Minerals and Rocks [UPSC Geography Notes]

Geography is an important part of the UPSC syllabus. These notes on Geography for IAS exam will help aspirants score good marks in both the prelims and mains examinations. In this article, you can read all about minerals and rocks, characteristics of minerals, and the rock cycle. This is an important topic for the UPSC exam.

Minerals and Rocks

About 98% of the total crust of the earth is composed of eight elements namely, oxygen, sodium, calcium, iron, magnesium, silicon, aluminium and potassium, and the rest is constituted by titanium, hydrogen, phosphorus, manganese, carbon, sulfur, nickel and other elements. These elements combine with other elements to form substances called minerals.

Definition of a mineral – A mineral is defined as a naturally occurring organic and inorganic substance, having an orderly arrangement of atoms and a definite composition and physical properties.

• The elements found in the lithosphere combine in different ways to form minerals of various types. About 2000 minerals have been found in the earth’s crust, but most of the commonly occurring ones are related to six major mineral groups that are called major rock-forming minerals.
• The hot magma in the interior of the earth is the main source of all minerals. The magma cools and crystals of minerals get formed. A systemic series of minerals are formed in sequence to solidify so as to form rocks.

Read about some major minerals and their characteristics in the linked article.

Physical Characteristics of Minerals

In this section, we discuss the physical characteristics of minerals.

1. External crystal form – External shape of a mineral is determined by its internal arrangement of molecules. It can be cubic, tetrahedral, tabular, hexagonal, etc.
2. Cleavage – The property of breaking along specific planes is called cleavage. Crystals have one plane along which the bonding between the atoms is weaker than along other planes.
3. Fracture – When the internal molecular arrangement is so complex that there are no planes of molecules, the crystal will break in an irregular manner.
4. Lustre – Lustre describes the shining quality of an object. Each mineral has a distinctive lustre like metallic, glossy, silky, etc.
5. Colour – The colour of a mineral depends on its molecular structure. Some minerals show specific colours like malachite (green), azurite (blue), chalcopyrite (golden yellow), etc. Some minerals due to the presence of different impurities show different colours, such as quartz – it can be red, white, green, etc.
6. Streak – It is the colour of the ground powder of any mineral. It may be of the same colour as the mineral or different. For example, Malachite is green and gives a green streak, Fluorite is purple or green but gives a white streak, chromite and magnetite are almost black and can be distinguished by their streaks – brown for chromite and black for magnetite.
7. Specific gravity – It is the ratio between the weight of a given mineral and the weight of an equal amount of water. Since it is a ratio, it has no units. For instance, the specific gravity of quartz is 2.65.
8. Hardness – The hardness of a mineral is measured by its ability to resist scratching. In order to have a standard method of expressing the hardness of minerals, a standard scale called the Mohs scale is commonly adopted.
   1. In the sequence of increasing hardness from 1 – 10, the following minerals are used as the standard of comparison – talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum and diamond. Compared to this, a fingernail is 2.5 and glass or knife blade is 5.5.
9. Structure – It refers to the particular arrangement of the individual crystals.
10. Transparency – Transparent, when the light rays pass through so that the objects can be seen. Translucent, when light rays pass through but get diffused and the objects cannot be seen. Opaque, when the light will not pass through.

Know more about the physical characteristics of minerals in the linked article.

Classification of Minerals

Broadly, minerals can be classified into metallic and non-metallic minerals.

Metallic Minerals 

These minerals are composed of metals and can be divided into three subtypes –

1. Precious metals – Platinum, gold, silver, etc.
2. Ferrous metals – Iron mixed with other metals.
3. Non-ferrous metals – Metals other than iron like copper, aluminium, lead, zinc, tin, etc.

Metallic minerals are generally obtained from igneous rocks, and are malleable and ductile.

Non-Metallic Minerals 

These minerals are composed of non-metals like sulphur, silicon, phosphorus. For example, cement is a mixture of non-metallic minerals. Non-metallic minerals are generally obtained from sedimentary rocks, lacking malleability and ductility.

