Introduction
Coal is an organic sedimentary rock of immense geological and economic importance. From a geological perspective, coal represents the preserved and altered remains of ancient vegetation deposited in peat-forming environments and subsequently transformed through diagenesis, burial, and coalification. Understanding coal is essential for geologists involved in coal exploration, basin analysis, stratigraphy, mining geology, and energy resource evaluation.
Geological Definition of Coal
Coal is a carbonaceous sedimentary rock formed by biochemical accumulation and physicochemical alteration of plant material under reducing conditions. It mainly consists of:
- Organic matter (macerals)
- Mineral matter (clay, quartz, pyrite)
- Moisture and volatile compounds
Coal-Forming Environments (Paleoenvironment)
Coal forms predominantly in low-energy, anoxic depositional environments, such as:
- Coastal swamps
- Delta plains
- Back-barrier lagoons
- Alluvial floodplains
Key conditions required:
High plant productivity
Low clastic sediment input
Waterlogged conditions preventing oxidation
Stable subsidence for peat preservation
Coalification Process (Geological Transformation)
Coalification is the progressive transformation of peat → coal due to:
- Increase in temperature
- Increase in pressure
- Increase in burial depth
- Geological time
Stages of Coalification
1. Peat – Biochemical alteration
2. Lignite – Early diagenesis
3. Bituminous coal – Catagenesis
4. Anthracite – Metagenesis
With increasing rank:
Carbon % increases
Moisture & volatile matter decrease
Reflectance of vitrinite increases
Coal Rank vs Coal Grade (Geological Perspective)
Rank: Degree of coalification (metamorphic maturity)
Grade: Quality based on ash, sulphur, and moisture
Rank is controlled by thermal history, while grade depends on depositional and post-depositional conditions.
Types of Coal (Based on Rank)
1. Peat (Pre-Coal Stage)
- Unconsolidated organic material
- Preserves plant structures
- Carbon content ~50%
- High moisture (>80%)
Geological significance: Indicates active or fossil peat swamps.
2. Lignite (Low Rank Coal)
- Brown to dark brown
- High inherent moisture
- Low vitrinite reflectance
- Poor compaction
- Carbon content: 60–70%
Depositional implication: Shallow burial, limited geothermal gradient.
3. Bituminous Coal (Medium Rank)
- Black, compact, well-banded
- High volatile matter
- Economically most important
- Carbon content: 70–85%
Sub-types (ASTM classification):
- High volatile
- Medium volatile
- Low volatile bituminous
Indian Gondwana coalfields mostly contain bituminous coal.
4. Anthracite (High Rank Coal)
- Hard, shiny, semi-metallic luster
- Carbon content >90%
- Low volatile matter
- Highest calorific value
Geological condition: High tectonic stress and thermal metamorphism.
Coal Petrography (Maceral Composition)
Coal is studied microscopically using reflected light microscopy.
Major Maceral Groups
1. Vitrinite
Derived from woody tissues
Indicates humification
Most important for rank determination
2. Liptinite (Exinite)
Derived from spores, resins, algae
High hydrogen content
3. Inertinite
Derived from oxidized or charred plant material
Indicates wildfire or oxidation
Vitrinite Reflectance (Ro%)
Key parameter for:
Coal rank determination
Thermal maturity
Hydrocarbon generation studies
Ro% Coal Rank
<0.4 Lignite
0.5–1.3 Bituminous
>2.0 Anthracite
Coal Types Based on Maceral Dominance
Humic coal – Derived from terrestrial plants
Sapropelic coal – Derived from algae (rare)
Cannel coal – Fine-grained, liptinite-rich
Coal Basins and Stratigraphy (India)
Indian coal is mainly found in Gondwana basins (Permian age).
Major Coal-Bearing Formations
- Barakar Formation (main coal seams)
- Raniganj Formation (upper coal seams)
Important Coalfields
Jharia
Raniganj
Bokaro
Talcher
Ib Valley
Structural Controls on Coal Seams
Faulting → seam displacement
Folding → seam thickening/thinning
Igneous intrusions → coking & rank increase
Example: Jharia coalfield shows natural coke due to igneous intrusions.
Coal Bed Methane (CBM)
Coal acts as both:
Source rock
Reservoir rock
Gas is stored by adsorption on maceral surfaces, especially vitrinite.
Economic and Applied Geology Importance
- Coal exploration & resource estimation
- Mine planning and seam correlation
- Basin evolution studies
- Energy security and fuel geology
Environmental Geology Aspects
- Acid mine drainage (pyrite oxidation)
- Subsidence
- Spontaneous combustion
- Trace element release (Hg, As)
Uses of Coal
- Electricity generation (Thermal power plants)
- Iron and steel industry (Coke)
- Cement manufacturing
- Chemical industry (Coal tar, gas)
- Domestic fuel (limited use today)
Advantages of Coal
- Abundant and easily available
- Low cost compared to other fuels
- Reliable base-load power source
- Established mining and transport infrastructure
From a geological standpoint, coal is not merely a fuel but a record of Earth’s paleo-climate, depositional environments, and tectonic evolution. Detailed study of coal rank, petrography, and basin geology is critical for effective resource utilization and environmental management.