Pyrite formation in Louisiana coastal marshes: Scanning electron microscopy and X-ray diffraction evidence

The highly reducing chemical environments within coastal marsh soils provide favorable conditions for pyrite (FeS) formation. However, when exposed and oxidized, pyrite can bring about adverse effects by creating extremely acidic conditions within the marsh ecosystem. Therefore, evidence of pyrite formation within these marshes would caution against dredging activities in these areas and should be considered when making land management decisions. This study investigated saline and brackish marsh types within the Barataria Bay basin located in Lafourche Parish, Louisiana. Previous studies, based on such indirect biogeochemical approaches as Fe-based methods and sulfur speciation, have determined the presence of pyrite in these marsh soils. in this study, we used scanning electron microscopy and X-ray diffraction (XRD) to find pyrite crystals in marsh soils as evidence of pyrite formation in the coastal marshes. Pyrite framboids (6 to 8 mum diameter) were found within saline marsh soils. Pyrite crystals were found in close association with organic material. The XRD analysis confirmed the presence of pyrite in both saline and brackish marsh samples. The differential X-ray diffraction (DXRD) patterns revealed that pyrite crystals were more abundant within the coarser soil fraction (2-20 mum) compared with the finer soil fraction (<2 mum). The DXRD also showed a greater abundance of pyrite in saline marsh than in brackish marsh. The micrographs of pyrite crystals detected in this study provide morphological evidence for pyrite occurring in the marsh, further documenting the pyrite and clay mineralogy in these marsh soils.