Rocks are the main constituent of the earth's crust. A rock may be defined as any mass of natural deposit present in the solid mass of the earth's crust. Most rocks are made of aggregates of minerals. These minerals are specifically referred to as the rock-forming minerals. A mineral is a naturally occurring inorganic substance possessing certain physical properties, a definite chemical composition and a definite atomic structure. Many minerals have a tendency to form crystals which are bounded by plane surfaces arranged in a regular and symmetrical manner. Some physical properties like clevage, hardness, specific gravity and colour are useful for identification of minerals. Usually, minerals are composed of two or more than two elements, but some minerals have only one element. For instance, sulphur, graphite, gold etc. are called one-element minerals.

Some of the rock forming minerals are as follows:

1. Feldspar: Half the crust is composed of feldspar. It has a light colour and its main constituents are silicon, oxygen, sodium, potassium, calcium, aluminium. It is of three types orthoclase, plagioclase, microline.

2. Quartz: It has two elements, silicon and oxygen. It has a hexagonal crystalline structure. It is uncleavaged, white or colourless. It cracks like glass and is present in sand and granite. It is used in manifacture of radio and radar.

3. Pyroxene: It is a mineral with green or dull black lustre. Calcium, aluminium, magnesium, iron, silica are its main constituents.

4. Amphibole: A fibrous mineral with a hexagonal structure which has a green or black glittering appearance. Its main constituents are calcium, magnesium, iron, aluminium, silica.

5. Mica: This is a layered, cleavaged, white, black or colourless mineral. It used in electrical appliances. Its constituents are potassium, aluminium, magnesium, iron, silica.

6. Olivine: Its components are magnesium, iron, silica etc. It is a glassy, green or yellow mineral with crystalline structure.

7. Apatite: A complex compound containing calcium phosphate. It is red, brown, yellow or green in colour. Phosphorus and fluorine are derived from it.

8. Barite: It is barium sulphate and has a white or brown colour. It has a crystalline structure.

9. Bauxite: A hydrous oxide of aluminium, it is the ore of aluminium. It is non-crystalline and occurs in small pellets.

10. Calcite: An important ingredient of limestone, chalk and marble, it is calcium carbonate. It is white or colourless.

11. Chlorite: It is hydrous magnesium, iron, aluminium silicate. It has a cleavaged structure.

12. Cinnabar: It is mercury sulphide and mercury is derived from it It has a brownish colour.

13. Corundum: It is aluminium oxide and is present in form of ruby and sapphire. It has a hexagonal structure.

14. Dolomite: A double carbonate of calcium and magnesium, it is used in cement and iron and steel industries. It is white in colour.

15. Galena: It is lead sulphate and lead is derived from it.

16. Gypsum: It is hydrous calcium sulphate and lead is used in cement, fertiliser and chemical industries.

17. Haematite: It is a red ore of iron.

18. Kaolinite: China clay, it is basically aluminium silicate.

19. Magnesite: It is magnesium carbonate and has a non-crystalline structure.

20. Magnetite: It is the black ore (or iron oxide) of iron.

21. Pyrite: It is iron sulphide. Iron and sulphuric acid are obtained from it.

• Written by Sasikanth.
• In category Nature.
• 12 years ago.

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