(i) Minerals are naturally occurring solid inorganic substances having definite chemical composition and physical properties. They are generally crystalline in appearance. Minerals are homogeneous in form while rocks are heterogeneous in their composition. There are atleast 2000 types of minerals that are named and identified in the Earth’s crust.
The classes of minerals are as follows:-
• Feldspar- It is one of the widely spread minerals. Silicon and oxygen are the two common elements. It is used for ceramics and gloss making
• Quartz- It consists of silica and forms one of the most important components of sand and granite. It is white and colourless and used in radio and radar.
• Pyroxene- It consists of calcium, magnesium, aluminium, iron and silica. It is of black and green colour
• Amphibole- It forms about 7% of the earth’s crust and consists of mainly aluminium, silica, iron, magnesium. It is used in asbestos industry.
• Mica- It consists of potassium, aluminium, magnesium, iron, silicon etc and forms 4% of the earth’s crust. It is found in igneous and metamorphic rocks and is used in electrical instruments.
• Olivine- Magnesium, iron, silica are the main elements. It is found in rocks.
(ii) There are three chief types of rocks at the earth’s crust:
• Igneous Rock- The word ‘igneous’ means ‘ire’ derived from the Latin word ‘ignis’ or Sanskrit word ‘agni’. Igneous rocks are of thermal origin and are formed by the process of solidification of molten rock material called magma. Such molten magma originates from the interior of the Earth. Such magma erupts during volcanic eruptions and on reaching the Earth’s surface, gets solidified by cooling. So they are referred to as primary rocks. They are hard and crystalline in nature and have various textures according to the shape and size.
• Sedimentary Rocks- Sedimentary rocks are called secondary rocks or derived rocks because they are derived by the denudation of other pre-existing primary or parent rocks. Sedimentary rocks are formed by denudation, weathering, erosion, transportation and deposition. These consolidated layered strata are known as stratified rocks. Lithification is the cementation, compaction, and hardening of sediments into sedimentary rocks. As the sediments are deposited in water bodies, they get sorted out according to their size. These sediments accumulate in different layers or strata arranged one above the other and thus form sedimentary rocks. They accumulate in different strata one above the other and thus called stratified rocks. Presence of plants and animals between layers of sediments i.e. fossils.
• Metamorphic Rocks- Metamorphic rocks are those which are formed by the process of metamorphism or alteration of pre-existing rocks. Metamorphism means change of form which may be physical or chemical or both. Such metamorphism occurs when rocks are subjected to high temperature, pressure or both. During the process of metamorphism, the original minerals get altered and new minerals are formed. They combine together to form rocks which are harder and more resistant than the original rocks. They are the source to many important mineral deposits.
(iii) The word ‘metamorphic’ means change of form. Metamorphic rocks are those which are formed by the process of metamorphism or alteration of pre-existing rocks. Such metamorphism occurs when rocks are subjected to high temperature, pressure or both. During the process of metamorphism, the original minerals get altered and new minerals are formed. They combine together to form rocks which are harder and more resistant than the original rocks. They are the source to many important mineral deposits like marble slates, mica, phyllites. Metamorphic rocks have their minerals arranged in a series of bands called foliation.
There are two types of metamorphic rocks:-
• Foliated metamorphic rocks- These rocks have a layer of banded appearance that is produced by exposure to heats and direct pressure. Example- slate, phyllites.
• Non-Foliated metamorphic rocks- These types of rocks have no foliation or any appearance and thus they are subjected to heat and pressure. Example- Marble, Quartzite.
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