Minerals and Rocks | CUET PG Geography | Notes

A rock is a solid combination of minerals. The basic source of all minerals and rocks is the hot magma in the interior of the earth. When magma cools, crystals of minerals appear and this series of minerals solidify to form rocks. Minerals such as coal, petroleum and natural gas are organic substances found in solid, liquid and gaseous forms respectively. 

Minerals

• A mineral is a naturally occurring substance that can be either organic or inorganic.
• It has an orderly atomic structure along with definite chemical composition and physical properties.
• A mineral is made up of two or more elements. For example, Quartz is made up of two elements: oxygen and silicon.
• However, single-element minerals like sulphur, copper, silver, gold, graphite etc. are also found.

Rocks

• Two or more minerals are combined together through geological processes to form rocks.
• It is categorized by the minerals & chemical composition and the way in which it is formed.
• The science dealing with the study of rocks is called Petrology.
• There are three main types of rock namely: sedimentary rocks, igneous rocks, and metamorphic rocks.

Physical Characteristics of Minerals

• External Crystal Form: It is determined by the internal arrangement of the molecules – cubes, octahedrons, hexagonal prisms, etc.
• Cleavage: It is the tendency to break in given directions producing relatively plane surfaces – the result of the internal arrangement of the molecules — that may cleave in one or more directions and at any angle to each other.
• Fracture: It is an internal molecular arrangement so complex there are no planes of molecules; the crystal will break in an irregular manner, not along planes of cleavage.
• Lustre: Lustre is the appearance of material without regard to colour; each mineral has a distinctive lustre like metallic, silky, glossy etc.
• Colour: Some minerals have characteristic colour determined by their molecular structure – malachite, azurite, chalcopyrite etc., and some minerals are coloured by impurities. For example, because of impurities quartz may be white, green, red, yellow etc.
• Streak: The colour of the ground powder of any mineral. It may be of the same colour as the mineral or may differ — malachite is green and gives a green streak, and fluorite is purple or green but gives a white streak.
• Transparency: Transparency is the tendency of light rays to pass through so that objects can be seen plainly.
• Structure: It is the particular arrangement of the individual crystals; fine, medium or coarse-grained; fibrous – separable, divergent, radiating.
• Hardness: Hardness is the relative resistance that is being scratched; ten minerals are selected to measure the degree of hardness from 1-10. They are: 1. Talc; 2. Gypsum; 3. Calcite; 4. Fluorite; 5. Apatite; 6. Feldspar; 7. Quartz; 8. Topaz; 9. Corundum; 10. Diamond.
• Specific Gravity: Specific gravity is the ratio between the weight of a given object and the weight of an equal volume of water. Objects are weighed in air and then weighed in water and divided by weight in the air by the difference between the two weights.