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Concretionary pyrite refers to pyrite (iron disulfide, FeS2) that has formed as a concretion within sedimentary rocks. These concretions are typically spherical, discoidal (lens-shaped), or irregularly shaped masses that have grown within the host sediment. They are characterized by their metallic luster and brassy yellow color, often exhibiting radial or concentric internal structures. The size can vary from a few millimeters to several tens of centimeters in diameter. The most well-known forms are 'pyrite suns' or 'pyrite dollars,' which are flattened, discoidal concretions with radiating pyrite crystals, commonly found in certain shale formations.
How to Identify
- Color
- Brassy yellow to golden metallic, often tarnishing to a duller bronze or iridescent hue upon exposure to air and moisture.
- Luster
- Distinctive metallic luster.
- Texture
- Typically smooth to slightly rough on the exterior, often showing radial or concentric growth patterns on fractured surfaces. 'Pyrite suns' have a characteristic radiating crystalline texture.
- Crystal Form
- While the overall concretion is massive or discoidal, individual pyrite crystals within the concretion are typically euhedral to subhedral, often cubic, pyritohedral, or octahedral, sometimes radiating outwards from a central point.
- Cleavage
- Poor to indistinct on individual crystals; the concretion itself does not exhibit cleavage.
- Geological Environment
- Most commonly found in fine-grained sedimentary rocks such as shales, mudstones, and siltstones, particularly those rich in organic matter and deposited in anoxic marine or lacustrine environments. Can also occur in coal seams.
Key Facts
- Hardness: 6 to 6.5 on the Mohs scale
- Specific Gravity: 4.95 to 5.03 g/cm³
- Crystal System: Isometric (cubic)
- Color: Pale brass-yellow, golden metallic
- Luster: Metallic
- Transparency: Opaque
- Fracture: Conchoidal to uneven
- Cleavage: Poor to indistinct {100}
- Composition: Iron disulfide (FeS2)
Quick Check
- Color: Brassy yellow to golden metallic
- Luster: Metallic
- Streak: Greenish-black to brownish-black
Physical Characteristics
- Crystal Habit: Concretionary, massive, discoidal, radiating, granular. Individual crystals are typically cubic, pyritohedral, or octahedral.
- Cleavage Type: Poor to indistinct, typically not observed in concretions.
- Fracture Type: Conchoidal to uneven, brittle.
- Tenacity: Brittle
- Luster Type: Metallic
Formation
Concretionary pyrite forms through diagenetic processes in sedimentary environments, particularly in fine-grained sediments (shales, mudstones, siltstones) rich in organic matter. The process begins with the decomposition of organic material by sulfate-reducing bacteria in anoxic conditions. This bacterial activity produces hydrogen sulfide (H2S). Iron, typically present as iron oxides or hydroxides in the sediment, reacts with the hydrogen sulfide to form iron monosulfides (e.g., mackinawite, greigite). Over time, these unstable iron monosulfides recrystallize into more stable pyrite (FeS2) through further reactions with sulfur species. The concretionary form arises from the localized precipitation and growth of pyrite around a nucleus (e.g., a fossil fragment, a grain of organic matter, or a mineral grain) within the sediment, often displacing the surrounding sediment as it grows. The spherical, discoidal, or irregular shapes are a result of this radial growth.
Usage
Concretionary pyrite has limited industrial use due to its form. However, it is highly valued by mineral collectors for its aesthetic appeal and unique shapes, particularly the 'pyrite suns' or 'pyrite dollars'. Historically, pyrite in general was used as a source of sulfur for sulfuric acid production and, to a lesser extent, for iron. It was also used as a spark-producing material in early firearms (flintlock mechanisms).
Age Distribution
Concretionary pyrite can form in sedimentary rocks of various ages, from the Precambrian to the Cenozoic, wherever anoxic conditions and organic matter are present.
Where to Find
Sparta, Illinois, USA
Famous for producing 'pyrite suns' or 'pyrite dollars' from the Pennsylvanian-age Francis Creek Shale, often associated with coal seams.
Germany
Various localities, particularly in the Posidonia Shale (Lower Jurassic), yield well-formed pyrite concretions, sometimes preserving fossils.
United Kingdom
Jurassic shales along the coast (e.g., Yorkshire, Dorset) are known for pyrite concretions, often encasing ammonites and other fossils.
Russia
Certain sedimentary basins contain significant occurrences of concretionary pyrite.
Finding Tips
Look in Shale and Mudstone Outcrops
Concretionary pyrite is typically found embedded within dark, fine-grained sedimentary rocks. Examine weathered outcrops where concretions may have eroded out or are visible on exposed surfaces.
Search for Anoxic Environments
Focus on geological formations that indicate ancient anoxic conditions, such as black shales, coal-bearing strata, or areas with abundant fossilized organic matter.
Check for Associated Fossils
Pyrite concretions often form around organic nuclei, so they can be found associated with or even encasing fossils (e.g., ammonites, bivalves, plant remains).
Use a Geologist's Hammer and Chisel
Carefully extract concretions from the host rock. They are often harder than the surrounding sediment, making them stand out.
Be Aware of Oxidation
Freshly exposed pyrite can oxidize rapidly, especially in humid conditions, leading to the formation of sulfuric acid and iron oxides. Store specimens in dry, stable environments.
Similar Rocks
Marcasite Concretion
Marcasite (FeS2) in a concretionary form
Also known as: White Iron Pyrites Concretion
Siderite Concretion
Siderite (FeCO3) in a concretionary form
Also known as: Iron Carbonate Concretion
Chert Nodule
Chert (SiO2) in a nodular form
Also known as: Flint Nodule
Scientific Classification
- Mineral Class
- Sulfides
- Group
- Pyrite Group
- Crystal System
- Isometric
- Chemical Formula
- FeS2
- Composition
- Iron (Fe) 46.55%, Sulfur (S) 53.45%
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