Pyrite micromorphology in three Australian Holocene sediments

The distribution and morphology of pyrite in 3 Holocene estuarine sediments were examined using light microscopy, analytical scanning electron microscopy, and X-ray diffraction. The distribution and morphologies of pyrite were similar with pyrite being dispersed throughout the soil matrices as well as concentrated in root remnants and other macropores. The pyrite occurred in both framboidal and irregularly shaped dense clusters from 4 to 15 μm diameter, and also in loose clusters. Individual crystals in these clusters ranged in size from 1 to 2 μm. Pyrite crystals exhibited octahedral, pyritohedral, and equant anhedral (i.e. rounded globular) habit. Organic matter coatings and clay coatings were common around pyrite at each site. Thin coatings of monosulfide with an acicular morphology occurred around some framboidal clusters at one site. The distribution of framboids and individual crystals in the clayey matrix, root remnants, and other macropores indicates that the pyrite in these sediments has formed in situ. These data have important implications for the rate of sulfide oxidation and acid production from these soils.