Minerals and Rocks

Minerals

-Inorganic substances occurring naturally at or below the earth’s surface.

Characteristics of Minerals

1. Different degrees of hardness e.g. some are very hard e.g. diamond while others are very soft e.g. talc.
2. Some have atoms arranged in an orderly manner to form crystals e.g. quartz form a 6- sided prism.
3. Varying number of elements e.g. gold has one (Av) while quartz has 2 (SiO&sub2;).
4. Different abilities to allow light to pass through e.g. some are transparent, opaque or translucent.
5. Specific colours e.g. gold is shiny yellow while copper is brown.
6. Have specific surface appearance (lustre) when they reflect light i.e. metallic (shiny) or non-metallic (glass like).
7. Definite chemical composition or constant ratio of elements e.g. quartz has one atom of silicon and two atoms of oxygen.
8. Tendency to break along certain lines or cleavage) e.g. flint has cleavage like that of glass.
9. Different densities e.g. some are very heavy e.g. lead while others are light e.g. silicate minerals.
10. Some minerals conduct electricity while others don’t e.g. copper conducts while diamond doesn’t.
11. Some can be pressed into different shapes while others can’t e.g. copper is malleable while flint isn’t.

Types of Minerals

1. Metallic minerals
2. Ferrous Minerals – limonite, magnetite, siderite and haematite
   
   A Limonite Ore (Picture Courtesy)
3. Non-ferrous Minerals – copper, aluminium, gold, lead, etc.
4. Non-metallic Minerals – graphite, diamond, asbestos, coal, etc.
5. Energy minerals – petroleum, coal and uranium.

Rocks

– A consolidated material composed of grains of one or more minerals.

Classification of Rocks

1. Igneous Rocks

-Rocks formed when molten material from the earth’s interior cools and solidifies on or beneath the earth’s surface.
Types of Igneous Rocks

a) Intrusive Igneous Rocks

– Rocks formed when magma cools and solidifies below the earth’s surface e.g. granite, diorite, gabbro, peridotite.
– Have coarse texture as a result of slow cooling giving minerals more time to form large crystals.
– Are classified further into two:

• Hypabyssal rocks – intrusive igneous rocks which are near the earth’s surface.
• Plutonic rocks – intrusive igneous rocks which are deep below the surface.

(b) Extrusive Igneous Rocks

-Rocks formed when lava solidifies on the earth’s surface.
-Have fine texture due to fast cooling giving minerals less time to collect together to form larger crystals.
They are of two types namely:

• Volcanic Ejecta
  -Extrusive igneous rocks are formed When ash and lava ejected from underground as they fall on the earth’s surface e.g. pumice. When dust and ash ejected settle on the ground and get compressed to form a rock e.g. tuff.
• Lava Flows
  -Extrusive igneous rocks formed when basic lava flows over a considerable distance then cools and solidifies e.g. basalt and obsidian.

2. Sedimentary Rocks

-Rocks formed when particles of other rocks are laid down and compressed into layers or when plant and animal remains are buried and compressed and compacted.

A Sandstone- Type of a Sedimentary Rock (Picture Courtesy)

• When they are laid down a layer is formed.
• As deposition continues additional layers are formed which compress the lower layers into a hard mass.

Types of Sedimentary Rocks

a) Mechanically Formed Sedimentary Rocks
-Sedimentary rocks formed when weathered igneous or metamorphic rocks are deposited and compacted e.g. sandstone and shale.

b) Organically formed Sedimentary Rocks
-Sedimentary rocks formed when animal and plant or animal remains are buried, compressed and compacted.
– Organically formed sedimentary rocks can be classified into:

1. Calcareous rocks – rich in calcium carbonate e.g. chalk and limestone.
   Coral rocks are formed from remains of sea polyps which extract lime from the sea, build shells for protection, attach themselves to each other and rocks to live in colonies, then die and shells to form coral rocks.
2. Ferruginous Rocks – rich in iron e.g. ironstone.
3. Siliceous Rocks – rich in silica e.g. diatomite.
4. Carbonaceous Rocks – rich in carbon e.g. coal.

c) Chemically formed Sedimentary Rocks
-Sedimentary rocks formed when materials dissolved in water chemically react forming new substances then water evaporated leaving layers of those salts.
Chemically formed sedimentary rocks can be classified into:

1. Carbonates e.g. trona and dolomite
2. Sulphates – sulphate compounds
3. Chlorides e.g. halite
4. Silicates e.g. flint
5. Iron stones e.g. haematite and limonite

3. Metamorphic Rocks

– Rocks which have changed their physical appearance and chemical properties as a result of subjection to great heat and pressure e.g

• Gneiss from granite
• Slate from clay
• Marble from limestone
• Quartzite from sandstones

A Quartzite rock – A Chemically Formed Rock (Picture Courtesy)

Distribution of Major Rocks in Kenya

Eastern Kenya region

The major rocks are:
– Metamorphic rocks e.g. marble in parts of Machakos and schist and gneiss in parts of Kitui.
– Volcanic rocks in Yatta plateau and Kapiti plans. – Sedimentary rocks e.g. limestone rocks used in Bamburi for cement manufacturing.

Coastal Region

– Major rocks are sedimentary rocks e.g. limestone used in Bamburi for cement manufacture.
– There are volcanic rocks in Tsavo rich in ground water resources.

Northern and N.E Region

– Dominated by sedimentary sands.
– There are volcanic rocks in Mt. Marsabit and around Rift Valley.

Rift Valley and Kenya Highlands

– Dominated by volcanic rocks.
– There are metamorphic rocks which have resulted from changing of igneous rocks.

L. Victoria Basin

– Granite and gneiss dominate Western Kenya where they form high rocky hills called granitic tors common in Kisii, Maragoli and Bunyore areas.
– Sedimentary rocks deposited by rivers e.g. Nyando, Nzoia, Yala and Sondu.

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 tors of W. Kenya and high volcanic peaks such as those of Mt.
   Kenya are a tourist attraction which brings foreign exchange
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 is associated with sedimentary rocks