Iron (II) sulfide or ferrous sulfide is a compound with the formula FeS. In practice, iron sulfides are often non-stoichiometric. Powdered iron sulfide is pyrophoric (ignites spontaneously in air).
As organic matter decays under low-oxygen (or hypoxic) conditions such as in swamps or dead zones of lakes and oceans, sulfate-reducing bacteria will use the sulfates present in the water to oxidize the organic matter, producing hydrogen as waste. Some of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides, which are not water soluble. These metal sulfides, such as iron (II) sulfide, are often black or brown, leading to the color of sludge.

Iron (II) sulfide or ferrous sulfide is a compound with the formula FeS. In practice, iron sulfides are often non-stoichiometric. Powdered iron sulfide is pyrophoric (ignites spontaneously in air).
As organic matter decays under low-oxygen (or hypoxic) conditions such as in swamps or dead zones of lakes and oceans, sulfate-reducing bacteria will use the sulfates present in the water to oxidize the organic matter, producing hydrogen as waste. Some of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides, which are not water soluble. These metal sulfides, such as iron (II) sulfide, are often black or brown, leading to the color of sludge.