PYRITE FORMATION IN 2 FRESH-WATER SYSTEMS IN THE NETHERLANDS

Cycling of Fe and S was investigated in sediments of Gerritsfles and K liplo, two acidic shallow moorland pools in the Netherlands, in order to reveal the importance of pyrite as a pool of inorganic S. The porew ater chemistry in both pool sediments is extremely dynamic as, e.g., F e2+ concentrations may increase twentyfold within a time span of a mon th. Periodic oxidized conditions occur in the top several centimetres of the sediment. The redox status appeared to be a main factor in dete rmining whether pyrite or a monosulfide, defined by the content of aci d volatile sulfur (AVS), is formed. In Gerritsfles and Kliplo, pyrite was the most important iron sulfide phase; sedimentary FeS2-S/AVS-S mo lar ratios were 32 and 55, respectively. In other lakes, where stratif ication caused anaerobic conditions in the hypolimnion, FeS2/AVS ratio s were < 1. Morphological investigation by light and scanning electron microscopy indicated that pyrite was present as single crystals and a s framboids. The framboids, mainly associated with organic matter, wer e thought to form in microsites where they may form slowly. The presen ce of a single crystal morphology point to a rapid formation of pyrite .