◄UNIT 4 : WAVE EROSION AND DEPOSITIONUNIT 6 : CLASSIFICATION OF SOILS AND SOIL FORMATIO►

UNIT 5 : ROCKS AND MINERALS

  • UNIT 5 : ROCKS AND MINERALS

     Key Unit competence
     By the end of this unit, I should be able to compare different types of rocks 

    and minerals and evaluate their importance.

     Introductory activity

    Observe the rock provided below and answer the following questions: 

    1. Identify the types of rocks given above.
    2. In which category can they be classified? 
    3. Which properties can help to identify these rocks and their minerals? 

    4. Explain the economic advantages of the rocks and minerals.

     5.1. Rocks: Definition, types and characteristics
    Learning activity  5.1
     Make a field trip in your environment; observe the rock and identify their 
    types and distinctive characteristics.
     
    5.1.1. Definition 
    A rock is a natural aggregate of minerals in the solid state; usually hard and 
    consisting of one, two, or more mineral varieties. Rocks form the solid part of the 
    earth’s crust. Rocks may also include substances like clay, sandstones, 
    shells and corals. Rocks which contain metallic compounds are called ores. 

    5.1.2. Types of rocks  
    There are three major groups of rocks namely igneous rocks, sedimentary rocks 
    and metamorphic rocks. Their classification is based on the mode of formation 
    and the nature of constituting minerals. Characteristics of each rock group are 
    briefly described below.  

    i) Igneous rocks
    The word igneous comes from the Latin word ignis, which means fire. Igneous 
    rocks are rocks formed by cooling of molten material from a volcano or from 
    deep inside the earth. This molten material from inside the earth is known as 
    magma. Igneous rocks are also called magmatic rocks or volcanic rocks. Their 
    formation is associated with the cooling and hardening of molten material from 
    the interior of the earth. 

    ii) Sedimentary rocks
    Sedimentary rocks are the result of the accumulation of small pieces broken 
    off from pre-existing rocks (igneous rocks, metamorphic rocks and sedimentary 
    rocks) or precipitation of dissolved minerals. Sedimentary rocks form when 
    sediments become pressed or cemented together or when sediments precipitate 

    out of solution. 

    iii) Metamorphic rocks
     The metamorphic rocks get their name from “meta” (change) and “morph” (form). 
    Metamorphic rocks are formed from pre-existing rocks due to increases in heat 
    and pressure which alter rock structure and chemical composition. Therefore, 

    sedimentary and igneous rocks can become metamorphic rocks.   

    There are four factors that contribute to the formation of metamorphic rocks:  
    – Heat or high temperature: this speeds up the chemical reactions that 
    result in metamorphic rocks. The heat is from magma, steam from hot 
    water and rocks sinking deeper into the warmer layer of the crust
    – High pressure which changes the mineral and feel of the original rock.
    – Nature of the parent rock which determines how resistance it is to change.
    – Time which determines the period required for the chemical 

    reactions to take place.

     5.1.3. Characteristics of rocks
     A. Characteristics of igneous rocks
     Igneous rocks have the following characteristics:
    – They are hard, and water does not pass through their joints easily, that 
    is why they are less affected by erosion;
    – Igneous rocks have a lot of minerals;
    – They do not have strata or layers;
    – They do not contain fossils (fossils are remains of plants and animals 
    fixed in rocks);
    – The number of joints increases upwards in any igneous rock;
    – Igneous rocks are mostly associated with volcanic activities and are 
    mainly found in the volcanic zones. That is why they are also called 
    volcanic rocks.

    Igneous rocks can also be classified based on the chemical and mineralogical 
    compositions, texture of grains, forms and size of grains, and the mode of origin, 

    igneous rocks are classified as follows:

     1) Classification based on the amount of silica
     • Acidic igneous rocks: they contain more silica: (≥65% of SiO2 );

     • Basic igneous rocks: they contain low amount of silica (≤ 45% of SiO2 ).

     2) Classification based on the chemical and mineral composition
     • Felsic igneous rocks: they are composed of the dominant minerals of 
    the light group (e.g. Silicon, Aluminum).
     • Mafic igneous: they arecomposed of the dominant mineral of dark 
    group (magnesium and iron).
    3)  Classification based on texture of grains 
    • Pegmatitic igneous rocks: they are very coarse grained: (e.g. granite);
     • Phaneritic igneous rocks: grains of minerals are of intermediate size;• 
    Aphanitic igneous rocks: they are fine grained igneous rocks);
     • Glassy igneous rocks: they don’t contain a defined grain size;

     • Porphyritic igneous: they have mixed graine sizes.

