The nature of the bonding in pyrite is considered in relation to the origin of the quadrupole splitting observed by means of the M6ssbauer effect and the angular variation of the recoil-free fraction. It is found that neither crystal-field effects nor non-stoichiometry are enough to account for the observed splitting. Consideration of molecular orbital effects, however, shows that a very small amount of electron delocalization is enough to cause the observed splitting because of the strong effects the valence electrons have on the iron nucleus. It is also found that, contrary to previously published data, the recoil-free fraction is practically independent of angle. This result is derived from both M6ssbauer resonance measurements and an X-ray structure determination. The structure determination confirms earlier reports of interatomic distances and angles and also determines the anisotropic thermal parameters for both Fe and S atoms in naturally occurring pyrite. The temperature variation of the isomer shift is measured and interpreted to mean that vibrational modes corresponding to Raman modes are not excited in the pyrite lattice between 10 and 300 K. This complements published IR spectra which show no such modes between 190 and 660 cm -1 (equivalent to 274 and 950 K). The isomer shift of pyrite is found to be 0"074 + 0"001 mm/s relative to Fe in Cu (source and absorber at room temperature) and the quadrupole splitting is 0"634+ 0"006 mm/s at room temperature. The ratio of intensities of the ~}--~ ½ and ½ ~-} transitions of the M6ssbauer spectra was typically 1.004 + 0.011.