Violarites of the general composition (Fe, Ni) 3 S 4 from a variety of natural Fe-Ni sulfide assemblages have been studied by electron microprobe analysis. The violarite solid solution extends from the Ni-rich end member polydymite to at least 17 at. percent Ni, compositions more Fe-rich than FeNi 2 S 4 being characteristic of pyrrhotite-bearing assemblages and compositions more Ni-rich than FeNi 2 S 4 of Ni-rich assemblages that do not contain pyrrhotite. Most of the violarites have compositions in the range of 56 to 57 at. percent S and thus are slightly S-deficient relative to the ideal M 3 S 4 composition.The predominant mode of formation of violarite is by alteration of pentlandite. Violarite also forms from millerite, but alteration of pyrrhotite to violarite has not been confirmed in the present study. A single crystal X-ray diffraction study of a partially violaritized pentlandite grain has shown that violarite has the same crystallographic orientation as the host pentlandite and the alteration can be effected by removal of excess metal atoms from pentlandite and redistribution of the remainder. It is suggested that violarite forms readily from pentlandite because of the ease with which the pentlandite structure can be converted to that of violarite.The stability field of violarite in the Fe-Ni-S system must enlarge to more Fe-rich compositions, toward the Fe-S join, as the temperature decreases. Likely phase relations at temperatures sufficiently low to include smythite and the extension of the violarite field to 17 at. percent Ni have been schematically presented.