Marcasite
Image: Robert M. Lavinsky (CC BY-SA 3.0)
Mineral NameMarcasite
Chemical FormulaFeS₂
Mohs Hardness6
Crystal Systemorthorhombic
Lustermetallic
Streak Colorgreenish-black to brownish-black
Cleavagepoor/indistinct
Fractureuneven to conchoidal
Specific Gravity4.89
Colorspale bronze-yellow, brass-yellow, tarnished iridescent
Transparencyopaque
Type LocalityPanasqueira, Covilhã, Castelo Branco District, Portugal
Mineral GroupSulfides
Usesjewelry (historically), sulfur ore, collector
Associated Mineralspyrite, galena, sphalerite, calcite, quartz
Rarityuncommon
Description
This specimen displays a cluster of radiating, spear-shaped orthorhombic marcasite crystals, exhibiting a pale brassy-yellow metallic luster, characteristic of its coxcomb or cockscomb habit.
Geological Context
Marcasite forms in low-temperature, acidic environments, often found in sedimentary rocks, coal seams, and hydrothermal veins. It is a polymorph of pyrite, but less stable and prone to decomposition.
