Hypersulfidic material
This soil material is rich in iron sulfides (commonly pyrite) and exists below a permanent water-table under anaerobic conditions. In its saturated state it is uniformly dark grey to black in colour. If exposed to atmospheric oxygen following drainage or exposure, the iron sulfides become unstable and generate sulfuric acid to form Sulfuric material. It is also known as potential acid sulfate soil.
Hypersulfidic material
Photo credit: Queensland Department of Resources
Black hypersulfidic material below the grey topsoil in this profile
Photo credit: Bernie Powell