Origin of Coal And its Microscopic Constituents


Coal is a flammable black hard rock used as a solid fossil fuel. It is mainly made up of 65-95 % of carbon and also contains Hydrogen, Sulphur, Oxygen, and Nitrogen. Basically the Carbon rich mineral deposit formed from the remains of plant fossils. These are initially deposited as peat but with burial and increase in temperature at depth bring about physical and chemical changes.

The process of Coalification results in production of coal of different ranks (Coal Series) from peat, through the bituminous coal and lignite to anthracite.

Each rank marks a reduction in the percentage of carbon. They are termed “woody or Humic coals” if formed from fragments of trees or bushes. If the major constituent of coal or pollen grains and /or finely divided plant debris, the term “Sapropelic Coal” is used.

  1. Peat:
  • It contain less than 40 to 55 % carbon.
  • Peat contain sufficient volatile matter and lots of moisture content.
  • It burns like wood, gives less heat, emits more smoke and leaves a lot of ash.
  1. Lignite:
  • It contain 60 to 70 % carbon on dry ash free basis.
  • It has high moisture content of more than 35 %.
  • It is also known as Brown Coal and is a low grade coal.
  • It undergoes spontaneous combustion and creates fire accidents in mines.
  1. Bituminous Coal:
  • It is the most abundant rank of coal.
  • It is refered to as soft coal and Black Coal.
  • It contain 40 to 80 % of
  • It conatin moisture and volatile content upto 15 to 40 %.
  • It is dense compact and does have traces of originally vegetable material.
  • It is used in production of Coke and Gas.
  1. Anthracite:
  • It is the highest rank of Coal as it contain carbon content ranging from 80 to 95%.
  • It is also known as Hard Coal.
  • It contain very little volatile matter and has semi metallic lustre.
  • It ignites slowly with less loss of heat.

Origin of Coal

Coal formation began during the Carboniferous period known as the first coal age which spanned 360 million to 290 million years ago.

The build-up of the silt and other sediments together with movements in the earth’s crust (known as tectonic movements) buried swamps and peat bogs, often to great depth. With burial, the plant material was subjected to high pressure and temperature which causes physical and chemical changes in the vegetation, transforming it to Peat and then into Coal. The degree of change undergone by a coal as it matures from peat to anthracite is known as Coalification.

The Quality of each coal deposit is determined by:

  1. Varying types of vegetation from which the coal is originated
  2. Depth of burial
  3. Pressure and temperature at that depth
  4. Length of time the coal has been formed in the deposit.

Microscopic Constituent of Coal

The microscopic Analysis of coal is carried out in terms of group maceral composition. Polished blocks of coal are studied under reflected light with oil immersion lens using a Leitz Microscopic.

  • Vitrinite Group: The dominant maceral group in the Sulphur rich Eocene coal is vitrinite. Collinite is the dominant maceral of Vitrinite group and bears no plant Structure. Collinite is light grey in colour and shows low to moderate reflectance and occurs as groundmass and sometimes in definite bands.
  • Exinite Group: The macerals of this group are derived from spores, cuticles, resins and algae. The Exinite group of macerals are represented by
  1. Sporonite and
  2. Resinite.
  • Inertinite Group: Inertinite macerals show highest reflectance and exhibit well preserved cell structure. This group of macerals are represented by
  1. Fusinite
  2. Semifusinite
  3. Sclerotinite
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