Sulphides (sulfides) are stable under waterlogged (anaerobic) conditions but when oxygen is introduced into the system, these sulphides are oxidised to sulphuric acid.

Chemical oxidation of pyrite is slow but the reaction is mediated by iron-oxidising bacteria, in particular Thiobacillus ferrooxidans, so that the optimum conditions for sulphide oxidation are the optimum conditions for Thiobacillus ferrooxidans, namely oxygen concentration >0.01 Mole fraction (1%); temperature 5-55°C, optimally 30°C; and pH 1.5-5.0, optimally 3.2 (Jaynes et al. 1984).

Progressive oxidation following drainage of compact sulphidic material. Oxidation appears to be proceeding concentrically from the ped faces. The ped cores remain reduced and dark in colour. The pale band is presumed to be a diffusion zone through which iron and acid move outwards and oxidant move inwards.  Pleistocene marine terrace near Lelydorp, Surinam (Photo: Robert Brinkman)