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Minerals and Rocks - Geography Form 1


Minerals and Rocks

Learning objective: by the end of the session, you should be able to;

a)define the terms minerals and rocks.

b)state the characteristics of minerals.

Minerals and Rocks

Welcome to our lesson on minerals and rocks. In this session we shall define minerals and rocks, state their characteristics, and classify rocks.These are some of the materials made from minerals and rocks;

Minerals and rocks can be found in our surroundings.

Definition of Minerals and Rocks

Minerals can be defined as organic or inorganic substances which occur naturally on or beneath the earths surface.
Minerals have a definite chemical composition and physical properties.
They contain common elements such as oxygen, silicon, aluminum, iron, calcium, magnesium, sodium, potassium, titanium and carbon
A rock is a substance that is a mixture of mineral particles forming part of the earth crust.

Hardness

Hardness refers to the resistance of a mineral to abrasion or scratching. Minerals have different degrees of hardness. some are hard while others are soft. For instance, when talc is hit is easily crumbles, when Quartz is hit only few particles break but when diamond it hit, it does not break.

.


Shape

Different minerals have distinct shapes.
Some minerals like pyrite and quartz have very unique shapes.

Ability to allow light to pass through

Minerals can be transparent, translucent or opaque.This depends on the color of the constituent mineral.For instance obsidian rock is dark and glassy while quartz is white and glassy.

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Lustre

This is the appearance or quality of light reflected from the surface of a mineral.The lustre can be described as glassy, pearly, silky or dull.

Color

Different minerals have different colors. For example Copper is brown, sulphur is yellow and malachite is bright green.

Cleavage

This is the tendency of a mineral to break along a plane of weak bonding.

For instance, when mica breaks it forms thin flat sheets while granite breaks into granules.

Fracture

 It is a way a substance breaks following the line of weakness.For example Quartz which has no clearage.

Streak

This is the color left after rubbing the mineral on a white surface. Metallic minerals have a dense dark streak.



Tenacity

This is the resistance of minerals to tearing, bending or stretching. Minerals have different degrees of tenacity and can be described as brittle, elastic or flexible.


Specific Gravity

This is the ratio of the weight of a mineral to the weight of an equal volume of water.

Shape

Different minerals have distinct shapes.
Some minerals like pyrite and quartz have very unique shapes.

Lustre

This is the appearance or quality of light reflected form the surface of a mineral.
The lustre can be described as glassy, pearly, silky or dull. (label approximately)
Provide photographs of well illuminated area where the different mineral surface show quality of reflecting light. The minerals are:
Glassy-flourite
Pearly-marble
Silky-phyllite
Dull-rhyolite

(Organize animation in such a way that amount of light reflected will vary from different surfaces. Name each of the surfaces accordingly)

Color

Different minerals have different colors. Copper is brown, sulphur is yellow and malachite is bright green.

classification of rocks according to their mode of formation.
By the end of the lesson the learner should be able to,

  1. classify rocks according to mode of formation.
  2. state the characteristics of rocks

Intrusive igneous rocks

These are rocks which were formed when magma cooled and solidified below the surface of the earth.Igneous intrusive rocks are formed near the surface of the earth are called hypabyssal rocks. Those that form deep below the surface are called plutonic rocks and are deep rooted. Examples include; granite, diorite, peridotite, gabbros and syenite.


Extrusive igneous rock


These are igneous rocks that are formed on the earth's crust after magma has cooled very fast. They are also referred to as volcanic rocks.
Examples of igneous rocks are:

Hypabyssal

The rocks that were formed after gradual cooling of the magma are referred to as hypabyssal. These rocks are in shallow depth of the earth crust.

Characteristics of igneous rocks

Some igneous rocks have a fine-grained texture.These rocks are generally light in weight and color.


Other igneous rocks have large crystals and are usually heavy.

Some Igneous rocks formed when magma cools rapidly such rocks have a glassy texture and are dark colored.


Igneous rocks such as basalt have vesicular texture.They are formed when air escapes slowly during lava flow leaving behind rocks with many air spaces.

When fragments of individual rocks such ash and bombs consolidate, they form rocks such as tuff which have a pyroclastic texture.

Another important characteristic of above igneous rocks is that they contain mineral called silica.Igneous rocks with high silica content are said to be acidic.
Igneous rocks with low silica content are said to be basic.

While some rocks have smooth surfaces, others have many joints which may result from tension caused earth movements.



Characteristics of Igneous rocks
We have already seen that igneous rocks are formed through Igneous activity. These rocks have the following characteristics.
(Animation) ground swelling and magma rising from magma chamber. Some magma is erupted and some of it cools within crust.

  • Some igneous rocks: have a fine-grained texture.

Photo A. Fig 3.3gg 53.Book; The earth, 3rd edition an introduction to Physical Geo. Tarbuch lutgens

  • The rocks are generally light in weight and color.

Photos B, C fig 3.3 pg 53 in the book referred to above.

  • Other igneous rocks have large crystals and are usually heavy.
  • Some Igneous rocks formed when magma cools rapidly such rocks  have a glassy texture and are dark colored.

