Pyrite formation in two freshwater systems in the Netherlands

Cycling of Fe and S was investigated in sediments of Gerritsfles and Kliplo, two acidic shallow moorland pools in the Netherlands, in order to reveal the importance of pyrite as a pool of inorganic S. The porewater chemistry in both pool sediments is extremely dynamic as, e.g., Fe 2+ concentrations may increase twentyfold within a time span of a month. Periodic oxidized conditions occur in the top several centimetres of the sediment. The redox status appeared to be a main factor in determining whether pyrite or a monosulfide, defined by the content of acid volatile sulfur (AVS), is formed. In Gerritsfles and Kliplo, pyrite was the most important iron sulfide phase; sedimentary FeS 2-S AVS-S molar ratios were 32 and 55, respectively. In other lakes, where stratification caused anaerobic conditions in the hypolimnion, FeS 2 AVS ratios were < 1. Morphological investigation by light and scanning electron microscopy indicated that pyrite was present as single crystals and as framboids. The framboids, mainly associated with organic matter, were thought to form in microsites where they may form slowly. The presence of a single crystal morphology point to a rapid formation of pyrite.