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Coal is a combustible black or brownish-black sedimentary rock composed primarily of carbon, along with varying amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. It is formed from the remains of ancient plants that have been subjected to heat and pressure over geological timescales. Its properties vary significantly depending on its rank (degree of coalification), from low-rank lignite to high-rank anthracite.
How to Identify
- Color
- Typically black to brownish-black. Lignite can be dark brown, while anthracite is jet black.
- Luster
- Dull, earthy, waxy, or vitreous (glassy) depending on the rank. Anthracite often has a bright, submetallic to vitreous luster.
- Texture
- Can be earthy, granular, or compact. Often exhibits banding or layering from original plant material. Anthracite is very hard and dense.
- Crystal Form
- Amorphous; does not form crystals. It is a rock, not a mineral.
- Cleavage
- None, but often exhibits fracture along bedding planes or jointing. Anthracite can show conchoidal fracture.
- Geological Environment
- Forms in ancient swamps, peat bogs, and deltaic environments where rapid burial of organic matter occurred under anoxic conditions, preventing complete decomposition. Found in sedimentary basins.
Key Facts
- Hardness: 1-2.5 (Mohs scale) for lignite to bituminous; 2.5-3 for anthracite. It is relatively soft.
- Specific Gravity: 1.1-1.8 (varies with rank and ash content). Lignite is typically lower, anthracite higher.
- Crystal System: Amorphous (non-crystalline)
- Color: Black, dark brown, brownish-black
- Luster: Dull, earthy, waxy, submetallic, vitreous
- Transparency: Opaque
- Fracture: Irregular, conchoidal (especially anthracite), or splintery
- Cleavage: None
- Composition: Primarily carbon (C), with varying amounts of hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S), plus inorganic ash constituents.
Quick Check
- Color: Black to brownish-black
- Luster: Dull, earthy, waxy, or vitreous
- Streak: Black to brownish-black
Physical Characteristics
- Crystal Habit: Massive, layered, or banded; amorphous
- Cleavage Type: None
- Fracture Type: Irregular, conchoidal, or splintery
- Tenacity: Brittle
- Luster Type: Dull, earthy, waxy, submetallic, vitreous
Formation
Coal forms from the accumulation and burial of plant matter in anoxic (oxygen-deficient) environments, typically swamps or peat bogs. Over millions of years, heat and pressure from overlying sediments transform the peat through a process called coalification, increasing its carbon content and reducing its moisture and volatile matter. The stages of coalification are peat, lignite, sub-bituminous coal, bituminous coal, and anthracite.
Usage
Primarily used as a fossil fuel for electricity generation, steel production (as coking coal), cement manufacturing, and various industrial processes. Historically used for heating and as a chemical feedstock.
Age Distribution
Predominantly Carboniferous (359 to 299 million years ago), Permian, Triassic, Jurassic, Cretaceous, and Cenozoic periods.
Where to Find
United States
Major coal basins include the Appalachian Basin (Pennsylvania, West Virginia, Kentucky), Illinois Basin (Illinois, Indiana, Kentucky), Powder River Basin (Wyoming, Montana), and Western Interior Coal Region.
China
World's largest producer and consumer of coal, with significant reserves in provinces like Shanxi, Inner Mongolia, and Shaanxi.
India
Large coal reserves, primarily in the eastern states such as Jharkhand, Odisha, and Chhattisgarh.
Australia
Significant producer and exporter, with major deposits in Queensland and New South Wales.
Russia
Vast coal reserves, particularly in the Kuznetsk Basin (Kuzbass) and the Tunguska Basin.
South Africa
Major coal producer, with deposits concentrated in the Highveld and Waterberg coalfields.
Finding Tips
Geological Maps
Consult geological maps of sedimentary basins, particularly those indicating Carboniferous, Permian, or Cretaceous age strata, as these are common periods for coal formation.
Outcrops and Road Cuts
Look for black, layered rock exposures in road cuts, stream banks, and mining areas. Coal seams are often interbedded with shales, sandstones, and limestones.
Associated Rocks
Coal is frequently found with fossilized plant remains, pyrite (iron sulfide), and clay minerals. The presence of these can indicate proximity to a coal seam.
Safety Precautions
When exploring old mine sites or natural outcrops, be aware of unstable ground, potential for methane gas accumulation (especially in enclosed spaces), and the presence of heavy metals or radioactive elements in some coal deposits. Always prioritize safety and avoid entering abandoned mines.
Similar Rocks
Oil Shale
Oil Shale
Also known as: Kerogen Shale
Peat
Peat
Also known as: Turf
Bitumen
Bitumen
Also known as: Asphalt
Scientific Classification
- Mineral Class
- Organic sedimentary rock (not a true mineral)
- Group
- Fossil fuel
- Crystal System
- Amorphous
- Chemical Formula
- Variable, primarily C with H, O, N, S
- Composition
- Complex mixture of organic compounds, predominantly carbonaceous material derived from plant remains.
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