Sedimentary Rock (Metamorphosed)
Anthracite Coal
Anthracite (a variety of coal, predominantly carbon)
Hardness: 2.5-2.75 (Mohs scale), Color: Jet-black, often with a semi-metallic sheen. Luster: Vitreous to sub-metallic, sometimes dull. Crystal structure: Amorphous (non-crystalline), effectively a rock, not a mineral in the strict sense. Cleavage: None, fractures conchoidally. Specific Gravity: 1.3-1.8.
- Hardness
- 2
- Color
- Jet-black, often with a semi-metallic sheen
- Luster
- Vitreous to sub-metallic, sometimes dull
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Physical properties
Hardness: 2.5-2.75 (Mohs scale), Color: Jet-black, often with a semi-metallic sheen. Luster: Vitreous to sub-metallic, sometimes dull. Crystal structure: Amorphous (non-crystalline), effectively a rock, not a mineral in the strict sense. Cleavage: None, fractures conchoidally. Specific Gravity: 1.3-1.8.
Formation & geological history
Anthracite is the highest rank of coal. It forms from peat, which transforms into lignite, then sub-bituminous coal, then bituminous coal, and finally anthracite due to high pressure and temperature during metamorphism. This process, called coalification, typically takes millions of years, often beginning in vast swampy environments during geological periods like the Carboniferous (approx. 359 to 299 million years ago).
Uses & applications
Primarily used as a clean-burning fuel for residential and commercial heating due to its high carbon content and low volatile matter. It is also used in metallurgy (e.g., as a carbon source in steel production) and water filtration.
Geological facts
Anthracite is the purest form of coal, containing 92-98% carbon. It burns with a short, blue, smokeless flame, producing high heat and very little ash, making it an environmentally cleaner option than other coal types. Its formation requires specific geological conditions, making it less common than lower-rank coals.
Field identification & locations
In the field, anthracite is identified by its intense black color, shiny or semi-metallic luster, and its relative hardness compared to other coals. It often has a distinct conchoidal fracture (shell-like breaks). It does not soil the hands like bituminous coal. Common locations include eastern Pennsylvania (USA), parts of Wales (UK), and areas in China and Russia. For collectors, good quality specimens with a high luster are sought after.
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