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Coal with pyrite is a sedimentary rock, specifically bituminous coal, characterized by the presence of the mineral pyrite (iron disulfide, FeS2) disseminated within its matrix or as distinct nodules, lenses, or bands. Bituminous coal is a black, relatively hard coal with a bright to dull luster, typically exhibiting banding. Pyrite appears as brassy yellow, metallic crystals or masses, often cubic, pyritohedral, or octahedral, or as framboids (microscopic spherical aggregates). The amount of pyrite can vary from trace amounts to several weight percent, significantly impacting the coal's sulfur content.
How to Identify
- Color
- Coal is typically black to dark brownish-black. Pyrite is brassy yellow, pale gold, or silvery-yellow.
- Luster
- Coal can range from vitreous (glassy) to dull or earthy. Pyrite has a distinct metallic luster.
- Texture
- Coal is generally brittle, often with a blocky or layered texture. Pyrite is hard and can be granular, massive, or crystalline.
- Crystal Form
- Coal does not have a crystal form. Pyrite commonly forms cubes, octahedra, or pyritohedra, or can be massive or granular.
- Cleavage
- Coal exhibits no true cleavage but has a fracture pattern. Pyrite has indistinct cleavage on {100} and {110}.
- Geological Environment
- Found in sedimentary basins where ancient peat swamps were buried and coalified. Pyrite forms within these coal seams, often indicating marine or brackish water influence during peat deposition or later diagenetic processes.
Key Facts
- Hardness: Coal: 2.5-3 (Mohs); Pyrite: 6-6.5 (Mohs)
- Specific Gravity: Coal: 1.2-1.5 g/cm³; Pyrite: 5.0-5.2 g/cm³
- Crystal System: Coal: Amorphous; Pyrite: Isometric
- Color: Black (coal); Brassy yellow, pale gold (pyrite)
- Luster: Vitreous to dull/earthy (coal); Metallic (pyrite)
- Transparency: Opaque
- Fracture: Conchoidal to irregular (coal); Uneven to conchoidal (pyrite)
- Cleavage: None (coal); Indistinct on {100} and {110} (pyrite)
- Composition: Coal: Primarily carbon (75-90% by weight) with varying amounts of hydrogen, oxygen, nitrogen, and sulfur; Pyrite: Iron disulfide (FeS2), 46.55% Fe, 53.45% S
Quick Check
- Color: Black (coal) with brassy yellow (pyrite)
- Luster: Vitreous to dull (coal) and metallic (pyrite)
- Streak: Black to brownish-black (coal) and greenish-black to brownish-black (pyrite)
Physical Characteristics
- Crystal Habit: Coal: Massive, banded; Pyrite: Cubes, octahedra, pyritohedra, massive, granular, framboidal
- Cleavage Type: Coal: None; Pyrite: Indistinct
- Fracture Type: Coal: Conchoidal to irregular; Pyrite: Uneven to conchoidal
- Tenacity: Coal: Brittle; Pyrite: Brittle
- Luster Type: Coal: Vitreous to dull/earthy; Pyrite: Metallic
Formation
Bituminous coal forms from the compaction and alteration of plant matter in anoxic swamp environments over millions of years (coalification). Pyrite (FeS2) forms within the coal seam through two primary mechanisms: 1) Syngenetic pyrite forms early during peat accumulation from the reaction of iron-rich sediments with hydrogen sulfide produced by sulfate-reducing bacteria in marine or brackish water influence. 2) Epigenetic pyrite forms later, after coalification, from iron-rich solutions migrating through fractures and pores in the coal, reacting with sulfur compounds.
Usage
Historically and currently used as a fuel for electricity generation, industrial processes, and steel production (coking coal). However, the presence of pyrite significantly reduces its quality due to the release of sulfur dioxide (SO2) upon combustion, contributing to acid rain and air pollution. Pyrite also contributes to spontaneous combustion of coal and acid mine drainage.
Age Distribution
Predominantly Carboniferous (Mississippian and Pennsylvanian periods, ~359 to 299 million years ago), but also found in Permian, Jurassic, Cretaceous, and Tertiary coal deposits.
Where to Find
Appalachian Basin, USA
Extensive bituminous coal deposits, particularly in states like West Virginia, Pennsylvania, Kentucky, and Ohio, often contain significant pyrite due to marine incursions during the Pennsylvanian period.
Illinois Basin, USA
Coal seams in Illinois, Indiana, and Kentucky are known for their high sulfur content, largely attributed to abundant pyrite.
Ruhr Basin, Germany
Historically significant coal-producing region with Carboniferous bituminous coals, often containing pyrite.
Donbas Basin, Ukraine/Russia
Major coalfield with high-quality bituminous coals, where pyrite is a common accessory mineral.
Bowen Basin, Australia
Permian coal deposits, including coking coals, can contain varying amounts of pyrite.
Finding Tips
Visual Inspection
Look for distinct brassy yellow, metallic specks, nodules, or bands within the black coal matrix. These can range from microscopic to several centimeters in size.
Density
Coal with significant pyrite will feel noticeably heavier than pure coal due to pyrite's higher specific gravity (5.0-5.2 g/cm³ vs. coal's 1.2-1.5 g/cm³).
Hardness Test
Pyrite is much harder than coal (6-6.5 on Mohs scale vs. 2.5-3 for coal). A steel knife will scratch coal but not pyrite.
Streak Test
Pyrite leaves a greenish-black to brownish-black streak on an unglazed porcelain plate, while coal leaves a black or brownish-black streak.
Acid Test (Caution!)
Dilute hydrochloric acid (HCl) will not react with pyrite, but it can be used to distinguish it from other sulfide minerals if necessary. However, this is generally not needed for identification in coal.
Similar Rocks
Anthracite
Anthracite
Also known as: Hard Coal
Lignite
Lignite
Also known as: Brown Coal
Shale with Pyrite
Shale with Pyrite (FeS2)
Also known as: Pyritic Shale
Maracas
Siderite Concretions
Also known as: Ironstone Concretions
Scientific Classification
- Mineral Class
- Coal: Organic sedimentary rock; Pyrite: Sulfide mineral
- Group
- Coal: Coal group; Pyrite: Pyrite group
- Crystal System
- Coal: Amorphous; Pyrite: Isometric (Cubic)
- Chemical Formula
- Coal: (C,H,O,N,S)x; Pyrite: FeS2
- Composition
- Coal: Carbonaceous material; Pyrite: Iron disulfide
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