[6] The narrower, modern scientific definition for marcasite as specifically orthorhombic iron sulfide dates from 1845.

Commonly associated minerals include pyrite, pyrrhotite, galena, sphalerite, fluorite, dolomite, and calcite.

[8] Ab initio calculations suggest that this is due to pyrite having a higher surface energy (thus being less thermodynamically stable) than marcasite at low pH.

However, sedimentary pore waters below the modern ocean are typically buffered at near-neutral to slightly alkaline pH by dissolved carbonate species.

[10] This raises the question of how sedimentary pore waters became sufficiently acidic to promote marcasite formation in the past.

Several theories have been proposed for the formation of early diagenetic marcasite, including: partial oxidation of primary pyrite by molecular oxygen infiltrating from the overlying water column,[11] and rapid anoxic organic matter decomposition and organic acid generation by fermentation and methanogenesis.

[12] Blueite (S.H.Emmons): Nickel variety of marcasite, found in Denison Drury and Townships, Sudbury District, Ontario, Canada.

Sperkise derives from the German Speerkies (Speer meaning spear and Kies gravel or stone).