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1. UNIT 3. ROCKS AND MINERALS
3.1. Definitions of Rocks and Minerals
Rock is the solid mineral material forming the surface of the earth. A rock is
composed of one or more minerals. Petrology is the science of rocks which
includes the studying of mineral composition, structure, texture, origin,
occurrence, alteration and relationship with other rocks.
Rocks are formed due to the combination of minerals. Some rocks may
contain only one mineral but many rocks are composed of different minerals.
Rocks are known by different names which are related to the combination of
minerals.
The hot magma in the interior of the earth is the main source of all minerals.
The magma cools and crystals of minerals get formed. A systemic series of
minerals are formed in sequence to solidify so as to form rocks.

2. Cont.
Minerals are the chemical compounds of different elements. For e.g. Silica (Si
O2).
Some minerals contain only one element e.g. Sulphur and carbon (diamond).
Mineral: naturally occurring inorganic substance having an orderly atomic
structure and a definite chemical composition and physical properties.
Mining- Process of extracting valuable minerals from the earths surface.
It is composed of two or three minerals /single element ex. S, Cu, Ag, Au,
Graphite.
There are at least 2000 minerals in the crust. There are at least six mineral
groups which form rocks in the crust.
The basic source of all minerals is the hot magma in the interior of the earth.
Coal, petroleum and natural gas are organic minerals.
Some minerals are crystalline because the atoms forming the crystals are
arranged in a definite manner e.g. quartz.

3. Cont..
• Some minerals are non-crystalline as the atoms forming these minerals
are not arranged in definite manner.
• Some common minerals are found everywhere on the earth and hence are
termed as ‘Rock forming minerals’, while some minerals are scarce and
are found only at particular location.
• These minerals are costly and are termed as Economic minerals.
• Metals are extracted from some minerals which are known as ‘Ore’ of that
metal e.g. Bauxite is an ore of Aluminum.

4. 3.1.1. Physical Characteristics of Minerals
• In this section, we discuss the physical characteristics of minerals.
1. External crystal form – External shape of a mineral is determined by its internal
arrangement of molecules. It can be cubic, tetrahedral, tabular, hexagonal, etc.
2. Cleavage – The property of breaking along specific planes is called cleavage.
Crystals have one plane along which the bonding between the atoms is weaker
than along other planes.
3. Fracture – When the internal molecular arrangement is so complex that there
are no planes of molecules, the crystal will break in an irregular manner.
4. Lustre – Lustre describes the shining quality of an object. Each mineral has a
distinctive Lustre like metallic, glossy, silky, etc.

5. Cont..
5. Colour – The colour of a mineral depends on its molecular structure. Some
minerals show specific colours like malachite (green), azurite (blue),
chalcopyrite (golden yellow), etc. Some minerals due to the presence of
different impurities show different colours, such as quartz – it can be red,
white, green, etc.
6. Streak – It is the colour of the ground powder of any mineral. It may be of the
same colour as the mineral or different. For example, Malachite is green and
gives a green streak, Fluorite is purple or green but gives a white streak,
chromite and magnetite are almost black and can be distinguished by their
streaks – brown for chromite and black for magnetite.
7. Specific gravity – It is the ratio between the weight of a given mineral and
the weight of an equal amount of water. Since it is a ratio, it has no units.

6. Cont..
8. Hardness – The hardness of a mineral is measured by its ability to resist
scratching. In order to have a standard method of expressing the hardness of
minerals, a standard scale called the Mohs scale is commonly adopted. In the
sequence of increasing hardness from 1 – 10, the following minerals are used
as the standard of comparison – talc, gypsum, calcite, fluorite, feldspar, quartz,
topaz, corundum and diamond. Compared to this, a fingernail is 2.5 and glass
or knife blade is 5.5.
9. Structure – It refers to the particular arrangement of the individual crystals.
10.Transparency – Transparent, when the light rays pass through so that the
objects can be seen. Translucent, when light rays pass through but get diffused
and the objects cannot be seen. Opaque, when the light will not pass through.

7. 3.2. Types of minerals and their distributions
3.2.1. Types of Minerals (Classification of Minerals)
Broadly, minerals can be classified into metallic and non-metallic minerals.
Metallic Minerals
These minerals are composed of metals and can be divided into three subtypes –
1. Precious metals – Platinum, gold, silver, etc.
2. Ferrous metals – Iron mixed with other metals.
3. Non-ferrous metals – Metals other than iron like copper, aluminium, lead, zinc, tin, etc.
Metallic minerals are generally obtained from igneous rocks, and are malleable and ductile.
Non-Metallic Minerals
These minerals are composed of non-metals like sulphur, silicon, phosphorus. For example,
cement is a mixture of non-metallic minerals. Non-metallic minerals are generally obtained from
sedimentary rocks, lacking malleability and ductility.

8. 3.2.2. Distribution of Minerals
• The table below shows the minerals, their uses and where they are found. The
countries shown are just some among the many other ones. below there is a
table showing major world minerals and their locations along with their major
uses.

