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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 a common and widespread mineral, found in various geological settings. It crystallizes in the isometric system, often forming distinctive cubes, octahedra, or pyritohedra, sometimes with characteristic striations on crystal faces. While it contains iron, it is not an economically viable ore of iron due to the difficulty of separating iron from sulfur. Its presence in coal and other sedimentary rocks can lead to environmental issues like acid mine drainage when exposed to oxygen and water.
How to Identify
- Color
- Pale brass-yellow to golden-yellow, often tarnishing to a darker, duller brassy color.
- Luster
- Distinctive metallic luster, very bright and reflective.
- Texture
- Typically forms well-defined crystals; granular or massive aggregates also occur. Crystal faces are often striated.
- Crystal Form
- Commonly forms cubes, octahedra, and pyritohedra. Penetration twins are also observed. Striations on cubic faces are perpendicular to each other.
- 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 ore deposits.
Key Facts
- Hardness: 6-6.5 on the Mohs scale
- Specific Gravity: 4.95-5.10 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: Brassy yellow
- Luster: Metallic
- Streak: Greenish-black to brownish-black
Physical Characteristics
- Crystal Habit: Cubic, octahedral, pyritohedral, massive, granular, reniform, radiating, stalactitic. Cube faces often striated.
- Cleavage Type: Poor/Indistinct
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Metallic
Formation
Pyrite forms under a wide range of conditions, typically in anoxic (oxygen-deficient) environments. It can precipitate from hydrothermal fluids, form in sedimentary rocks (especially shales and coals) through bacterial sulfate reduction, or occur as an accessory mineral in igneous and metamorphic rocks. It is common in sulfide ore deposits.
Usage
Historically, pyrite was used as a source of sulfur for sulfuric acid production and as a source of iron. It has also been used as a spark-producing material in early firearms (flintlock mechanisms). Today, its primary industrial use is limited, but it is collected for its aesthetic crystal forms and is studied for its role in environmental geochemistry (e.g., acid mine drainage).
Age Distribution
Pyrite forms throughout Earth's geological history, from Precambrian to present-day sedimentary environments.
Where to Find
Navajún, La Rioja, Spain
Famous for producing some of the world's finest cubic pyrite crystals, often perfectly formed and embedded in marl.
Elba Island, Italy
Known for well-formed pyrite crystals, often associated with hematite.
Peruvian Andes, Peru
Numerous localities produce excellent pyrite specimens, including large, complex crystals.
Various coal-bearing regions worldwide
Commonly found as concretions and disseminated grains in coal seams and associated shales (e.g., Appalachian Basin, USA; Ruhr Basin, Germany).
Hydrothermal ore deposits globally
A ubiquitous sulfide mineral in many types of ore deposits, including porphyry copper deposits, volcanogenic massive sulfide (VMS) deposits, and mesothermal gold deposits.
Finding Tips
Look for Metallic Luster
Pyrite's bright, brassy metallic luster is a key identifier. It will stand out against duller rock matrices.
Check for Crystal Forms
Well-formed cubic, octahedral, or pyritohedral crystals are highly characteristic. Look for striations on cubic faces.
Perform a Streak Test
A greenish-black to brownish-black streak is diagnostic and differentiates it from gold (yellow streak) and chalcopyrite (greenish-black streak).
Consider 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 fingernail or copper penny.
Examine Associated Minerals
Pyrite often occurs with other sulfide minerals (e.g., galena, sphalerite, chalcopyrite) and quartz in hydrothermal veins, or with organic matter in sedimentary rocks.
Safety Precautions
When collecting pyrite, especially from old mine workings or areas with acid mine drainage, be aware that it can react with oxygen and water to produce sulfuric acid. Avoid inhaling dust. Wash hands after handling. Do not ingest. Pyrite can also contain trace amounts of arsenic, which is toxic. Handle with gloves if concerned about prolonged exposure or if processing samples.
Similar Rocks
Chalcopyrite
Copper iron sulfide (CuFeS2)
Also known as: Copper Pyrites
Gold
Gold (Au)
Also known as: Native Gold
Marcasite
Iron sulfide (FeS2)
Also known as: White Iron Pyrites
Pyrrhotite
Iron sulfide (Fe1-xS)
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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