Pyrite

Description

Pyrite is an iron sulfide mineral with the chemical formula FeS2. It is known for its distinctive metallic luster and brass-yellow color, which often leads to its misidentification as gold, hence the nickname 'Fool's Gold'. It crystallizes in the isometric system, commonly forming cubes, octahedra, and pyritohedra, often with striated faces. Pyrite is a common and widespread mineral found in various geological settings, including igneous, metamorphic, and sedimentary rocks. It is relatively hard and brittle. A significant environmental concern associated with pyrite is its oxidation when exposed to air and water, which can lead to the formation of sulfuric acid and acid mine drainage.

How to Identify

Color

Pale brass-yellow to golden-yellow, often with a tarnished or iridescent surface.

Luster

Distinctive metallic luster, very bright and reflective.

Texture

Typically forms well-defined crystals; granular or massive aggregates are also common.

Crystal Form

Cubes, octahedra, and pyritohedra are characteristic. Cube faces often show striations perpendicular to adjacent faces. Can also be massive, reniform, globular, or stalactitic.

Cleavage

Poor to indistinct cleavage on {100}. Typically fractures conchoidally or unevenly.

Geological Environment

Found in igneous rocks (as an accessory mineral), metamorphic rocks (e.g., schists, gneisses), and sedimentary rocks (especially black shales, coal seams, and limestones). Common in hydrothermal veins, contact metamorphic deposits, and as concretions.

Key Facts

• Hardness: 6-6.5 (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 {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, reniform, globular, 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 geological conditions, primarily in reducing environments. It commonly precipitates from hydrothermal fluids in veins and replacement deposits, in sedimentary rocks (especially shales and coal seams) where organic matter provides reducing conditions and sulfur is available, and in metamorphic rocks. It can also form in igneous rocks as an accessory mineral. The formation process involves the reaction of iron-bearing minerals or dissolved iron with sulfide ions (S2-), often mediated by sulfate-reducing bacteria in low-temperature sedimentary environments.

Usage

Historically, pyrite was a primary ore for sulfur, used in the production of sulfuric acid. It was also used as a source of iron, though less efficiently than iron oxides. In modern times, its industrial uses are limited due to environmental concerns (acid mine drainage) and the availability of more efficient sources for sulfur and iron. It is occasionally used as a semiconductor in some specialized applications and is a popular collector's mineral due to its attractive metallic luster and crystal forms. It has also been used in early radio receivers (cat's-whisker detectors).

Age Distribution

Pyrite forms across a vast range of geological time, from Precambrian to recent, wherever suitable conditions for its precipitation exist.

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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