MINERALS & ROCKS: Igneous, Sedimentary & Metamorphic

Minerals

• Mineral is a naturally occurring organic and inorganic substance, having an orderly atomic structure and a definite chemical composition and physical properties. 
• A mineral is composed of two or more elements. But sometimes single element minerals like sulphur, copper, silver, gold, graphite etc. are found.
• Though the number of elements making up the lithosphere are limited they are combined in many ways to make up many varieties of minerals.

Whole Earth		Crust
Iron	33.3	Oxygen	45.2
Oxygen	29.8	Silicon	27.2
Silicon	15.6	Aluminum	8.2
Magnesium	13.9	Iron	5.8
Nickel	2.0	Calcium	5.1
Calcium	1.8	Magnesium	2.8
Aluminum	1.5	Sodium	2.3
Sodium	0.2	Potassium	1.7
Fig: Chemical composition of the entire Earth and Earth’s crust respectively

The 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 to form rocks.

ROCKS

• A rock is an aggregate of one or more minerals.
• Rocks do not have definite composition of mineral constituents.
• Feldspar and quartz are the most common minerals found in rocks.
• There are three family of rocks classified based on mode of formation:
  • Igneous Rocks
  • Sedimentary Rocks
  • Metamorphic Rocks

IGNEOUS ROCKS

• They are formed out of magma and lava (when it cools and solidifies) from the interior of the earth, hence are known as primary rocks. 
• The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth.
• If molten material is cooled slowly at great depths, mineral grains may be very large. 
• Sudden cooling (at the surface) results in small and smooth grains.
• Intermediate conditions of cooling would result in intermediate sizes of grains.
• Examples of igneous rocks- Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff.

SEDIMENTARY ROCKS

• They are formed by the process of lithification- The rocks (Igneous, sedimentary and metamorphic) undergo denudation and the sediments are transported and deposited at a place which under continuous accumulation and compaction change into solid rocks. 
• Sometimes, we see several layers of varying thickness in sedimentary rocks like sandstone, shale etc.
• Depending upon the mode of formation, sedimentary rocks are classified into three major groups: 
  • Mechanically formed — sandstone, conglomerate, limestone, shale, loess etc.
  • Organically formed—geyserite, chalk, limestone, coal etc. 
  • Chemically formed — chert, limestone, halite, potash etc.

METAMORPHIC ROCKS

• The word metamorphic means ‘change of form.’ 
• Metamorphism is a process by which already consolidated rocks undergo recrystallisation and reorganisation of materials within original rocks due to change of temperature, pressure and volume. 
• Ex. schist, gneiss, quartzite and marble. 

• Contact Metamorphism: When rocks meet hot intruding magma and lava and the rock materials recrystallise under high temperatures.
• Regional metamorphism: When rocks undergo recrystallisation due to deformation caused by tectonic shearing together with high temperature or pressure or both. 
• Examples of metamorphic rocks Gneissoid, granite, syenite, slate, schist, marble, quartzite etc.

ROCK CYCLE

• A continuous process through which old rocks are transformed into new ones.
• Igneous rocks are primary rocks and other rocks (sedimentary and metamorphic) form from these primary rocks. 
• Igneous rocks can be changed into metamorphic rocks. Fragments derived out of these rocks form into sedimentary rocks. Sedimentary rocks themselves can turn into fragments and the fragments can be a source for formation of new sedimentary rocks. 
• Crustal rocks (igneous, metamorphic and sedimentary) once formed may be carried down into the mantle and the same melt down due to increase in temperature and turn into molten magma, the original source for igneous rocks.