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15. 3.3. Formations in Which Minerals Occur
1. Veins and Lodes
- Occurrence of minerals in crevices, cracks or faults in igneous
rocks.
• They are said to occur in veins if they occur there in small
quantities.
• Said to occur in lodes if they occur there in large quantities e.g.
zinc, copper and silver.
2. Reefs
- Veins and lodes which are exposed on the surface.
3. Seams/Layers/Beds
-Occurrence of minerals as sedimentary or as a result of
compression of accumulated organic or inorganic material e.g.
coal and halite.

16. Cont..
4. Alluvial Deposits
-Occurrence of minerals while mixed with materials such as sand, gravel,
silt, etc.
-These were minerals which were detached from the veins by weathering
and carried away by streams and rivers and got deposited e.g. gold,
diamond and platinum.
5. Weathering Products
- Minerals formed by deep weathering of rocks then leaching carried
minerals from the top to lower layers where they accumulated e.g.
aluminum, nickel, iron and manganese.
6. Oil pools/Wells
-Occurrence of minerals in pools or wells in sedimentary rocks e.g.
petroleum and natural gas.

17. 3.3.1. Conditions Necessary for Formation of Petroleum
A.Presence of fossils or organic remains.
B.Presence of sedimentary rocks for burying organic remains.
C.Presence of pressure to compress organic remains to cook
the oil and natural gas out of organic matter.
D.Presence of a porous reservoir rock to store and transmit
petroleum to the oil pools e.g. limestone and sandstone.
E.Presence of a trap like a syncline to hold petroleum in a
reservoir to prevent its escape.
F.Presence of impermeable rocks below the trap or syncline to
prevent petroleum from percolating further underground.

18. 3.3.2. Factors Influencing Exploitation of Minerals
1. Value of Mineral
Minerals of high value will be mined even if they occur in small
quantities because one sold it will be possible to offset mining costs
and make a profit and vice versa.
2. Quality of Ore
Mining can be done if the mineral deposits have high mineral content
because they are economical to work on but deposits with low mineral
content are rarely worked on except if the mineral in them is rare e.g.
uranium.

19. Cont..
3. Size of Deposit
Minerals which aren’t of high value have to occur in large quantities for them
to be mined so that it will be a possible to recover mining costs and make a
profit.
4. Capital
Lack of capital causes developing countries not to exploit minerals and leave
it to international companies because a lot of money is needed for
exploration, infrastructure, salaries, energy etc. e.g. titanium mining at Kwale
is being done by Tiomin company from Canada.

20. Cont..
5. Method of Mining
A mineral requiring open cast mining will be mined even if the mineral
deposit is large but one requiring underground mining will be extracted if
it’s in large deposit or if it’s of high value or rare.
6. Transport costs
Minerals occurring in remote areas far from the markets are not likely to be
exploited if the transport system is poorly developed since mineral ore is
heavy and bulky and transporting it by road and railway is expensive.

21. Cont..
7. Market for the Mineral
Mining can be done if the mineral is in demand and if the prices are reasonable so
that mining costs are offset and a profit is realized.
8. Political Influence
Mineral deposits at the borders of two countries may not be exploited as a dispute
may arise concerning whom mine it e.g. dispute between Iraq and Kuwait over
Rumaila should oil field.
9. Labour
Exploitation of some minerals require skilled workers and if they lack it may not be
done as is the case in developing countries because expatriates have to be
engaged and are very expensive to pay which may reduce the profits accruing
from mining.

22. 3.4. Classification of Rocks and Rock Cycle
Based on the origin, rocks are of three types – igneous, sedimentary and
metamorphic.
Igneous Rocks
 “Ignis” in Latin means ‘fire’. Igneous rocks are formed out of magma and lava
from the interior of the earth. When magma in its upward movement cools and
turns into solid form, it is called igneous rock.
 There are two types of igneous rocks – intrusive rocks e.g., granite and
extrusive rocks e.g., basalt Deccan Traps.
o Intrusive rocks are formed when magma rises and cools within the crust
which gives rise to various forms like batholiths, laccoliths, dyke, etc.
o Extrusive rocks are formed when cooling and solidification takes place on the
surface of the earth.

23. Cont..
 Igneous rocks are also classified based on the texture, size
and arrangement of grains or other physical conditions of the
materials. If the magma cools slowly at great depths, mineral
grains increase in their size. Sudden cooling at the surface
results in small and smooth grains.
 The igneous rocks are the oldest of all the
rocks. Pegmatite, gabbro, granite, basalt, tuff are some of the
examples of igneous rocks.

24. Types of Rocks

25. Sedimentary Rocks
 Sedimentary rocks are also called detrital rocks.
 The word ‘sedimentary’ is derived from the Latin word sedimentum, which
means settling. Rocks of the earth’s surface undergo denudation and are
broken into various fragments. These fragments are transported by different
exogenous forces and deposited. These deposits through compaction turn
into sedimentary rocks. The process is called lithification. They are layered or
stratified of varying thickness.

