The earth is composed of various kinds of elements. These elements are in solid form in the outer layer of the earth and n hot and molten form in the interior. bout 98 per cent of the total crust of the earth is composed of eight elements like oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium (Table 5.1), and the rest is constituted by titanium, hydrogen, phosphorous, manganese, sulphur, carbon, nickel and other elements. The elements in the earth’s crust are rarely found exclusively but are usually combined with other elements to make various substances. These substances are recognised as minerals. Though the number of elements making up the lithosphere are limited they are combined in many different ways to make up many varieties of minerals. There are at least 2,000 minerals that have been named and identified in the earth crust; but almost all the commonly occurring ones are related to six major mineral groups that are known as major rock forming minerals. he basic source of all minerals is the hot magma in the interior of the earth. When
magma cools, crystals of minerals appear and a systematic series of minerals are formed in sequence to solidify so as to form rocks. Minerals such as coal, petroleum and natural gas are organic substances found in solid,
liquid and gaseous forms respectively.
SOME MAJOR MINERALS AND THEIR CHARACTERISTICS
Feldspar
Silicon and oxygen are common elements in all types of feldspar and sodium, potassium, calcium, aluminium etc. are found in specific feldspar variety. Half of the earth’s crust is composed of feldspar. It has light cream to
salmon pink colour. It is used in ceramics and glass making.
Quartz
It is one of the most important components of sand and granite. It consists of silica. It is a hard mineral virtually insoluble in water. It is white or colourless and used in radio and radar. It is one of the most important components of granite.
Pyroxene
Pyroxene consists of calcium, aluminum, magnesium, iron and silica. Pyroxene forms 10 per cent of the earth’s crust. It is commonly found in meteorites. It is in green or black colour.
Amphibole
Aluminium, calcium, silica, iron, magnesium are the major elements of amphiboles. They form 7 per cent of the earth’s crust. It is in green or black colour and is used in asbestos industry. Hornblende is another form of
amphiboles.
Mica
It comprises of potassium, aluminium, magnesium, iron, silica etc. It forms 4 per cent of the earth’s crust. It is commonly found in igneous and metamorphic rocks. It is used in electrical instruments.
Olivine
Magnesium, iron and silica are major elements f olivine. It is used in jewellery. It is usually a greenish crystal, often found in basaltic rocks. Besides these main minerals, other minerals like chlorite, calcite, magnetite, haematite, bauxite and barite are also.
Metallic Minerals
These minerals contain metal content and can be sub-divided into three types: (i) Precious metals : gold, silver, platinum etc.
(ii) Ferrous metals : iron and other metals often mixed with iron to form various kinds of steel.
(iii) Non-ferrous metals : include metals like copper, lead, zinc, tin, aluminium etc.
Non-Metallic Minerals
These minerals do not contain metal content. Sulphur, phosphates and nitrates are examples of non-metallic minerals. Cement is a mixture of non-metallic minerals.
ROCKS
The earth’s crust is composed of rocks. A rock is an aggregate of one or more minerals. Rock may be hard or soft and in varied colours. For example, granite is hard, soapstone is soft. Gabbro is black and quartzite can be
milky white. Rocks do not have definite composition of mineral constituents. Feldspar and quartz are the most common minerals found in rocks.
Excercise
1. Multiple choice questions.
(i) Which one of the following are the two main constituents of granite?
(a) Iron and nickel (c) Silica and aluminium
(b) Iron and silver (d) Iron Oxide and potassium
(ii) Which one of the following is the salient feature of metamorphic rocks?
(a) Changeable (c) Crystalline
(b) Quite (d) Foliation
(iii) Which one of the following is not a single element mineral?
(a) Gold (c) Mica
(b) Silver (d) Graphite
(iv) Which one of the following is the hardest mineral?
(a) Topaz (c) Quartz
(b) Diamond (d) Feldspar
(v) Which one of the following is not a sedimentary rock?
(a) Tillite (c) Breccia
(b) Borax (d) Marble
2. Answer the following questions in about 30 words.
(i) What do you mean by rocks? Name the three major classes of rocks.
(ii) What is an igneous rock? Describe the method of formation and characteristics of igneous rock.
(iii) What is meant by sedimentary rock? Describe the mode of formation of sedimentary rock.
(iv) What relationship explained by rock cycle between the major type of rock?
3. Answer the following questions in about 150 words.
(i) Define the term ‘mineral’ and name the major classes of minerals with their physical characteristics.
(ii) Describe the nature and mode of origin of the chief types of rock at the earth’s crust. How will you distinguish them?
(iii) What are metamorphic rocks? Describe the types of metamorphic rock and how are they formed?
Please refer to attached file for NCERT Class 11 Geography Minerals And Rocks
