MarcasiteEdit
Marcasite is an iron sulfide mineral with the chemical composition FeS2. It is the orthorhombic polymorph of iron sulfide, a close cousin of the more common pyrite, which has the isometric crystal system. Marcasite occurs in a variety of sedimentary and hydrothermal environments, often forming small, sparkly crystals in limestones, shales, and coal beds. Because marcasite is less stable at surface, specimens and jewelry made from it require careful handling.
Crystallography and physical properties
- Crystal system: orthorhombic; mineral class of sulfide minerals. Marcasite is the orthorhombic polymorph of FeS2, distinct from the isometric form of pyrite pyrite.
- Chemistry: iron sulfide with the same formula as pyrite but a different crystal structure. See also FeS2.
- Color and luster: pale-brass to yellowish in fresh surfaces; metallic to mantling luster.
- Hardness: around Mohs 6–6.5, making it relatively hard but brittle.
- Streak: typically a pale brown to grayish color.
- Density: roughly 4.95–5.2 g/cm3.
- Stability: marcasite tends to weather or alter more readily than pyrite when exposed to air and moisture, often turning to oxidation products such as goethite or limonite over time. See weathering and oxidation processes in sulfide minerals.
Occurrence and geology
Marcasite forms in low-temperature, reducing environments and is commonly found in sedimentary rocks such as limestones and shales, where it can fill small cavities as well-formed crystals, and in various hydrothermal veins. It is frequently associated with other sulfide minerals and carbonates, and it can occur as framboids, nodules, or euhedral crystals within host rocks. Due to its relative instability at surface conditions, marcasite is less common in freshly collected samples than pyrite, and it is more prone to alteration during weathering and handling.
Geographically, marcasite has been reported in many parts of the world, including Europe, the Americas, and Asia, with historically notable finds in areas with carbonate-rich beds. For mineral collectors and researchers, distinguishing marcasite from its close cousin pyrite is a routine task, often relying on crystal habit, luster, and stability under weathering.
Formation, alteration, and stability
Marcasite forms when sedimentary rocks experience diagenetic or low-temperature hydrothermal conditions that favor orthorhombic FeS2 over the cubic form. Its structure is more prone to instability in air and humidity than that of pyrite, leading to alteration to secondary iron oxides or sulfates in surface-exposed specimens. This instability is a practical consideration for collectors and for uses that involve surface exposure, such as jewelry.
Uses and cultural context
Marcasite has long been used in decorative contexts, most famously in marcasite jewelry. In these ornaments, small, faceted crystals of marcasite are set in metal settings (often sterling silver or gold) to evoke the look of more precious stones at a lower cost. The practice became popular in various periods, notably in late 19th to early 20th-century fashion. Because marcasite is relatively brittle and susceptible to tarnish, care is required to preserve these pieces, and some modern jewelry uses pyrite or other substitutes to imitate marcasite in contemporary designs. See jewellery and marcasite jewelry for related discussions.
In mineral collecting and geology, marcasite is valued for its crystal habits and its role as a natural phase illustrating the polymorphism of FeS2. Its appearances in fossil-rich sedimentary sequences can also provide information about the diagenetic history of host rocks. See also mineralogy and geology for broader context.