    4) Classification based on the mode of occurrence
     • Intrusive igneous rocks: They are formed when the rising magma, 
    during a volcanic activity, does not reach the earth’s surface but rather 
    cools and solidifies below the surface of the earth. Intrusive igneous 
    rocks fall into two categories:
     a) Plutonic igneous rocks: they are formed due to the cooling of 
    magma very deep inside the earth.
     b) Hypabyssal igneous rocks: they are formed due to the cooling 
    and solidification of rising magma during volcanic activity in cracks, 
    pores, crevices and hollow places just beneath the earth’s surface.
     •  Extrusive igneous rocks: They are formed due to the cooling and 
    solidification of hot and molten lava on the earth’s surface (examples 
    are basalt, Gabbro). Extrusive igneous rocks are further divided into 

    two major subcategories:
    a) Explosive type: The igneous rocks formed by a mixture of volcanic 
    materials ejected during explosive or violent volcanic eruptions.
    b) Quiet type: The appearance of lava through minor cracks and 
    openings on the earth’s surface is called ‘lava flow’. The lava forms 

    basallic igneous rocks after cooling and solidifying.

     B) Characteristics of sedimentary rocks 
    Sedimentary rocks have the following characteristics: 
    – They are the product of other rocks that have already formed; 
    – They appear in the form of layers or strata; 
    – Sedimentary rocks are formed of fragment from materials from older 
    rocks, plant and animal remains;
    – They are found over the largest surface area of the earth;
    – Sedimentary rocks have various minerals because they are a product 
    of different sources; 
    – Most of the sedimentary rocks allow liquids and gases to pass through 
    them (permeable and porous);
    – Sedimentary rocks are characterized by different sizes of joints;
    – Sedimentation units in the sedimentary rocks having a thickness of 
    greater than one centimetre are called beds;
    – As highlighted in the figure below, the composition of sedimentary 
    rocks includes clay, sand, rounded pebbles, angular fragments, calcium 

    deposits and organic carbon.

     C) Characteristics of Metamorphic Rocks 
      The following are the characteristics of metamorphic rocks:
    – They are harder than the original rocks. Therefore, they are not easily 
    eroded;– They do not split easily; 
    – They contain minerals;
    – Some are made up of just one mineral, for example, marble; 
    – They have a different texture or feel from the original rock. 

    Metamorphic rocks present two distinctive physical characteristics:     Foliated 
    metamorphic rocks     and Non-foliated metamorphic rocks. Foliated 
    metamorphic rocks such as gneiss, phyllite, schist and slate have a layered or 
    banded appearance that is produced by exposure to the heat and pressure. 
    Non-foliated metamorphic rocks such as hornfels, marble, quartzite do not have 

    a layered or banded appearance. 

    Application activity 5.1
     1. In which area of Rwanda do we find igneous rocks? Explain their 
    characteristics. 
    2. Observe rocks found in your environment and examine their major 

    rock groups.

     5.2. Composition, properties of rocks and Impact of rocks
         Learning activity  5.2
     Rocks are composed of physical and chemicals elements. describe the 

    physical and chemical properties of rocks.

     5.2.1. Composition of rocks  
    All rocks are composed of minerals. Composition refers to both the types of 
    minerals within a rock and the overall chemical makeup of the rock. The mineral 

    that compose the three types of rocks are presented in the table below. 

    Types of rocks and their forming minerals


    5.2.2. Properties of rocks
     i) Physical properties of rocks
    Physical properties of a rock can be intensive (hardness and softness) and 
    extensive (volume, total mass and weight). Rocks, whether igneous, sedimentary 
    or metamorphic, are subject to powerful stress or pressure by tectonic forces 
    and the weight of overlying rocks. The physical properties of rocks determine 
    their behaviour and respective deformations when a rock is subject to stress 
    such as folding, faulting or warping, and their resulting landscape deformation. 
    – Stress refers to forces that constantly push, pull, or twist the earth 
    crust. There are three types of stress: tension (stretching), compression 
    (shortening), and shear (twisting or tearing). 
    – Strain is how rocks respond to stress whether by stretching, shortening, 
    shearing. 
    – The surface expressions refer to the structure of landforms resulting 
    from the stress depending on whether the rock is brittle (hard) or ductile 
    (pliable). Surface expressions can be folding (bending) or faulting 
    (breaking). Brittle rock breaks (brittle deformation) while ductile 
    rocks like clay bend or flow (ductile deformation).