26. Cont..
 Sedimentary rocks are three types depending upon the mode of
formation –
o Mechanically formed sedimentary rocks – For example,
conglomerate, loess, sandstone, etc.
o Chemically formed – For example, potash, halite, etc.
o Organically formed – For example, chalk, coal, limestone, etc.

27. Metamorphic Rocks
 The word metamorphic means ‘change of form’. The metamorphic rocks
form under the action of pressure and heat.
 Metamorphism is a process by which the already consolidated rocks
undergo recrystallisation and reorganization of materials within original
rocks.
 The breaking and crushing of the original minerals within rocks without
any significant chemical changes is called dynamic metamorphism.

28. Cont..
 When the materials of the rocks alter chemically and
recrystallize, the process is known as thermal
metamorphism. Thermal metamorphism is of two types –
contact metamorphism and regional metamorphism.
o Contact Metamorphism – In this case, the rocks come
in contact with hot magma and lava as a result of which
rock materials recrystallize under high temperatures.
Generally, new materials form when lava/magma
interacts with the rocks.

29. o Regional Metamorphism – Due to deformation caused by tectonic
shearing together with high temperature or pressure or both, rocks
undergo recrystallisation which is known as regional metamorphism.
 Sometimes rock grains or minerals form layers or lines during the process
of metamorphism. Such an arrangement in metamorphic rocks is called
foliation or alienation.
 Sometimes minerals of different nature form alternating arrangements of
thin and thick layers which appear in light and dark shades. Such an
arrangement in metamorphic rocks is called banding and such rocks are
called banded rocks.
 Slate, diamond, marble, quartzite, schist, gneiss are some examples of
metamorphic rocks.

30. 3.4.1. Rock Cycle

31.  The Rock Cycle is a continuous process through which old rocks are
transformed into new ones. Igneous rocks are primary rocks and other rocks,
sedimentary and metamorphic form from these igneous rocks.
 These primary rocks under the influence of high pressure/temperature
transform into metamorphic rocks. The igneous and metamorphic rocks can
break into fragments and these fragments can be the source of sedimentary
rocks.
 The crustal rocks – igneous, sedimentary and metamorphic once formed may
be carried down into the mantle (interior of the earth) through the subduction
process and the same melt and turn into magma which is the source of
igneous rocks. In this way, the rock cycle is a continuous process.

32. 3.4.Methods of Mining
1. Open Cast Mining
• -Method of extracting minerals which are near the earth’s surface.
• -Using boring instruments known as augers to drill out mineral deposit and
bring it to the surface.
2. underground Mining
• -Method employed when the mineral lies very deep below the surface and
the overburden is too thick to be removed by mechanical means.
3. Alluvial/ Placer mining. Method used to extract minerals occurring in
alluvial deposits e.g. gold, tin, diamonds and platinum.

33. 3.5. Significances of Rocks and Minerals
These are importance of rocks and minerals.
A) Significance of Minerals
1) A Country earns foreign exchange from exportation of minerals which is
used to import goods and services and fund development projects.
2) Mining is a source of employment to people such as those who work in
mines, in cement factories, in transport sector, etc.
3) Mining has led to development of industries by providing raw materials
used in those industries e.g. limestone used in cement factories, coal
used in iron and steel industries, soda ash used in glass industry, etc.

34. Cont..
4)Mining has led to development of transport system to make
mining areas accessible e.g. Magadi soda mine is connected to
the main Mombasa-Nairobi railway line.
5)Mining has led to development of settlements e.g. Magadi town
which originated from the mining of soda ash.
6)Mining is a source of market for goods and services e.g. there
are shops and markets, banking and insurance services offered
to people working in mines and related industries.
7)Has led to development of social amenities by providing social
facilities such as housing, health, electricity, water and education
alongside infrastructure.

35. B) Significance of Rocks
1)Rocks weather to form soil which is important in agriculture.
2)Form aquifers which store ground water which forms springs
which form rivers and wells which provide water for domestic
and industrial use.
3)Some rocks are sources of building materials e.g. igneous rocks
are used to make ballast and limestone rocks are used as
building blocks and raw material in cement manufacturing.
4)Phosphate and nitrate rocks are used to make fertilizer used in
agriculture.
5)Granitic rocks are a tourist attraction which brings foreign
exchange.

36. 6) Pumice is used as a scrubbing stone.
7) A rock such as coal is used as fuel for heating, smelting of iron and
thermal electricity generation.
8) Source of minerals e.g. oil and coal are associated with sedimentary
rocks.
Practice Questions on Rocks and Minerals
Q1. Distinguish between metallic and non-metallic minerals.
Q2. What are veins and lodes?
Q3. What is a placer deposit?
Q4. Minerals are unevenly distributed in Somalia. Justify your answer.