape and are a white to
grayish color.

50. The feldspars grains are mostly potassium and sodium rich
varieties with individual rectangular shaped grains. The
feldspars are often white, grey, or pinkish in color depending
on the chemical composition.
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 equi-granular. But some shows in
equigranular, textures and are called granite porphyries.

51. 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.

53. 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.

54.  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.

55. USES OF GRANITE:
Granite is desirable as foundation rock, building
stone, road metal, railway ballast or for flooring.
In the past its idely used as pillars, beams, slabs etc.,
in temples and forts.
More over being more competent rocks granite does
not require any lining during tunneling operations.

56. 2. 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.
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.

59. 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.

60. 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.
ENGINEERING POINT OF VIEW:
 Since pegmatite minerals are large in size and the rock mass cannot
behave uniform throughout. Further, the presence of mica which has
excellent cleavages obviously makes the rock weak. So it is unsuitable
to be used as a building stone and also undesirable at the site of
foundation of major constructions. However, pegmatites are
economically very important due to the presence of rare and valuable
minerals.

62. 3. DOLERITE
The term dolerite was coined by Hauy to refer a dark, heavy, fine
grained igneous rock. This was the most commonly found hypabyssal
rock. It is intermediate in composition and characteristically
melonocratic. Mineralogically and chemically it is similar to gabbro
and basalt.
Dolerite is a dark, fine grained black or dark greenish black igneous
rock.
Composition:
Dolerite consists of Plagioclase Feldspars and pyroxene(augite).
Iron oxides, hypersthene and biotite occur as common accessory
minerals. Olivine is some times found if the parent magma was
deficit of silica.

63. Texture:
 Dolerite is a massive and compact rock. It is neither porous nor
permeable.
 The texture in dolerites is generally equigranular. Interlocking texture
is also common in dolerite.
 Under the microscope dolerite exhibit Ophitic or subophitic texture.
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.

65. Types of Dolerite:
 A massive rock with melanocratic, equigranular, compact phaneritic
medium to fine grained texture is typical normal dolerite.
 Slightly un saturated and olivine bearing type is called Olivine –
Dolerite.
 Slightly over saturated and quartz bearing type is called “Quartz-
Dolerite”
 Normal dolerite having prophyritic texture is called “ Dolerite –
Porphyry”
 Dolerite with completely weathered minerals is called “Diabase”
 Plutonic equivalent of dolerite i.e., coarse grained type is called
“Basalt”
 Glassy equivalent of dolerite is called “Trachylyte”.

66. 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 than granites.
 Dolerite dykes are generally vertical or steeply inclined with
limited width quarrying them is naturally difficult.
 The greater toughness and lack of weak planes does not make
them easily workable.
 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.

67. 4.BASALT:
Basalt is a mafic, extrusive fine-grained dark green to black volcanic
rock with a porphyritic texture.
Basalt is composed primarily of pyroxene, and calcium-rich
plagioclase with small amounts of olivine and amphibole.
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.
 Pyrite may also seen sometimes. Either quartz or olivine may appear
in small amounts depending on the silica content of parent lava.

69. Structure and texture:
 Vesicular and amygdaloidal structures are the most common in
basalt. Columnar structures and flow structure are also observed
in some cases. Basalts exhibit aphanitic texture in hand
specimens. ( ie the minerals are too fine).
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.

70. Appearance in Hand specimens:
Basalt is typically black or greenish grey or greenish black. Non-
vesicular, massive in nature. Exhibit a typical aphanitic texture ie
extremely fine grained with or without vesicles. Basalts are always
unstratified, unfossiliferous and do not react with acids.
VESICULAR BASALT: it is characterized by the presence of
empty cavities or vesicles.
AMYGDALOIDAL BASALTS is a vesicular basalt with cavities
filled up by secondary minerals of silica (quartz, amethyst, opal,
agate); zeolites, calcite. Among these, silica minerals may be used
as semi-precious gemstones.

71. VARITIES OF BASALT:
SPILLITE is a soda-rich basalt in which plagioclase
feldspar is albite or oligoclase in stead of labradorite.
Dolerite is the hypabyssal equivalent of basalt .
Gabbro is plutonic equivalent of Basalt .
Trachylite is equivalent of glassy basalt
Alkali Basalt is unsaturated basalt
Tholeite is oversaturated basalt

72. 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.

73. 5.GABBARO
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic
igneous rocks chemically equivalent to basalt.
The rocks are plutonic, formed when molten magma is trapped beneath
the Earth's surface and cools into a crystalline mass.
Gabbro is dense, greenish colored and contains pyroxene, plagioclase,
amphibole, and olivine (olivine gabbro when olivine is present in a
large amount).
The pyroxene is mostly clinopyroxene; small amounts of
orthopyroxene may be present. If the amount of orthopyroxene is
substantially greater than the amount of clinopyroxene, the rock is then
a norite

75. Quartz gabbros are also known to occur and are probably derived from
magma that was over-saturated with silica.
Essexites represent gabbros whose parent magma was under-saturated with
silica, resulting in the formation of the feldspathoid mineral nepheline.
Gabbros contain minor amounts, of iron-titanium oxides such as magnetite,
ilmenite.
Gabbro is generally coarse grained, with crystals in the size range of 1 mm
or greater. Finer grained equivalents of gabbro are called diabase.
Gabbro is usually equigranular in texture, although it may be porphyritic at
times, especially when plagioclase oikocrysts have grown earlier than the
groundmass minerals.

76. USES:
Gabbro often contains valuable amounts of chromium, nickel,
cobalt, gold, silver, platinum, and copper sulfides.
Gabbro is widely used as crushed stone for concrete aggregate,
road base material, and railroad ballast.
Smaller quantities are cut and polished for dimension stone and
called black granite.

77. THANK YOU