Introduction
The earth’s solid layer, called lithosphere, is about 100 km in thickness. The word ‘lithosphere’ literally means ‘a sphere of rocks’. Down to a depth of 16 km from the earth’s surface, 95 per cent of the earth materials consist of rocks. The rocks are made ofindividual solid substances called minerals. Each mineral usually contains two or more elements, of which the whole earth is made of.
The earth’s solid layer, called lithosphere, is about 100 km in thickness. The word ‘lithosphere’ literally means ‘a sphere of rocks’. Down to a depth of 16 km from the earth’s surface, 95 per cent of the earth materials consist of rocks. The rocks are made ofindividual solid substances called minerals. Each mineral usually contains two or more elements, of which the whole earth is made of.
ROCKS:-
Petrologists
(scientists who study rocks) define a rock as any natural mass of mineral
matter that makes up the earth’s crust. All rocks are not hard. Clay, for
example, is a type of rock that is soft. Chalk is still softer.
Rock Types
There are three main
groups of rocks:
1. Igneous;
2. Sedimentary;and
3. Metamorphic Rocks.
1. Igneous Rocks:-
These
are formed from lava hurled out of a volcano or from the cooling of hot magma
below the crust. Granite is coarse-grained igneous rock that is formed by the
slow cooling of magma. Basalt is fine-grained igneous rock, almost black, that
is formed by quick cooling of lava. Igneous rocks are classified on the basis
of chemical composition and texture. Chemical differentiation of magma gives
rise to mafic and felsic types of igneous rocks. Texture relates to the sizes
and patterns of the mineral crystals present in the rock. The size of mineral
crystals in an igneous rock depends largely upon the rate of cooling of magma.
As a general rule, rapid cooling results in small crystals and slow cooling in
large crystals. Extremely sudden cooling will result in the formation of a
natural glass which is noncrystalline. Large bodies of magma, trapped beneath
the surface, cool very slowly because the surrounding rock conduct the heat
slowly. Rapid cooling occurs in lava that loses heat rapidly to the atmosphere
or to the overlying ocean water.
The crystals large enough to be seen with naked
eyes or with the help of a hand lens are called phaneritic textured crystals
whereas those too small to be distinguished without the aid of a microscope are
called aphanitic textured crystals of the igneous rocks. Where crystals in the
rock are all within the same size range, the texture is described as
equigranular. Where a few large crystals, called as phenocrysts, are embedded
in a martix or groundmass of smaller crystals, the texture is porphyritic.
2. Sedimentary Rocks:-
These
are made from sediments formed by the erosion and weathering of other rock
types. Wind, water and snow erode rocks and carry the sediments to low lying
areas. When deposited in the sea they are compressed and hardened to form
layers of rocks. Sediment is fragmented mineral and organic matter derived
directly or indirectly from pre-existing rocks and from life processes,
transported and deposited by air, water or snow/ice. Sandstone is made from
grains of sand, which have been naturally cemented together. Chalk is made up
of millions of tiny calcium carbonate (lime) skeletons of microorganisms.
Throughout the geologic past, layers of sediment
have accumulated to great thickness in certain favourable areas on the
continents and on the ocean floor. As new layers are added, the underlying
layers experience progressively deeper burial. Under increasing pressures
imposed by the overlying load, water is excluded from the sediment. They become
denser and strongly coherent. Ultimately, hard rock layers are produced, a
process termed lithification. In some instances, chemical changes also occur in
the sediments following deposition. All processes of physical and chemical
change affecting sediments during its conversion into solid rock are called
diagenesis.
The first level of classification of sedimentary rocks is into clastic and non-clastic divisions. The adjective “clastic” comes from the Greek word “klastos”, meaning “broken” and describes clastic sediments consisting of particles removed individually from a parent rock source. The naming of clastic rocks depends in part on the size of component mineral grains. A system of grading of mineral grains has been established by geologists, which is known as Wentworth Scale.
