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Pyrite is an iron sulfide mineral with the chemical formula FeS2. It is known for its distinctive metallic luster and brass-yellow color, which often leads to its misidentification as gold, hence the nickname 'Fool's Gold'. Pyrite crystallizes in the isometric system, commonly forming cubes, octahedra, and pyritohedra, often with striated faces. It is a common and widespread mineral found in various geological settings.
How to Identify
- Color
- Brass-yellow to golden-yellow, sometimes with a tarnished or iridescent surface.
- Luster
- Distinctive metallic luster, very bright and reflective.
- Texture
- Typically forms well-defined crystals; massive or granular aggregates are also common. Crystal faces can be striated.
- Crystal Form
- Cubic, octahedral, or pyritohedral crystals are characteristic. Penetration twins are also observed. Can also be massive, granular, or radiating.
- Cleavage
- Poor to indistinct cleavage on {001}. It typically fractures conchoidally or unevenly rather than cleaving.
- Geological Environment
- Found in igneous, metamorphic, and sedimentary rocks. Common in hydrothermal veins, contact metamorphic deposits, black shales, coal seams, and as an accessory mineral in various rock types. Often associated with other sulfide minerals.
Key Facts
- Hardness: 6-6.5 on the Mohs scale
- Specific Gravity: 4.95-5.03 g/cm³
- Crystal System: Isometric (Cubic)
- Color: Pale brass-yellow, golden-yellow
- Luster: Metallic
- Transparency: Opaque
- Fracture: Conchoidal to uneven
- Cleavage: Poor to indistinct on {001}
- Composition: Iron sulfide (FeS2)
Quick Check
- Color: Brass-yellow to golden-yellow
- Luster: Metallic
- Streak: Greenish-black
Physical Characteristics
- Crystal Habit: Cubic, octahedral, pyritohedral, massive, granular, radiating, stalactitic, concretions
- Cleavage Type: Poor/Indistinct
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Metallic
Formation
Pyrite forms under both high and low-temperature conditions. It commonly crystallizes from hydrothermal solutions, in sedimentary environments (often associated with organic matter and anoxic conditions), in metamorphic rocks, and as a primary mineral in igneous rocks. Its formation is often linked to the presence of iron and sulfur, with microbial activity playing a significant role in many sedimentary and diagenetic occurrences.
Usage
Historically, pyrite was a primary source of sulfur for sulfuric acid production and iron for iron ore, though its iron content is too low for efficient extraction today. It is still used in some specialized applications, such as in lithium-iron-sulfide batteries. It is also collected as a mineral specimen due to its attractive metallic luster and crystal forms. In the past, it was used as a spark-producing material in flintlock mechanisms.
Age Distribution
Pyrite forms throughout geological time, from the Precambrian to the present day, in a wide range of geological environments.
Where to Find
Spain
The Rio Tinto district is famous for large, well-formed pyrite crystals, often cubic.
Peru
Various localities, including Huaron and Huanzala, produce excellent pyrite specimens, often with complex crystal habits.
Italy
Elba Island is known for its beautiful, often iridescent, pyrite crystals.
USA
Colorado (e.g., Leadville, Alma) and Illinois (e.g., Sparta) are known for significant pyrite occurrences, including concretions and well-formed crystals.
Germany
Various mining districts, such as Freiberg, have yielded fine pyrite specimens.
Finding Tips
Look for Metallic Luster
Pyrite's bright, metallic luster is a key identifier. It often stands out against duller host rocks.
Check for Crystal Forms
Cubic, octahedral, or pyritohedral crystals are very characteristic. Look for these geometric shapes.
Perform a Streak Test
Pyrite leaves a greenish-black streak, which distinguishes it from gold (yellow streak) and chalcopyrite (greenish-black streak, but softer and often tarnished).
Assess Hardness
Pyrite is harder than gold (Mohs 2.5-3) and chalcopyrite (Mohs 3.5-4). It will scratch glass and cannot be scratched by a knife.
Consider Associated Minerals
Pyrite often occurs with quartz, calcite, galena, sphalerite, and other sulfide minerals. Its presence can indicate a mineralized zone.
Safety Warning: Acid Generation
Pyrite can react with oxygen and water to form sulfuric acid and iron hydroxides (acid mine drainage). This process can be accelerated by bacteria. When collecting or storing pyrite, especially in humid environments, be aware of potential acid generation. Avoid prolonged skin contact with weathered pyrite and wash hands after handling. Do not ingest. Inhalation of dust should be avoided, as with any mineral.
Similar Rocks
Chalcopyrite
Chalcopyrite (Copper iron sulfide, CuFeS2)
Also known as: Copper Pyrites
Gold
Gold (Au)
Also known as: Native Gold
Marcasite
Marcasite (Iron sulfide, FeS2)
Also known as: White Iron Pyrites
Pyrrhotite
Pyrrhotite (Iron sulfide, Fe(1-x)S)
Also known as: Magnetic Pyrites
Scientific Classification
- Mineral Class
- Sulfide
- Group
- Pyrite Group
- Crystal System
- Isometric (Cubic)
- Chemical Formula
- FeS2
- Composition
- Iron (Fe) 46.55%, Sulfur (S) 53.45%
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