FeS, or iron(II) sulfide, is a chemical compound made up of iron and sulfur. It typically appears as a black solid and is known for its use in various industrial applications, including the production of sulfuric acid and as a precursor for other iron compounds. In nature, FeS can be found in some mineral deposits and is often associated with the decomposition of organic matter. It can also form in anaerobic environments, such as marshes and sediments, where sulfate-reducing bacteria convert sulfate into sulfide.