EPITAXIAL OVERGROWTHS OF MARCASITE 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 t he Des Plaines unit of the Chicago Tunnel and Reservoir Project. The m arcasite occurs as thin tabular crystals, up to 300 mum on edge, domin ated by {010}. These grow around the [001] zone of pyrite octahedra su ch that {100} [001] pyrite \\ {101} [010] marcasite. Development of ov ergrowth varies from initial marcasite formation to complete coverage of the pyrite. The marcasite apparently grows around the pyrite octahe dra by a layer growth mechanism as evidenced by macrosteps terminating partially developed layers on the marcasite {010}. Other forms presen t on the marcasite are {110}, {011}, {101}, {001}, and {100}. Thermody namically, pyrite is the stable form of FeS2. However, it is well docu mented that FeS2 crystallization from solutions with pH less than 5 re sults in marcasite. These samples show that marcasite can directly pre cipitate in an epitaxial relationship onto a pyrite substrate. It is s uggested that the epitaxy observed in this study might be initiated th rough 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, Fe-S-S-Fe ... Periodic bond cha ins. This proposed reconstruction of the pyrite surface into a marcasi te-like arrangement, rather than simply continued growth of the thermo dynamically stable phase (pyrite), may be due to interactions of a pro tonated disulfide species with the pyrite during growth.