    The figure below presents different types of stresses that are naturally applied 

    on rocks, their resulting strains and surface expressions. 


    ii) Chemical properties of rocks
    A) Sedimentary rocks

    All water falling onto the earth as rain and running over the earth surface carries 
    minerals in solution. These minerals may precipitate by direct evaporation of 
    water, chemical interaction or by the release of pressure where underground 
    water reaches the surface. Sedimentary rocks formed as chemical precipitates 
    include halite, gypsum, silcretes, ferricretes, limestone, and dolomite. The table 

    below gives details on their chemical composition. 

    Chemically formed sedimentary rocks and their composition


    B) Metamorphic rocks
     Metamorphism involves the alteration of existing rocks either by excessive 
    heat and pressure or through the chemical action of fluids. This alteration can 
    cause chemical changes or structural modification to the minerals making up 
    the rock. Metamorphism process results in the creation of new minerals by 
    the substitution, removal, or addition of chemical ions. Metamorphism may 
    consist of three minerals, kyanite, andalusite and sillimanite. These are 
    all aluminium silicates having the same chemical formula (Al2SiO5 ) but different 
    crystal structures and physical properties. 

    Below is an example of a simplified representation of sediments products and 

    resulting metamorphic rocks from sea beaches to far shelf.

    Lateral representation of metamorphic rocks form beach to far sea shelf


    Igneous Rocks
    The major indicator for the chemical classification of igneous rocks is the amount 
    of Silica (SiO2). Igneous rocks with a high proportion of silica exceeding 65% 
    are said to be acidic or felsic, for example, the granite found on an extensive 
    part of Muhanga District of the Southern Province. Where the amount of silica 
    is very low (less than 45%), the rocks are said to be ultramafic or ultrabasic. 
    Rock having intermediate silica content comprised between 65% and 45% are 
    said to be mafic or basic rocks.     

    Igneous rocks are classified according to their forming minerals (see the table 
    below). Mineral groups include Felsic minerals (feldspars and silica), mafic 
    minerals (magnesium and iron), and ultramafic minerals (low silica content). 
    Some of these rocks form underneath the earth’s crust and are known as 
    intrusive magmatic rocks, whereas other form from the volcanic lave that 

    reached the earth’s surface, forming extrusive volcanic rocks.

    Families of igneous rocks and constituting minerals




    5.2.3. Impact of rocks: advantages and disadvantages on the 
    landscape and man
     A) Advantages of rocks on the landscape and on the man
     Rocks have a wide variety of uses. Many of them are used as building materials 
    of houses and infrastructures such as roads and rail ways.
    – Some rocks are more resistant to weathering and others are less resistant. 
    This difference in rock resistances provides various landscapes such 
    as alternation of elevated topographies (hills, mountains or interfluves) 
    and depressions (valleys and low-lying areas) which are sometimes 
    drained; 
    – Gravel and sand, being among products of rock weathering make 
    beautiful landscape at some location of the earth. Also, the weathering 
    of rocks provides different types of soils including sand, silt and clay 
    which are useful at varying points for agriculture. 
    – Some rocks present beautiful landscapes which may attract tourists; 

    – Some rocks store, purify water and act as water sources to rivers.

    The table below shows usages of rocks.





    B) Disadvantages of rocks on the landscape and man
    – Hard and resistant rocks hinder the penetration of plant roots hence, 
    limiting the weathering process or hindering the growth of vegetation;
    – Rock forming minerals have different colours. The difference in colours 
    makes minerals to absorb differently the heat. Dark-coloured minerals 
    absorb much heat during daytimes and therefore expand, causing the 
    cracking and fragmentation of rocks.
    – The sand can blow; rocks can roll risking injury to people;
    – Light-coloured rocks reflect sunlight and increase the temperature 
    around the plants during the daytime;
    – Some environments such as sand rocks (dunes, erg, etc.) are not 
    suitable for human settlement because of lack of water and soils;
    – Some rocks may reflect landscape with steep slopes where human 
    activities such as agriculture or settlement cannot be possible.
     
