Bituminous Coal

Description

Bituminous coal is a dense, black, or dark brown sedimentary rock with a relatively high carbon content (typically 45-86% by weight on a dry, ash-free basis) and a high calorific value. It is characterized by distinct banding, often exhibiting alternating bright (vitrain, clarain) and dull (durain, fusain) layers, reflecting its heterogeneous plant origins. It is the most abundant rank of coal globally and is a major energy source.

How to Identify

Color

Typically black, sometimes very dark brown.

Luster

Vitreous (glassy) to dull, often with a greasy or pitch-like appearance. Bright bands (vitrain) are vitreous, while dull bands (durain) are dull or earthy.

Texture

Often banded, with alternating bright and dull layers. Can be brittle and friable, breaking into blocky or irregular fragments. May show fossil plant impressions.

Crystal Form

Amorphous; does not form crystals. Occurs as massive, layered, or blocky aggregates.

Cleavage

No true mineral cleavage. Exhibits a characteristic 'blocky' or 'cubical' fracture pattern, often with jointing perpendicular to bedding planes.

Geological Environment

Found in sedimentary basins, typically within sequences of shale, sandstone, and sometimes limestone. Forms in ancient swamp or bog environments that were subsequently buried and subjected to coalification. Often associated with deltaic, fluvial, or lacustrine depositional settings.

Key Facts

• Hardness: 1.0-2.5 (Mohs scale) - very soft and easily scratched.
• Specific Gravity: 1.2-1.5 g/cm³ (variable depending on ash content and moisture).
• Crystal System: Amorphous (non-crystalline).
• Color: Black, sometimes dark brown.
• Luster: Vitreous, sub-vitreous, dull, or greasy.
• Transparency: Opaque.
• Fracture: Conchoidal to irregular, often blocky or cubical.
• Cleavage: None (exhibits jointing or 'cleat' planes, not true mineral cleavage).
• Composition: Primarily carbon (45-86% dry, ash-free basis), with varying amounts of hydrogen, oxygen, nitrogen, sulfur, and inorganic ash-forming minerals (e.g., clays, quartz, pyrite).

Quick Check

• Color: Black to very dark brown
• Luster: Vitreous to dull, often greasy
• Streak: Black to brownish-black

Physical Characteristics

• Crystal Habit: Massive, layered, or blocky aggregates; no distinct crystal form.
• Cleavage Type: None (exhibits 'cleat' - a system of natural fractures or joints, typically orthogonal, which aids in mining).
• Fracture Type: Conchoidal to irregular, often blocky.
• Tenacity: Brittle.
• Luster Type: Vitreous (bright bands), dull (dull bands), greasy, or pitch-like.

Formation

Bituminous coal forms from the compaction and alteration of peat under moderate to high pressure and temperature conditions, a process known as coalification. Peat, composed of partially decayed plant matter, accumulates in anaerobic wetland environments (swamps, bogs). As overlying sediments accumulate, the peat is buried, increasing pressure and temperature. This leads to the expulsion of water and volatile organic compounds, increasing the carbon content. The coalification sequence typically progresses from peat to lignite, then to sub-bituminous coal, bituminous coal, and finally anthracite. Bituminous coal represents an intermediate to high rank in this process, characterized by a significant increase in carbon content and calorific value compared to lower ranks.

Usage

Historically and currently, bituminous coal is primarily used as a fuel for electricity generation in thermal power plants. It is also a crucial raw material in the steel industry, where it is converted into coke for use in blast furnaces. Other uses include industrial heating, cement production, and as a source of various chemical byproducts (e.g., coal tar, ammonia, benzene, toluene).

Age Distribution

Predominantly Carboniferous (359 to 299 million years ago), Permian (299 to 252 million years ago), and Cretaceous (145 to 66 million years ago) periods, but can be found in other geological ages.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Organic Sedimentary Rock (not a true mineral)

Group

Coal

Crystal System

Amorphous

Chemical Formula

Variable, primarily C with H, O, N, S (e.g., C137H97O9N3S for a typical bituminous coal macromolecule, but highly complex and variable).

Composition

Complex macromolecular structure derived from plant matter, predominantly carbon, with significant volatile matter (20-40% on a dry, ash-free basis) and moisture content (1-10%).

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