Gudmundite - Mineral almost identical to Arsenopyrite, but contains antimony in place of the arsenic. Formula = FeSbS (Whereas Arsenopyrite is FeAsS). Gudmundite is very similar in physical properties to Arsenopyrite, and is sometimes (wrongly) classified as a variety of it. It is found in Gudmundstorp, Sala, Sweden and in Ilimaussaq, Greenland.