Pyrite

Description

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. It typically forms well-developed crystals, most commonly cubes, octahedra, and pyritohedra, often with striated faces. Its distinctive appearance and widespread occurrence make it a significant mineral for both geological study and mineral collecting.

How to Identify

Color

Pale brass-yellow to golden-yellow, often tarnishing to a darker, duller brassy color.

Luster

Splendent metallic.

Texture

Typically crystalline, often forming distinct euhedral crystals. Can also be massive, granular, or botryoidal.

Crystal Form

Cubic, octahedral, and pyritohedral forms are common. Faces are often striated. Also found as radiating aggregates or concretions.

Cleavage

Poor/indistinct on {001}. Typically exhibits conchoidal to uneven fracture rather than true cleavage.

Geological Environment

Found in igneous rocks (as an accessory mineral, in magmatic sulfide deposits), metamorphic rocks (schists, gneisses), and sedimentary rocks (shales, coal seams, limestones, sandstones) where it forms under reducing conditions. Common in hydrothermal veins and contact metamorphic deposits.

Key Facts

• Hardness: 6-6.5 (Mohs scale)
• Specific Gravity: 4.95-5.03 g/cm³
• Crystal System: Isometric (Cubic)
• Color: Pale brass-yellow to golden-yellow
• Luster: Metallic
• Transparency: Opaque
• Fracture: Conchoidal to uneven
• Cleavage: Poor/indistinct on {001}
• Composition: Iron disulfide (FeS2), containing 46.55% Fe and 53.45% S by weight.

Quick Check

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

Physical Characteristics

• Crystal Habit: Cubic, octahedral, pyritohedral, massive, granular, radiating, botryoidal, stalactitic. Cube faces often striated.
• Cleavage Type: Poor/indistinct, typically fractures rather than cleaves.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Metallic, splendent.

Formation

Pyrite forms under a wide range of conditions, both igneous, metamorphic, and sedimentary. It commonly crystallizes from hydrothermal solutions, in magmatic sulfide deposits, and as an accessory mineral in igneous rocks. In metamorphic rocks, it can form during regional or contact metamorphism. In sedimentary environments, it forms in anoxic conditions, often associated with organic matter, where sulfate-reducing bacteria play a crucial role in its precipitation.

Usage

Historically, pyrite was used as a source of sulfur for sulfuric acid production and as a source of iron. Today, its primary industrial use is still as an ore for sulfur, particularly in countries lacking native sulfur deposits. It is also collected as a popular mineral specimen due to its attractive metallic luster and crystal forms. In some niche applications, it has been explored for its semiconductor properties and in photovoltaic devices.

Age Distribution

Found in rocks of all ages, from Precambrian to recent sedimentary deposits.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Sulfide

Group

Pyrite Group

Crystal System

Isometric (Cubic)

Chemical Formula

FeS2

Composition

Iron disulfide

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