In order to investigate the physical and chemical effects of neutron capture reaction, a neutron in-beam Mössbauer spectroscopic
study on two isomorphs of iron disulfide: pyrite and marcasite, were carried out with a parallel plate avalanche counter at
room temperature. In both compounds only two major products accounted for the obtained spectrum: one with Mössbauer parameters
close to the parent compound and the other one considered to be a new product. The yield of the parent-like species was different
in the two isomorphs.