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Pyrite is an iron sulfide mineral with the chemical formula FeS2. It 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 is one of the most common sulfide minerals and is found in a vast array of geological settings. Its distinctive cubic, octahedral, or pyritohedral crystal forms are highly prized by collectors. When exposed to air and moisture, pyrite can oxidize, producing sulfuric acid and iron oxides, which can lead to acid mine drainage in mining environments.
How to Identify
- Color
- Brassy yellow to golden yellow, sometimes with a tarnished, iridescent sheen.
- Luster
- Distinctive metallic luster, very bright and reflective.
- Texture
- Typically forms well-defined crystals with smooth, often striated faces. Massive forms can be granular or botryoidal.
- Crystal Form
- Commonly forms cubes, octahedra, and pyritohedra (a 12-faced form). Striations are often visible on cubic faces, running perpendicular to each other. Also found as massive, granular, radiating, or botryoidal aggregates.
- Cleavage
- Poor to indistinct cleavage, typically exhibiting conchoidal to uneven fracture rather than true cleavage.
- Geological Environment
- Found in igneous, metamorphic, and sedimentary rocks. Common in hydrothermal veins, contact metamorphic deposits, black shales, coal seams, and as concretions in sedimentary rocks. Often associated with other sulfide minerals, quartz, and carbonates.
Key Facts
- Hardness: 6-6.5 (Mohs scale)
- Specific Gravity: 4.95-5.02 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 {100}
- Composition: Iron sulfide (FeS2)
Quick Check
- Color: Brassy yellow
- Luster: Metallic
- Streak: Greenish-black to brownish-black
Physical Characteristics
- Crystal Habit: Cubic, octahedral, pyritohedral, massive, granular, radiating, botryoidal. Often striated on cube faces.
- Cleavage Type: Poor/Indistinct
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Metallic
Formation
Pyrite forms under a wide range of geological conditions, typically in anoxic (oxygen-deficient) environments. It can precipitate from hydrothermal fluids, form in sedimentary rocks (especially black shales and coal seams) through the action of sulfate-reducing bacteria, occur in metamorphic rocks, and as an accessory mineral in igneous rocks. It is often found in veins associated with other sulfide minerals and quartz.
Usage
Historically, pyrite was a primary source of sulfur for sulfuric acid production. It is also a minor ore of iron, though not economically viable compared to iron oxides. In modern times, it is sometimes used in thermoelectric devices and as a semiconductor. Its aesthetic appeal makes it popular in jewelry and mineral collections. It can also be a source of gold in some gold-bearing ore deposits, where gold is finely disseminated within the pyrite structure.
Age Distribution
Pyrite forms throughout Earth's geological history, from Precambrian to recent deposits. Its formation is not restricted to a specific geological age but rather to specific geochemical conditions.
Where to Find
Navajún, La Rioja, Spain
Renowned for producing some of the world's most perfect, large, and aesthetically pleasing cubic pyrite crystals, often embedded in marl.
Elba Island, Italy
Famous for well-formed pyrite crystals, often with iridescent tarnish.
Huánuco Department, Peru
Known for producing excellent pyrite specimens, including large, sharp crystals and aggregates.
Leadville, Colorado, USA
Historically significant mining district where pyrite is found in association with other sulfide ores.
Various coal seams and black shales worldwide
Pyrite concretions and disseminated crystals are common in these sedimentary environments, often forming through diagenetic processes.
Finding Tips
Look for Metallic Luster
Pyrite's strong metallic luster is a key identifier. It will reflect light much more brightly than most other minerals.
Check for Crystal Form
Well-formed cubic, octahedral, or pyritohedral crystals are highly characteristic of pyrite. Look for striations on cubic faces.
Perform a Streak Test
Pyrite leaves a greenish-black to brownish-black streak on an unglazed porcelain plate, which distinguishes it from gold (yellow streak) and chalcopyrite (greenish-black streak).
Consider Hardness
Pyrite is relatively hard (6-6.5 on Mohs scale), meaning it cannot be scratched by a steel knife or a copper coin, unlike gold (2.5-3) or chalcopyrite (3.5-4).
Assess Specific Gravity
Pyrite is dense (4.95-5.02 g/cm³), feeling noticeably heavy for its size, though less dense than gold (19.3 g/cm³).
Environmental Context
Pyrite is often found in association with other sulfide minerals in hydrothermal veins, or as concretions in sedimentary rocks like shales and coal.
Similar Rocks
Chalcopyrite
Chalcopyrite (CuFeS2)
Also known as: Copper Pyrites
Gold
Gold (Au)
Also known as: Native Gold
Marcasite
Marcasite (FeS2)
Also known as: White Iron Pyrites
Pyrrhotite
Pyrrhotite (Fe(1-x)S)
Also known as: Magnetic Pyrites
Scientific Classification
- Mineral Class
- Sulfides
- Group
- Pyrite Group
- Crystal System
- Isometric (Cubic)
- Chemical Formula
- FeS2
- Composition
- Iron (Fe) 46.55%, Sulfur (S) 53.45%
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