They can not be mistaken for each other however since pyrite is brassy yellow and sperrylite is tin white in color. Pyrite's and sperrylite's structure is analogous to galena's structure with a formula of PbS. Galena though has a higher symmetry. The difference between the two structures is that the single sulfur of galena is replaced by a pair of sulfurs in sperrylite. The sulfur pair are covalently bonded together in essentially an elemental bond. This pair disrupts the four fold symmetry that a single atom of sulfur would have preserved and thus gives sperrylite a lower symmetry than Galena.
Color is tin white.
Luster is metallic. Crystals are opaque. Crystals with combinations of these forms are more common. Also massive and reniform. Fracture is conchoidal.