Marcasite
Marcasite is an iron sulfide mineral with the same composition as pyrite but a different crystal structure. It appears as pale brass-yellow crystals, radiating groups, and botryoidal masses, and is prone to crumbling as it oxidizes over time.
Marcasite properties at a glance
| Type | Sulfide Minerals |
|---|---|
| Chemical formula | FeS2 |
| Mohs hardness | 6 |
| Crystal system | orthorhombic |
| Luster | metallic |
| Streak | Dark greenish gray to brownish black |
| Cleavage | Distinct on {101} |
| Fracture | Uneven |
| Specific gravity | 4.9 |
| Transparency | opaque |
| Common colors | Pale brass yellow, Tin white, Bronze |
| Rarity | common |
Geology & background
Marcasite forms from iron and sulfur in low-temperature, near-surface conditions, often precipitating in acidic environments within sedimentary rocks, coal seams, and shallow hydrothermal veins.
Marcasite has long been gathered for ornamental use and was once a source of sulfur and copperas. Its name was historically applied loosely to several iron sulfides before mineralogists separated it from pyrite.
How to identify Marcasite
- Look for pale brass-yellow color, often lighter than pyrite, in cockscomb or spear-shaped twinned crystals.
- Marcasite is brittle and tends to tarnish, crumble, and develop white sulfate crusts as it ages.
- Distinguish it from pyrite by crystal habit, paler color, and its tendency to decay in storage.
Where Marcasite is found
Marcasite occurs in sedimentary rocks, coal beds, and low-temperature veins worldwide, with notable specimen localities in the United States, Germany, the Czech Republic, France, the United Kingdom, and Mexico.
How much is Marcasite worth?
| Entry-level specimen | $5 |
|---|---|
| Typical collector specimen | $35 |
| Premium specimen | $200+ |
Value depends on crystal form, twinning, luster, locality, stability, and freedom from decay or damage.
Common uses
- Collector specimens
- Historical source of sulfur
- Mineralogical study
- Ornamental and decorative interest
Interesting facts
- Marcasite and pyrite share the formula FeS2 but differ in crystal structure, making them polymorphs.
- Specimens can break down into a powder and release sulfuric acid as they react with humidity, a process collectors call pyrite decay.
- Cockscomb and spear-shaped twinned crystals are characteristic and prized growth habits.
- Antique 'marcasite jewelry' was usually set with faceted pyrite rather than true marcasite.
Care & safety
Marcasite is brittle and chemically unstable, often crumbling as it slowly oxidizes in humid air.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
Stores poorly and can decay. Marcasite can oxidize and release sulfuric acid; wash hands after handling and keep specimens dry.
