The pyrite structure

                                    ferrous iron compound: structure of pyrite

 Schematic representation of the structure of pyrite, FeS2, as based on a cubic array of ferrous iron cations (Fe2+) and sulfur anions (S−).

FeS₂, C2, pyrite type structure.

Space group is Pa3. Pearson symbol is cP12.

Spherical modelling of the pyrite unit cell (space group Pa3). While Fe2+ ions (blue) form an fcc lattice, sulfur ions (yellow) reside on <111> axes, as described by positional parameter u. In FeS2 u amounts to 0.38484 in fractional coordinates of the unit cell [38]. The body diagonal is shown as an arrow. S ions form dumbbells, and there are four S2 dumbbells in the unit cell. Only the dumbbell at (uuu) and (ūūū) appears undivided, while the remaining S2 dimers all extend from the cell shown into neighboring cells. In order to visualize the different site symmetries, a mesh is inscribed into S ion spheres with poles lying on <111> symmetry axes. All ions are modelled by spheres with radii rMSP for Fe2+ and half the S–S bond distance dSS/2 for S−. With this setting, Fe and S spheres have contacts with none or only one neighbor, respectively. This result also holds for the other compounds listed in Table 1. Such a spherical packing would be unstable in terms of packing theory [2], demonstrating the failure of the ionic radius concept in the case of pyrite-type compounds.

Reference: http://www.mdpi.com/2073-4352/4/3/390/htm

Fe2 + ions form a face-centered cubic lattice; S- ions form octahedra around the Fe 2+ ions.

Reference: http://planet-terre.ens-lyon.fr/image-de-la-semaine/Img475-2014-11-10.xml