2. Minerals

Minerals are those substances which occur naturally in rocks. These are non-living solid substances which have a definite chemical composition.

Minerals are often classified as metallic and non metallic. The surface of the metallic minerals is generally slippe and glossy. Gold, copper and lead are metallic minerals. They are melted to obtain metals. The surface of the non metallic minerals is dull. They cannot reflect the sun-rays. Gypsum, quartz and mica are non-metallic minerals. Metals cannot be obtained from these minerals.

Rocks and minerals account for about 99 per cent of the materials found in the outer layer of the lithosphere. Some rocks have useful minerals, which provide us with metals and chemicals.

Out of about 2000 different minerals, only 12 are known as the rock-forming minerals. Oxygen and silicon account for about 75 per cent of the Earth’s crust by weight. These elements are essential for plant and animal life on the Earth.

PHYSICAL CHARACTERISTICS OF MINERALS: Minerals have distinct physical properties that can be used to correctly identify a mineral. These are

♤ Crystal structure: arrangement of atoms inside mineral

♤ Hardness: on the Mohs scale, a ten-point scale running from the softest, talc to the hardest, diamond.

♤ Lustre: appearance in light

♤ Colour

♤ Streak: colour of a mineral when it has been ground to a fine powder. Often tested by rubbing the specimen on an unglazed plate.

♤ Cleavage: how mineral splits along various planes

♤ Fracture: how it breaks against its natural cleavage planes

♤ Specific gravity: density compared with water