 Photo D fig 3.3 pg 53 in book referred to above.

  • Rocks with vesicular texture are formed when air escapes slowly during lava flow leaving behind rocks with many air spaces..

 Photo fig 3.4/3.5 pg. 54
When fragments of individual rocks consolidate, they form rocks with pyroclastic texture.
Photo fig 3.6 pg. 55 of text referred to above.
Mineral content
 Another important characteristic of above igneous rocks is that they contain mineral called silica.
Igneous rocks with high silica content are said to be acidic.
Igneous rocks with low silica content are said to be basic.
While some rocks have smooth surfaces, others have many joints which may result from tension caused earth movements.

Mechanically formed sedimentary rocks(clastic rocks)

These are rocks which have been formed by consolidation and cementation of sediments, which have been laid down on land or in water.The sediments are from existing weathered igneous and metarmorphic rocks.

They are transported and deposited on land or in the sea by water, wind or ice.

Examples are:


 

Organically formed sedimentary

These rocks are formed from accumulation of dead plants and animals.some remains of these organisims accumulated in lakes or oceans over along period of time to form various types of organic sedimentary rocks. These examples are coal, chalk, diatomite and limestone.The organically formed sedimentally rocks are further classified into;

  • Calcareous-rocks are rich in calcium carbonate
  • Ferruginous-rocks are rich in iron.
  • Siliceous -rocks are rich in silica
    • Carbonaceous-rocks are rich in carbon.


    Chemically formed sedimentary rocks

    These rocks were formed as a result of precipitation of minerals by various chemical reactions on already existing rocks to form new rocks.These rocks are classified into: Carbonates, Sulphates, Chlorides, Silicates and ironstones.

    examples;

    Carbonates-travertine,

    Dolomite and trona (soda ash)

    Sulphates-gypsum

    Chloride-rock salt

    Silicates-flint

    Ironstone-limonite and haematite


    Characteristics of sedimentary rocks

    There are two main characteristics of sedimentary rocks
    Occur in layers or strata.The layers are formed as sediments are deposited and they get cemented together

    .


    Some sedimentary rocks contain remains of living organisms which lead to formation of petroleum, natural gas and coal after many millions of years


    Metarmorphic Rocks

    Metamorphic rocks are formed from igneous or sedimentary rocks which have been subjected to Dynamic (due to pressure changes), Thermal (due to intense heat) and Thermal-dynamic (a combination of heat and pressure).Examples of metarmorphic rocks are:

    These rocks are also known as changed rocks. Most of the metamorphic rocks are the basis for the formation of precious minerals such as gold, diamonds, coal, marble

    Characteristics of Metamorphic rocks


    Metamorphic rocks are rocks that have undergone change as a result of great heat and pressure. They tend to be harder than the original rocks. During metamorphism, the minerals in the rocks sometimes change their patterns in a process called re-crystallization giving the rock a new structure.



    The most important characteristics of rocks depend mainly on how they originated. These characteristics include color, texture and structure especially jointing, density and mineral composition.Texture refers to the general appearance of the rock in the terms of the size and arrangement of its crystals.

    Characteristics of Rocks

    Learning objective:

    By the end of the session, you should be able to state the characteristics of rocks.

    We have already seen that rocks are formed in different ways. These are:

    Through Igneous activity

    (Animation) ground swelling and magma rising from magma chamber. Some magma is erupted and some of it cools within crust.

    Through metamorphism

    (Animation) Rock getting heated inside the crust and the crystals changing in pattern.

    Through sedimentation

    (Animation) uplands being eroded by running water and sediments being transported and deposited in layers in  a depression.

    The most important characteristics of rocks depend mainly on how they originated. These characteristics include color, texture and structure especially jointing, density and mineral composition.
    Texture refers to the general appearance of the rock in the terms of the size and arrangement of its crystals.



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    Activity
    Fill in the missing letters to complete the cross word.

    1. A mineral common in many igneous rocks
    2. Rocks with vesicular texture usually have a surface.
    3. rocks with low silica content are described as
    4. sedimentary rocks are usually arranged
    5. rocks with high silica content are described to as
    6. it can be used t distinguish different types of rocks
    7. remains of living organisms embedded in sedimentary rocks help in the formation of this
    8. metamorphic rocks are rocks that have undergone ­­­­­­­­­­­­­­­­­­­­­­­-­­­­­­­­­­­­____________________
    9. rocks with this texture are formed as fragments of  individual rocks
    10. structural features in rocks which may result from folding
    11. rocks formed when magma or lava cools slowly are usually
    12. the overall appearance of a rock in terms of the size and arrangements of its crystals.
    13. rocks with fine grains are usually in weight.
    14. the process by which rock mineral crystals are changed
    15. materials compacted together to form layers of rock.

    ANSWERS
    1.Silica            
        
    2. Rough
    3. Basic


    4. Strata
    5. Acidic
    6. Color
    7. Petroleum
    8. Change
    9. Pyroclastic
    10. Joints
    11.dense
    12. texture
    13.light
    14. re-crystalisation
    15. ediments

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