mineral
Marcasite on Agate / Sphalerite-Pyrite Agate Geode Slice
Marcasite (FeS2) / Pyrite on Quartz (SiO2)
Hardness: 6-6.5 (marcasite/pyrite), 7 (agate). Luster: Metallic (crust) to vitreous/waxy (agate). Color: Brass-yellow to silvery-grey metallic crystals with banded brown, cream, and white agate. Crystal structure: Orthorhombic (marcasite) or cubic (pyrite), Trigonal (quartz). Cleavage: Distinct in marcasite;…
- Hardness
- 6-6
- Luster
- Metallic (crust) to vitreous/waxy (agate)
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Physical properties
Hardness: 6-6.5 (marcasite/pyrite), 7 (agate). Luster: Metallic (crust) to vitreous/waxy (agate). Color: Brass-yellow to silvery-grey metallic crystals with banded brown, cream, and white agate. Crystal structure: Orthorhombic (marcasite) or cubic (pyrite), Trigonal (quartz). Cleavage: Distinct in marcasite; none in agate.
Formation & geological history
Formed in low-temperature hydrothermal veins and sedimentary environments where iron sulfide minerals precipitate onto preexisting agate or chalcedony cavity fillings.
Uses & applications
Popular as a decorative display specimen, slabbed geode slice for lapidary arts, and gemstone/jewelry collecting.
Geological facts
Marcasite is polymorphous with pyrite, meaning they share the same chemical formula (FeS2) but crystallize in different systems. Over time, marcasite can suffer from 'pyrite decay' if exposed to high humidity, oxidizing into iron sulfate and sulfuric acid.
Field identification & locations
Identified by the distinct metallic, fine-grained brassy-grey crystalline crust adhering to a clearly banded agate base. Frequently found in hydrothermal lead-zinc-iron deposits and geode-bearing volcanic or sedimentary rocks globally.
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