    Application activity 5.2
     1) Referring to the properties of rocks, explain how rocks react to the 
    stress and the resulting landscapes? 
    2) Analyse a sedimentary rock in your local environment and describe 
    the process under which it might have been formed. 
    3) With relevant examples, discuss the disadvantages of rocks on 

    landscape and society.

    5.3. Minerals
     Learning activity  5.3
     1) Account for the types and characteristics of minerals.
     2) Examine the use of minerals to your society.
     
    5.3.1. Definition, types and characteristics of a mineral
     A mineral is a solid inorganic substance that occurs naturally in the earth’s 
    crust. A mineral deposit is a concentration of naturally occurring solid material 
    in or on the earth’s crust. Mineral resources are non-renewable. 
    There are five characteristics shared by all minerals.  
    – All minerals are formed by natural processes. They can form when 
    magma cools, when liquids containing dissolved minerals evaporate, or 
    when particles precipitate from solution. 
    – Minerals are inorganic. They are not alive and are not made by life 
    processes. Coal, for instance, is made of carbon from living things. 
    Although geologists do not classify coal as a mineral, some people do. 
    Miners, for example, generally classify anything taken from the ground 
    that has the commercial value as a “mineral resource”.   
    – Minerals are solid and have a definite volume and shape. A gas such 
    as air and a liquid such as water aren’t minerals because they do not 
    have definite shape. 
    – Every mineral is an element or a compound with a chemical 
    composition unique to that mineral.
    – The atoms in a mineral are arranged in a pattern that is repeated 
    over and over again    .
    The table below shows two examples of mineral crystals (salt and quartz) with 
    defined shapes:

    Examples of mineral crystals with defined shapes 

    5.3.2. Types of minerals and ores

    The wide varieties of minerals that have been explored by man for general and 
    commercial purposes to satisfy his needs are of two types: metallic minerals 

    and non-metallic minerals. 

    1) Metallic minerals
     Metallic minerals include:
    – Industrial metallic minerals: iron ore
    – Ferroalloy metallic minerals: manganese, chromium, cobalt, molybdenum, 
    vanadium, nickel.

    – Precious metallic minerals: gold, silver and platinum

    2) Non-metallic minerals
     This category of non-metallic minerals includes salt, tin, potash, asbestos and 
    sulphur.

    Rocks or minerals worked because they contain valuable (profitable) elements 
    are usually called ore-deposits. Minerals are extracted in a mineral ore. For in
    stance, Aluminum comes from the ore bauxite. The iron comes from the mineral 
    ore Hematite. A mineral can also be called an ore, for example Hematite is a 
    mineral that can also be called an ore. A mineral is an ore if it contains useful 
    substance that can be mined at a high profit and be processed and refined into 
    more useful materials. For instance, Aluminum can be refined from bauxite, and 
    made into the useful products. These products are worth more money than the 

    cost of the mining, so bauxite is an ore. 


    5.3.3. Physical properties of minerals
     The most common minerals in earth’s crust can often be identified in the field 
    basing on their basic physical properties such as their form, hardness, fracture, 
    cleavage, colour, streak, density, luster, mass, taste, odour, feel, magnetism as 
    described below:

    1) Form:     Definite geometrical forms called crystals can be recognized 
    in minerals. These are for example: cubic, acicular (needle shaped), 

    columnar, fibrous, reniform (kidney shaped) and nodular forms.

    Pyrite (left) has a cubic form; Tourmaline (middle) is prismatic; azurite and 
    malachite (right) are often amorphous.

    2) Hardness:     The hardness of a mineral can be tested in several ways. 
    Most commonly, minerals are compared to an object of known hardness 
    using a scratch test developed by Friendrich Mohs. He assigned integer 
    numbers to each mineral, where 1 is the softest and 10 is the hardest. 

    This scale is shown below.