Rocks

Rock is the solid mineral material forming the surface of the earth. A rock is composed of one or more minerals. Petrology is the science of rocks which includes the studying of mineral composition, structure, texture, origin, occurrence, alteration and relationship with other rocks. The age of a rock is determined based on carbon-14 dating.

Classification of Rocks

Based on the origin, rocks are of three types – igneous, sedimentary and metamorphic.

Igneous Rocks 

• “Ignis” in Latin means ‘fire’. Igneous rocks are formed out of magma and lava from the interior of the earth. When magma in its upward movement cools and turns into solid form, it is called igneous rock.
• There are two types of igneous rocks – intrusive rocks e.g., granite and extrusive rocks e.g., basalt Deccan Traps.
  • Intrusive rocks are formed when magma rises and cools within the crust which gives rise to various forms like batholiths, laccoliths, dyke, etc.
  • Extrusive rocks are formed when cooling and solidification takes place on the surface of the earth.
• Igneous rocks are also classified based on the texture, size and arrangement of grains or other physical conditions of the materials. If the magma cools slowly at great depths, mineral grains increase in their size. Sudden cooling at the surface results in small and smooth grains.
• The igneous rocks are the oldest of all the rocks. Pegmatite, gabbro, granite, basalt, tuff are some of the examples of igneous rocks.

Sedimentary Rocks 

• Sedimentary rocks are also called detrital rocks.
• The word ‘sedimentary’ is derived from the Latin word sedimentum, which means settling. Rocks of the earth’s surface undergo denudation and are broken into various fragments. These fragments are transported by different exogenous forces and deposited. These deposits through compaction turn into sedimentary rocks. The process is called lithification.
• Sedimentary rocks occupy only 5% of the earth. They are layered or stratified of varying thickness.
• Sedimentary rocks are of three types depending upon the mode of formation –
  • Mechanically formed sedimentary rocks – For example, conglomerate, loess, limestone, sandstone, etc.
  • Chemically formed – For example, potash, halite, etc. 
  • Organically formed – For example, chalk, coal, limestone, geyserites, etc.

Metamorphic Rocks 

• The word metamorphic means ‘change of form’. The metamorphic rocks form under the action of pressure, volume and temperature (PVT change). 
• Metamorphism is a process by which the already consolidated rocks undergo recrystallisation and reorganization of materials within original rocks. The igneous and metamorphic rocks together account for 95% of the earth.
• The breaking and crushing of the original minerals within rocks without any significant chemical changes is called dynamic metamorphism.
• When the materials of the rocks alter chemically and recrystallise, the process is known as thermal metamorphism. Thermal metamorphism is of two types – contact metamorphism and regional metamorphism.
  • Contact Metamorphism – In this case, the rocks come in contact with hot magma and lava as a result of which rock materials recrystallise under high temperatures. Generally, new materials form when lava/magma interacts with the rocks.
  • Regional Metamorphism – Due to deformation caused by tectonic shearing together with high temperature or pressure or both, rocks undergo recrystallisation which is known as regional metamorphism.
• Sometimes rock grains or minerals form layers or lines during the process of metamorphism. Such an arrangement in metamorphic rocks is called foliation or alienation.
• Sometimes minerals of different nature form alternating arrangements of thin and thick layers which appear in light and dark shades. Such an arrangement in metamorphic rocks is called banding and such rocks are called banded rocks.
• Slate, diamond, marble, quartzite, schist, gneiss are some examples of metamorphic rocks.

Rock Cycle

The Rock Cycle is 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 igneous rocks. These primary rocks under the influence of high pressure/temperature transform into metamorphic rocks. The igneous and metamorphic rocks can break into fragments and these fragments can be the source of sedimentary rocks. The crustal rocks – igneous, sedimentary and metamorphic once formed may be carried down into the mantle (interior of the earth) through the subduction process and the same melt and turn into magma which is the source of igneous rocks. In this way, the rock cycle is a continuous process.

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