Pyrite

3320

June 23, 2025, 10:30 AM

Guide

Highlights at a glance

Pyrite, known as "Fool's Gold," is an iron disulfide (FeS₂) mineral with a brassy yellow color and metallic luster. It crystallizes in the cubic system and commonly forms cubes, octahedrons, or spherical framboidal aggregates. With a Mohs hardness of 6–6.5 and specific gravity of 4.95–5.10, pyrite has a greenish- to brownish-black streak and exhibits conchoidal fracture. Chemically, it contains 46.6% iron and 53.4% sulfur, oxidizing in moist air to produce iron hydroxides and sulfuric acid—contributing to acid mine drainage. It decomposes upon heating and dissolves in nitric acid but not in hydrochloric acid. Found in various geological settings, pyrite occurs as massive, disseminated, or framboidal forms and is associated with sulfide minerals like chalcopyrite and galena, as well as gangue minerals such as quartz and calcite. It often hosts trace elements including gold, silver, arsenic, and nickel, making it economically significant despite its deceptive appearance.

1.Chemical and Physical Properties

1.1 Chemical Identity

Mineral Name: Pyrite
Chemical Name: Iron Disulfide
Chemical Formula: FeS₂
Molecular Weight: 119.975 g/mol
Crystal System: Cubic (isometric)
Mineral Group: Sulfide minerals
Common Names: "Fool's Gold", Iron Pyrites, Mundic
Polymorphs: Marcasite (FeS₂ with orthorhombic structure)

1.2 Physical Properties

Appearance: Brassy to pale golden yellow with metallic luster
Crystal Habit:
- Cubic crystals (most common)
- Octahedral and pyritohedral forms
- Massive, granular, and nodular aggregates
- Framboidal structures (spherical aggregates)
Hardness: 6-6.5 on the Mohs scale
Specific Gravity: 4.95-5.10 g/cm³
Streak: Greenish-black to brownish-black
Cleavage: Poor to indistinct
Fracture: Conchoidal to uneven
Magnetic Properties: Non-magnetic to weakly magnetic

1.3 Chemical Properties

Composition:
- Iron (Fe): 46.6%
- Sulfur (S): 53.4%
Oxidation: Readily oxidizes in moist air
- 4FeS₂ + 15O₂ + 14H₂O → 4Fe(OH)₃ + 8H₂SO₄
Thermal Decomposition:
- FeS₂ → FeS + S (at ~500°C)
- 2FeS₂ + 5.5O₂ → Fe₂O₃ + 4SO₂ (roasting reaction)
Acid Reactivity:
- Soluble in nitric acid, insoluble in hydrochloric acid
Weathering:
- Forms iron oxides, sulfates, and sulfuric acid

1.4 Varieties and Associated Minerals

Crystalline Varieties:
- Coarse-grained massive pyrite
- Fine-grained disseminated pyrite
- Framboidal pyrite (sedimentary environments)
Associated Minerals:
- Chalcopyrite, galena, sphalerite (base metal sulfides)
- Quartz, calcite, dolomite (gangue minerals)
- Pyrrhotite, marcasite (other iron sulfides)
Trace Elements:
- May contain gold, silver, copper, nickel, cobalt, arsenic