Pyrite

Description

Pyrite is an iron sulfide mineral with a distinctive metallic luster and brass-yellow color, often leading to its nickname 'Fool's Gold'. It is one of the most common sulfide minerals and is found in a vast array of geological environments. Its crystal habit is typically cubic, pyritohedral, or octahedral, and it often forms well-developed crystals. Pyrite is relatively hard for a sulfide mineral and has a characteristic greenish-black streak. It is chemically unstable in oxidizing environments, leading to the formation of iron oxides and sulfuric acid, which can be a significant environmental concern in mining areas (acid mine drainage).

How to Identify

Color

Brass-yellow to pale brass-yellow, sometimes with a slight greenish tint. Can tarnish to a duller, darker brassy color.

Luster

Distinctive metallic luster, very bright and reflective.

Texture

Typically forms euhedral to subhedral crystals, granular aggregates, or massive forms. Crystal faces are often striated.

Crystal Form

Commonly cubic, pyritohedral (12-faced pentagonal dodecahedron), or octahedral. Also found as massive, granular, reniform, or stalactitic aggregates. Penetration twins are common.

Cleavage

Poor to indistinct on {001}. Typically fractures rather than cleaves.

Geological Environment

Found in igneous, metamorphic, and sedimentary rocks. Common in hydrothermal veins, disseminated in igneous intrusions, in black shales, coal seams, and as concretions in sedimentary rocks. Also found in massive sulfide deposits.

Key Facts

• Hardness: 6-6.5 (Mohs scale)
• Specific Gravity: 4.95-5.10 g/cm³
• Crystal System: Isometric (Cubic)
• Color: Pale brass-yellow to golden-yellow
• Luster: Metallic
• Transparency: Opaque
• Fracture: Conchoidal to uneven
• Cleavage: Poor to indistinct on {001}
• Composition: Iron sulfide (FeS₂)

Quick Check

• Color: Brass-yellow
• Luster: Metallic
• Streak: Greenish-black

Physical Characteristics

• Crystal Habit: Cubic, pyritohedral, octahedral, massive, granular, reniform, stalactitic. Striations often present on crystal faces.
• Cleavage Type: Poor/Indistinct
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Metallic

Formation

Pyrite forms under both high and low-temperature conditions. It commonly crystallizes from hydrothermal solutions, in sedimentary rocks under reducing conditions (e.g., black shales, coal seams), in metamorphic rocks, and as an accessory mineral in igneous rocks. It can also form biogenically through the activity of sulfate-reducing bacteria.

Usage

Historically, pyrite was used as a source of sulfur for sulfuric acid production and as a source of iron. It was also used in early radio receivers (cat's-whisker detectors) and as a spark-producing material in flintlock mechanisms. Today, its primary industrial use is still as a source of sulfur dioxide for sulfuric acid, though other sources are more common. It is also collected as a mineral specimen and used in some jewelry, though its tendency to tarnish and react with moisture limits its widespread use.

Age Distribution

Pyrite forms throughout Earth's geological history, from Precambrian to recent, in a wide variety of geological settings.

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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