Interatomic distances in pyrites

In the crystal structure of pyrite, which is modelled on the periclase structure (fig. 5.1), Fe2+ ions occupy Mg24- positions and the mid-points of (S-S)2- dimeric anions are located at the O2- positions. Each Fe2+ ion is in octahedral coordination with one sulphur atom belonging to six different (S-S)2- dimers, and the 

Fe2+ (245 pm). Similar bond-shortening due to 7t-bond formation also occurs in cattierite (CoS2) and vaesite (NiS2) in which observed (and radii sum) metal-sulphur distances are 232 pm (249 pm) and 240 pm (253 pm), respectively (cf. Appendix 3). Similar contracted bond-lengths in diarsenide and sulpharsenide minerals of Fe, Co and Ni may also be attributed to efficient 7t-bond formation. 

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