Arsenopyrite

Arsenopyrite is the most common arsenic mineral, once known as arsenical pyrites. Its color is silver-white to steel-gray on freshly broken surfaces. It yields a garlic odor when heated and the fumes can be toxic. It may tarnish to form an iridescent layer. Although it has a reputation for being poisonous to humans, complex arsenic compounds are used in the treatment of disease caused by microorganisms.