Marcasite and Pyrite (FeS₂)

Abstract

We first describe the structures of FeS₂-marcasite and FeS₂-pyrite, and the way in which they are related. It is then explained how this relationship can successfully account for several phenomena: pyrite- marcasite intergrowths, the reported anisotropy of (nominally cubic, and therefore isotropic) FeS₂-pyrite, stacking and deformation faults in pyrite parallel to {100}, and the observed structure of the pararammelsbergite polymorph of NiAs₂.