Arsenopyrite has, as the name suggests, the chemistry of pyrite (iron sulfide) plus some arsenic. Arsenopyrite is FeAsS - iron arsenic sulfide. It is the most common arsenic-bearing mineral on Earth. It has a metallic luster, silvery color, blackish streak, and a hardness of about 5.5 to 6. When heated, it gives off a garlic smell (the smell of arsenic).
Arsenopyrite (silvery) & dolomite (pale pink) (field of view 3.7 cm across) from Santa Eulalia Mining District, Chihuahua State, northern Mexico.
Arsenopyrite mass from Russia (public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA).