Marcasite inversion and the petrographic determination of pyrite ancestry

Pyrite formed by the inversion of marcasite or by oxidative leaching of pyrrhotite can be recognized in polished sections by the following characteristics, compiled from synthetic pyrite after marcasite and natural samples of pyrite after marcasite and after pyrrhotite. Pyrite formed by inversion from marcasite is characterized by approximately 2 modal percent pore space, no by-products filling the pores, optical anisotropy (dark green to dark red), anhedral shape, and polycrystalline daughter pyrite domains in two orientations relative to the marcasite parent crystal. Pyrite or marcasite formed by oxidative dissolution (as opposed to leaching) of pyrrhotite exhibits up to 32 modal percent pore space, gaps between the pyrite and parent pyrrhotite, pores and gaps filled with by-product siderite, magnetite, hisingerite, or other Fe-bearing phase, optically isotropic character (for pyrite), development of crystal faces, and random orientation relative to the parent pyrrhotite. Marcasite formed by oxidative leaching of Fe (as opposed to complete dissolution) from pyrrhotite also exhibits a large amount of pore space but has preferred orientation relative to the parent pyrrhotite.Pyrite after marcasite may be used as an indicator of pH < 5 and T < ca 240 degrees C during deposition of the parent marcasite. Marcasite precipitated following dissolution of pyrrhotite is also indicative of those conditions during alteration. Marcasite formed by oxidative leaching of pyrrhotite cannot be used as evidence of those conditions, however.