    If the gem minerals are excluded, the scale has only 7 numbers. Substitutes may 
    be used when the scale minerals are not available:
    – Easily scratched by nail;
    – Not so easily scratched;
    – Can be scratched by a piece (a copper coin);
    – Scratched easily by knife;
    – Can be scratched by knife with difficulty;
    – Scratched by window-glass;
    – Window-glass is scratched by the mineral.
    3) Fracture: Freshly broken surfaces of minerals present characteristic 
    fracture surfaces. The following important types are noted:
    – Conchoidal (vitreous):  the fracture surfaces are curved with a concave 
    or convex form; for example, quartz.
    – Even: the fracture surfaces are nearly flat; for example, in chert.
    – Uneven: the fracture surface is formed of minute elevations and 
    depressions; for example, most of minerals.
    4) Cleavage: This is how the mineral breaks. Certain minerals split easily 
    along certain planes called cleavage-planes. These planes are parallel 
    to certain faces of the mineral crystal, or to faces of a form in which the 
    mineral may crystallize.
    5) Colour: When a body absorbs all the seven colours that make up white 
    light it appears black, and when it reflects all the colours it appears white. 
    When a body reflects the green vibrations of white light and absorb the 
    other vibrations it appears green. Thus, the colour of a body depends on 
    the selective reflection and absorption of the different vibrations of white light.
    6) Streak: The colour of the powder of minerals sometimes differs from 
    the mineral in mass. Different specimens of the same mineral might show 
    variation in colour, yet the streak is fairly constant.
    7) Luster: The amount and the type of reflection from the surface of a 
    mineral determine its brightness. 
    8) Mass: The mass of a mineral can be used to identify its type.
    9) Density: The density of a mineral can also be used to determine its type.
    10) Taste: Some of the minerals which are soluble in water give distinctive 
    taste but the character is not very useful in identification of minerals 
    because there are only a few minerals which are soluble is water. For 
    example, we get a saline taste in case of common salt, and alkaline in 
    case of soda or potash.
    11) Odour: Only a few minerals give characteristic odour, e.g. the odour of 
    garlic from arsenic compounds.
    12) Feel: Minerals differ in the sensation they give by touch, e.g. minerals are 
    smooth, greasy or rough.
    13)  Magnetism: Generally, iron bearing minerals are magnetic, but not 
    necessarily all iron bearing minerals are magnetic.  Some non-magnetic 
    minerals like monazite are also slightly magnetic. The electromagnetic 
    minerals depend on the varying magnetism of different minerals. 

    5.3.4. Chemical properties of minerals
     Some minerals are affected by the variations in temperature and the pressure on 
    the earth’s surface. Others vary in the structure depending on the percentage of 
    water that they loose with the change of the temperature and the pressure. The 
    chemical composition influences the destruction of the rocks and development 
    of new minerals. 

    Chemical properties of minerals are identified from their chemical composition. 
    We refer to two elements that are Silicon and oxygen. These are the two most 
    abundant elements in the earth crust. They constitute approximately 90% of 
    the crust of the earth. Then we distinguish silicate minerals and non-silicate 
    minerals. Silicate minerals (silicates) are minerals containing Silicon and Oxygen 
    atoms usually with one or more other elements. Non-silicates are minerals other 

    than silicate minerals.

    Chemical properties of minerals


     5.3.5. The importance of minerals and manufactured products
     Minerals provide the material used to make most of the things of industrial-based 
    society; roads, cars, computers, fertilizers, etc. In more than 1600 minerals 
    identified in earth crust, only 200 are extracted for commercial and industrial 
    purposes and less than 1/3 are the most economically significant. 

    Some minerals have high economic value because of their uses or they are rare 
    and beautiful. For example, germs or Gemstones is a mineral with a distinctive 

    colour which makes it expensive. That is why it is used for jewellery. 

    The table showing manufactured products from minerals




    Application activity  5.3
    1. What are the five characteristics shared by all minerals?
    2. Differentiate a mineral from an ore.  
    3. Identify minerals that are extracted in your district and describe their 

    advantages and disadvantages.   

    Skills Lab
    Show how you are going to use available rocks for the economic 

    improvement of your    society.

     End unit assessment
     1. Classify the different types of rocks and their characteristics.
     2. Evaluate the economic importance of rock and minerals in your society.

     3. Identify the physical and the chemical properties of the minerals.

    ◄UNIT 4 : WAVE EROSION AND DEPOSITIONUNIT 6 : CLASSIFICATION OF SOILS AND SOIL FORMATIO►