How Pyrite are produced in Aqua Ponds?
he stagnation of water facilitates the formation of sulfide-containing sediments. It is known that the greater part of these oxidizable sulfide compounds are present in the form of pyrite, (FeS2). This is because pyrite is much more stable than ferrous sulfide, (FeS). When sedimentary pyrite is being formed, there must first be not only sulfate reduction, but also the formation of elemental sulfur. Van Breemen and Pons (1978) have suggested that the partial oxidation of sulfide to elemental sulfur may occur. 

This process is mainly anaerobic, alternated with a limited aerobic process. According to Stumm and Morgan (1970) however, under acid conditions, elemental sulfur is formed as a result of the reduction of SO4(2)- as an intermediate product. Once formed, it may persist as a stable solid phase. Thus, solid elemental sulfur is quite common in recent aqua sediments.