Arsenopyrite

Arsenopyrite is an iron arsenic sulfide mineral. It is the most abundant arsenic mineral and the primary ore of arsenic metal. In addition to being found in deposits that are large enough to be minable, arsenopyrite is widely distributed. However, it usually occurs in such small amounts and in such small particle sizes that it is easily overlooked. It is associated with other sulfide minerals in organic-rich sedimentary rocks, metamorphic rocks, and igneous rocks in many parts of the world. https://www.mindat.org/min-305.html