The non-clastic sedimentary rocks are made of
sediments of two basic types, chemical precipitates and organically derived
sediments. Chemical precipitates are solid mineral matters precipitated from an
aqueous solution in which the component ions have been transported. The
rock salt and gypsum are its examples. The organically derived sediments
consist of remains of plants or animals as well as mineral matters produced by
the activities of plants and animals. Coal and limestone are very good examples
of this kind. Important non-clastic sediments are the carbonate minerals,
compounds of calcium or magnesium ion or both. Sulphate compounds are also minerals
of the chemical sediments, anhydrite (calcium sulphate) and gypsum (hydrous
calcium sulphate) are important minerals.
3. Metamorphic Rocks:-
Metamorphic Rocks These types of rocks are formed when a rock is subjected to heat and/or pressure. The heat and/or pressure alter the characteristics of the original rock by forming new minerals. Clay when subjected to heat and/ or pressure gets metamorphosed to slate. Similarly, limestones give rise to marble. The metamorphic rocks can be broadly grouped into two major classes: cataclastic rocks and recrystallised rocks. Cataclastic rocks are formed by mechanical disruption (breaking and crushing) of the original minerals without appreciable chemical change. The process is described as dynamic metamorphism. The recrystallised rocks are formed by the recrystallisation of the original minerals. Minerals with different chemical formulae and crystal lattices are produced due to recrystallisation.
The recrystallised rocks are further divided into
two subclasses: contact and regional metamorphic rocks. The contact metamorphic
rocks are formed by recrystallisation under high temperature caused by the
intruding magma. The rocks are not subjected to bending and breaking but new
mineral/s, emanating from magma are added to metamorphosed rocks. The regional
metamorphic rocks undergo recrystallisation during the process of being
deformed by shearing, often under conditions of high pressure or high
temperature or both. As metamorphism continues a large percentage of the
minerals assume plate-like shape and are assembled in parallel orientation in
the rock, a structure known as foliation. Strongly developed foliation is known
as schists. In schists, crystals of certain minerals grow in size. These large
crystals are referred to as porphyroblasts. Another structure found in
metamorphic rocks is lineation. In this, mineral grains are drawn out into
long, thin, pencil-like objects, all in parallel alignment. In
another form of structure, known as banding, minerals of different
varieties or groups are segregated into alternate layers. These layers are
usually of light or dark shades, making the banding conspicuous. Gneiss and
schist have well marked banded structure.
MINERALS:-
The mineral is a naturally occurring homogeneous
solid that is an inorganic substance, having an orderly atomic structure and a
definite chemical composition. The basic source of all minerals is the hot
magma in the interior of the earth.
When
magma cools, crystals of minerals appear. These first crystals may sink in the
magma so that the composition of the magma changes with depth. Thus, a sequence
of minerals is formed in the rocks as the magma cools. Besides, certain
minerals such as coal, petroleum and natural gas are organic substances found
in solid, liquid and gaseous forms respectively.
Physical Properties of Minerals
Minerals can be identified by their hardness,
colour, the way they reflect light (lustre), the way they break (cleavage and
fracture), and their density.
Crystal Structure
Mineral crystals fall into six crystal systems,
each of which is defined in terms of its crystallographic axes.
(i) Isometric System :
Three axes of crystal are of equal length and lie
at right angle to each other, e.g. Halite mineral.
(ii) Tetragonal System :
Three axes make right angles with each other. The
two horizontal axes, a and b, are of equal length but the third, c
axis, is of different length, e.g Zircon mineral.
(iii) Hexagonal System :
Four axes are present. Three of the axes
are horizontal and of equal length, intersecting in angles of 600 and
1200. The fourth axes is perpendicular to other axes and is of different
length, e.g. Beryl mineral.
(iv) Orthorhombic System :
Three axes intersect at right angles to each
other, but they are of unequal lengths, e.g. Staurolite mineral.
