Rocks and Minerals & it’s Classification

1. Rocks :


“Rocks are hard mass of mineral matter comprising one or more rock forming minerals”. Rocks are the materials that form the essential part of the Earth’s solid crust. Rocks are formed from the molten material known as magma. Rock, in geology, naturally occurring and coherent aggregate of one or more minerals. Such aggregates constitute the basic unit of which the solid Earth is composed and typically form recognizable and mappable volumes. There are three kinds of rock: igneous, sedimentary, and metamorphic. Igneous rocks form when molten rock (magma or lava) cools and solidifies. Sedimentary rocks originate when particles settle out of water or air, or by precipitation of minerals from water. They accumulate in layers.

Formation of Rocks :

  1. Igneous / Primary Rocks.
  2. Sedimentary / Secondary Rocks.
  3. Metamorphic Rocks.

1. Igneous or Primary Rocks :

Cooling and consolidation of molten magma within or on the surface of earth.

A) Based on origin :

a) Extrusive Rocks or Volcanic Rocks :
  • These rocks are formed due to the consolidation of magma on the surface of the earth.
  • The magma, when it flows on the Earth surface is called LAVA.
  • e.g. Basalt, Gabbro.
b) Intrusive or Plutonic Rocks :
  • These rocks are produced due to solidification of magma below earth surface.
  • e.g. Granite, Syenite, Diorite, Gabbro.

B) Based on the silica content :

  1. Basic rocks : 40 to 55% SiO2 e.g. Gabbro, Basalt, Limestone.
  2. Intermediate : 56 to 65% SiO2 a) Sub Basic rocks : 56 tp 60% SiO2 e.g. Diorite and Andesite b) Sub Acidic rocks : 60 to 65% SiO2 e.g. Syenite and Trachyte
  3. Acid rocks : > 65% SiO2 e.g Granite, Rhyolite, Sandstone.

2. Sedimentary / Aqueous / Stratified Rocks :

  • Formed from Transportation and cementation of primary rocks.
  • Formed through the agency of water – so aqueous rocks.
  • The rocks are mostly deposited in layers or strata – so stratified rocks.
  • These rocks are formed from four stages :-
    • Weathering
    • Transportation
    • Deposition
    • Diagenesis

A) Based on chemical origin :

  1. Inorganically formed – Evaporation, Precipitation e.g. Gypsum, Limestone, Dolomite.
  2. Organically / Biochemically formed – e.g. Lignite, Anthresite.
  3. Mechnically formed e.g. Sandstone, Shale.

B) Based on formation :

  1. Arenaceous
  2. Argillaceous rocks
  3. Calcareous rocks
  4. Carbonaceous rocks
  5. Siliceous rocks
  6. Precipitated rocks

3) Metamorphic Rocks :

  • Alteration of the existing primary and secondary rocks.
  • These are formed from igneous and sedimentary rocks.
  • By heat, pressure, chemically active liquids and gases.
  • The changes due to-
    • Water – hydro metamorphosis
    • Pressure – dynamo metamorphosis
  • Sedimentary rocks to metamorphic rocks –
    • Sandstone : Quartzite
    • Shale : Slate / mica, Schist
    • Limestone : Marble
  • Igneous to metamorphic rocks –
    • Granite : Granite gneiss
    • Dolerite : Hornblende gneiss

Conclusion :

In conclusion, the formation of rocks is a complex geological process that occurs over millions of years and involves various forces and factors. Rocks are the building blocks of Earth’s crust and provide valuable insights into the planet’s history. Understanding the formation of rocks is essential for geologists and scientists seeking insights into Earth’s history, natural resources, and the forces shaping our planet. It also plays a crucial role in fields like mining, construction, and environmental science, where knowledge of rock types and their properties is fundamental. The study of rocks continues to uncover the mysteries of our planet’s past and informs our understanding of its future.

Minerals :


Minerals are naturally occuring solids. They have definite chemical composition and crystal structure. Minerals are solid substances composed of atoms having an orderly and regular arrangement. e.g. Quratz (SiO2). Minerals are those elements on the earth and in foods that our bodies need to develop and function normally. Those essential for health include calcium, phosphorus, potassium, sodium, chloride, magnesium, iron, zinc, iodine, chromium, copper, fluoride, molybdenum, manganese, and selenium.

Classification of Minerals :

A) Based on Formation :

1. Primary Minerals :

  • Minerals that are original components of rocks.
  • Formed from the parent rock. e.g. Quartz, Feldspar, Mica, Hornblende etc.

2. Secondary Minerals :

  • They are formed from changes in primary minerals and rocks. e.g. Clay minerals, Serpentine.

B) Based on Composition :

1. Essential Minerals :

  • Those minerals that are chief constituents of rocks. e.g. Feldspar, Pyroxenes micas

2. Accesory Minerals :

  • Those present in small quantities, whose presence or absence will not alter the properties of rocks. e.g. Tourmaline, Magnetite etc.

C) Based on Specific Gravity :

1. Light Minerals :

  • Specific gravity less than 2.85 e.g. Qurtz, Muscovite, Feldspar

2. Heavy Minerals :

  • Specific gravity more than 2.85 e.g. Hornblende, Pyrite, Heamatite.

D) Based on Chemical Composition :

  1. Native elements – Graphite, Gold, Copper, Sulphur
  2. Oxides and Hydroxide – Quartz, Hematite
  3. Halides – Rock Salts
  4. Silicates – Orthoclase, Mica, Olivine
  5. Sulphur Bearing
    • Sulphate Bearing – Gypsum
    • Sulphite Bearing – Pyrite

Conclusion :

In conclusion, minerals are fundamental components of the Earth’s crust, and they play a pivotal role in shaping our planet’s geology, supporting ecosystems, and meeting various human needs. Minerals are integral to Earth’s geology, the environment, industry, and human well-being. Their diverse properties and functions make them a critical area of study in both scientific and practical contexts. Balancing the sustainable use of mineral resources with environmental conservation is essential to ensure a prosperous and healthy future for our planet.

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