Pyrite

Description

Pyrite is an iron sulfide mineral with the chemical formula FeS2. It is one of the most common sulfide minerals and is characterized by its distinctive metallic luster and brassy-yellow color, which often leads to its misidentification as gold, hence the nickname 'Fool's Gold'. Pyrite typically crystallizes in the isometric system, forming cubes, octahedra, and pyritohedra, often with striated faces. It is a relatively hard and brittle mineral. Pyrite is chemically unstable in oxygenated, moist environments, where it oxidizes to form iron oxides and sulfuric acid, a process known as acid mine drainage when it occurs in mining contexts. This oxidation can also lead to the degradation of mineral specimens over time.

How to Identify

Color

Brassy-yellow to golden-yellow, sometimes with a brownish tarnish. The color is consistent and uniform.

Luster

Distinctive metallic luster, very bright and reflective.

Texture

Typically forms well-defined crystals, often cubic, octahedral, or pyritohedral. Can also be massive, granular, or radiating.

Crystal Form

Cubic crystals are most common, often with striations on the faces. Octahedral and pyritohedral forms are also frequent. Can also occur as radiating aggregates or massive forms.

Cleavage

Poor to indistinct cleavage on {100}. It typically fractures conchoidally or unevenly rather than cleaving.

Geological Environment

Found in a wide variety of geological settings: hydrothermal veins, sedimentary rocks (especially black shales, coal), metamorphic rocks, and as an accessory mineral in some igneous rocks. Often associated with other sulfide minerals, quartz, calcite, and various ore minerals.

Key Facts

• Hardness: 6-6.5 (Mohs scale)
• Specific Gravity: 4.95-5.03 g/cm³
• Crystal System: Isometric
• Color: Brassy-yellow, golden-yellow
• Luster: Metallic
• Transparency: Opaque
• Fracture: Conchoidal to uneven
• Cleavage: Poor to indistinct on {100}
• Composition: Iron disulfide (FeS2)

Quick Check

• Color: Brassy-yellow to golden-yellow
• Luster: Metallic
• Streak: Greenish-black to brownish-black

Physical Characteristics

• Crystal Habit: Cubic, octahedral, pyritohedral, massive, granular, radiating, stalactitic
• Cleavage Type: Poor/Indistinct {100}
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Metallic

Formation

Pyrite forms under a wide range of geological conditions, primarily in anoxic (oxygen-deficient) environments. It is commonly found in sedimentary rocks (especially black shales, coal seams, and evaporites), metamorphic rocks (schists, gneisses), and igneous rocks (hydrothermal veins, disseminated in mafic and ultramafic intrusions). It precipitates from hydrothermal fluids, often associated with ore deposits of gold, copper, and other base metals. It can also form diagenetically in sediments through the reduction of sulfate by bacteria in the presence of iron. Pyrite can also form as a primary mineral in some igneous rocks, though this is less common.

Usage

Historically, pyrite was used as a source of sulfur for sulfuric acid production and as a source of iron, though it is not a primary iron ore today due to the difficulty of extracting iron from its sulfide form. It has been used as a spark-producing material in early firearms (flintlock mechanisms). Today, it is primarily collected as a mineral specimen due to its attractive metallic luster and crystal forms. In some cases, it is a minor ore of gold, where gold occurs as microscopic inclusions within the pyrite lattice (invisible gold). It is also used in some thermoelectric applications and as a semiconductor in certain niche technologies.

Age Distribution

Pyrite forms throughout Earth's geological history, from Precambrian to recent sediments. Its formation is not restricted to a specific geological age but rather to specific geochemical conditions.

Where to Find

Finding Tips

Similar Rocks

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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