(v) Monoclinic System :
The three axes are of unequal length. Two axes
intersect at right angles whereas the third axis intersects obliquely forming
an acute angle, e.g. Gypsum mineral.
(vi) Triclinic System :
The three axes are of unequal length and intersect
at oblique angles, e.g. Albite mineral.
Cleavage
and Fracture
The smooth planar surfaces of weakness along which
a mineral has pronounced tendency to split is known as cleavage planes. It
bears a close relationship to internal atomic structure and external crystal
form of mineral. Minerals lacking cleavage break along various types of
fracture surfaces. For example, the curved fracture surfaces of a glass
constitute conchoidal fracture, that is seen in mineral quartz.
Specific Gravity
Each mineral has a certain specific gravity which
is the ratio of its density to the density of water at 40
Celsius. Specific gravity of minerals is a property of great importance because
it determines the density of a given rock and
rock density in turn determines the gross layered structure of the
earth.
Hardness
The degree to which a mineral surface resists
being scratched is known as its mineral hardness. Hardness is geologically
important because it determines how easily a mineral is worn away by the
abrasive action of streams, waves, wind and glaciers in the processes of
erosion and transportation. Ten standard minerals constitute the Mohs Scale of
hardness ranging from the softest to the hardest.
Lustre
The
appearance of a mineral surface under reflected light is referred to as its
mineral lustre. It is described by several descriptive adjectives, such as
metallic (metal like), adamantine (diamond like), vitreous (glass like),
resinous (oil like), pearly or silky (pearl or silk like).
Colour
Certain minerals possess a distinctive mineral colour that facilitates recognition. The impurities present in the mineral provides shades of colour to a mineral.
Certain minerals possess a distinctive mineral colour that facilitates recognition. The impurities present in the mineral provides shades of colour to a mineral.
Economic Importance of Minerals
Mineral resources can be divided into four main
groups: essential resources, energy resources, metal resources, and industrial
resources. The most basic group, essential resources, comprises soil and water.
Energy resources can be divided into the fossil fuels (crude oil, natural gas,
coal, oil shale and tar sand) and the nuclear fuels (including uranium, thorium
and geothermal power).
Streak
When a mineral is rubbed across the white
porceline plate, known as streak plate, it may leave a streak of mineral powder
of distinctive colour. The colour of powder of mineral may sometimes be
different than the colour of the mineral.
Besides physical properties, minerals are also
identified on the basis of their optical properties. These optical properties
are evaluated by means of a microscope using polarised light rays and are of
great value in mineral identification.
Metal resources range from structural metals such
as iron, aluminum and titanium to ornamental and industrial metals such as
gold, platinum and gallium. Industrial minerals include more than 30 minerals,
such as salt, asbestos and sand . Mineral deposits have two geological
characteristics that make them a real challenge to modern civilisation. First,
almost all of them are non-renewable resources. The geological processes that
form them are much slower than the rate at which we exploit them. There is no
likelihood of our ability to grow mineral deposits at a rate equal to our
consumption. Second, mineral deposits have a place value. We cannot decide
where to extract them; nature made that decision for us when the deposits were
formed.
Facts About Minerals
(1)There are at least 2,000 minerals that have
been named and identified. However, most rocks are made up of not more
than 12 different classes of minerals.
(2)The most common element in the earth’s crust is
oxygen. The second most common element is silicon. Mineral Quartz is
silicon dioxide and is very common mineral.
(3)The largest diamond ever found was the Cullinan
diamond, discovered in a mine in SouthAfrica in 1905. It was 3,106 carats, which
means it weighed more than 600 g.
(4)Diamond and Graphite are both forms of carbon
but their atoms are arranged differently. Diamond is the hardest mineral
whereas Graphite is soft, black and feels greasy.
(5)Under the sea, minerals dissolved in water
crystallise around the vents of faults or fissures.