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Pyrite is an iron sulfide mineral with the chemical formula FeS₂. It is one of the most common sulfide minerals and is often referred to as 'Fool's Gold' due to its brassy yellow color and metallic luster, which can be mistaken for gold by the inexperienced. Pyrite crystallizes in the isometric system, commonly forming cubes, octahedra, and pyritohedra, often with striated faces. It is a significant component in many ore deposits and is ubiquitous in various geological environments.
How to Identify
- Color
- Pale brass-yellow to golden-yellow, often tarnishing to a darker, duller brassy color.
- Luster
- Distinctive metallic luster, often bright and reflective.
- Texture
- Typically forms well-defined crystals, often cubic, octahedral, or pyritohedral. Can also be massive, granular, or radiating.
- Crystal Form
- Cubic, octahedral, or pyritohedral crystals are common, often with striations on cube faces. Also found as massive, granular, or radiating aggregates.
- Cleavage
- Poor to indistinct cleavage on {011}. Typically fractures conchoidally or unevenly.
- Geological Environment
- Found in igneous, metamorphic, and sedimentary rocks. Common in hydrothermal veins, magmatic sulfide deposits, contact metamorphic zones, black shales, coal seams, and as a detrital mineral in sediments.
Key Facts
- Hardness: 6-6.5 (Mohs scale)
- Specific Gravity: 4.95-5.10 g/cm³
- Crystal System: Isometric
- Color: Pale brass-yellow to golden-yellow
- Luster: Metallic
- Transparency: Opaque
- Fracture: Conchoidal to uneven
- Cleavage: Poor to indistinct on {011}
- Composition: Iron sulfide (FeS₂)
Quick Check
- Color: Brassy yellow
- Luster: Metallic
- Streak: Greenish-black
Physical Characteristics
- Crystal Habit: Cubic, octahedral, pyritohedral, massive, granular, radiating
- Cleavage Type: Poor/Indistinct
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Metallic
Formation
Pyrite forms under a wide range of conditions, both igneous, metamorphic, and sedimentary. It commonly crystallizes from hydrothermal solutions, in magmatic sulfide deposits, in contact metamorphic aureoles, and as a primary or diagenetic mineral in sedimentary rocks, particularly shales and coal seams, under reducing conditions. It can also form as a secondary mineral during weathering processes.
Usage
Historically, pyrite was used as a source of sulfur for sulfuric acid production and as an ore of iron, though it is not a primary iron ore today due to its sulfur content. It is occasionally used as a minor gemstone or ornamental stone. In modern applications, it is studied for its semiconductor properties and its role in environmental geochemistry, particularly acid mine drainage.
Age Distribution
Found in rocks of all ages, from Precambrian to recent sedimentary deposits.
Where to Find
Spain
The Rio Tinto district is famous for large, well-formed pyrite crystals, often associated with massive sulfide deposits.
Peru
Various mining districts, such as Huaron and Quiruvilca, produce excellent pyrite specimens, including cubic and pyritohedral forms.
Italy
Elba Island is known for its distinctive, often iridescent, pyrite crystals.
USA
Significant occurrences in Colorado (e.g., Leadville), Illinois (coal seams), and various localities in the Appalachian Mountains.
Germany
The Siegerland district has historically produced fine pyrite specimens.
Finding Tips
Look for Metallic Luster
Pyrite's bright, metallic luster is a key identifier. It often stands out against the duller matrix rock.
Check for Crystal Forms
Well-formed cubic, octahedral, or pyritohedral crystals are characteristic. Look for striations on cube faces.
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).
Consider the Geological Environment
Pyrite is common in sulfide-rich environments, hydrothermal veins, and sedimentary rocks formed under anoxic conditions (e.g., black shales, coal).
Test Hardness
Pyrite is relatively hard (6-6.5 on Mohs scale), which helps distinguish it from softer minerals like gold (2.5-3) and chalcopyrite (3.5-4).
Safety Warning: Acid Mine Drainage
Pyrite can react with oxygen and water to produce sulfuric acid and dissolved iron, a process known as acid mine drainage (AMD). This can be environmentally damaging and can occur in mine tailings or exposed rock. When collecting, be aware of potential AMD sites and avoid direct contact with acidic water. For collectors, long-term storage of pyrite specimens in humid environments can lead to 'pyrite disease' (decomposition due to oxidation), producing sulfuric acid and iron sulfates. Store specimens in dry, stable conditions, and consider sealing them with a clear acrylic spray if they show signs of degradation.
Similar Rocks
Chalcopyrite
Chalcopyrite
Also known as: Copper Pyrite
Gold
Gold
Also known as: Native Gold
Marcasite
Marcasite
Also known as: White Iron Pyrites
Pyrrhotite
Pyrrhotite
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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