Epitaxial overgrowths of marcasite on pyrite from the Tunnel and Reservoir Project, Chicago, Illinois, USA: Implications for marcasite growth

An unusual occurrence of marcasite epitaxially overgrown on pyrite was found along fractures in the Racine Dolomite, in the main tunnel of the Des Plaines unit of the Chicago Tunnel and Reservoir Project. The marcasite occurs as thin tabular crystals, up to 300 μm on edge, dominated by {010}. These grow around the [001] zone of pyrite octahedra such that {100} [001] pyrite | {101} [010] marcasite. Development of overgrowth varies from initial marcasite formation to complete coverage of the pyrite. The marcasite apparently grows around the pyrite octahedra by a layer growth mechanism as evidenced by macrosteps terminating partially developed layers on the marcasite {010}. Other forms present on the marcasite are {110}, {011}, {101}, {001}, and {100}. Thermodynmically, pyrite is the stable form of FeS₂. However, it is well documented that FeS₂ crystallization from solutions with pH less than 5 results in marcasite. These samples show that marcasite can directly precipitate in an epitaxial relationship onto a pyrite substrate. It is suggested that the epitaxy observed in this study might be initiated through a reconstruction of the surface most layers of the {100} face of pyrite to a configuration closely resembling the marcasite structure, while maintaining the common feature, FeSSFe ... periodic bond chains. This proposed reconstruction of the pyrite surface into a marcasite-like arrangement, rather than simply continued growth of the thermodynamically stable phase (pyrite), may he due to interactions of a protonated disulfide species with the pyrite during growth.