MORPHOLOGY AND BEHAVIOR OF GREIGITE FROM A HOLOCENE SEDIMENT IN EASTERN AUSTRALIA

Greigite (FeS1.34) was identified in the black magnetic fraction of a freeze-dried unoxidised estuarine sediment by X-ray diffraction and sc anning electron microscopy with quantitative energy dispersive X-ray a nalysis. The greigite crystals were tabular, unlike the cubic morpholo gy described in the literature, and ranged in size up to 1 mu m wide a nd 0.2 mu m thick. The stability of greigite was monitored by changes in peak intensities in X-ray diffractograms for a sample of greigite a fter exposure to a range of oxidising conditions. Moist greigite (with an approximate moisture content of 0.35 g/g) oxidised within hours un der ambient conditions at 25 degrees C and within minutes at 88 degree s C. However, greigite was relatively stable if kept air-dry. The resu lts indicate standard sample-handling and oven-drying procedures prese ntly used for sulfidic sediments to minimise oxidation actually enhanc e greigite oxidation and suggest greigite may have an important role i n the acidification of these sediments in the field.