Structural features and distribution of framboidal pyrite were studied during lithological, mineralogical and geochemical studies of sediments in the South Caspian Basin. The lithological and geochemical parameters affecting the intensity of the formation of authigenic pyrite in sediments of the South Caspian Basin under conditions of hydrogen sulfide contamination were determined. According to X-ray diffraction XRD, the content of pyrite in pelitic ooze reaches 4%. Microscopic studies have shown that pyrite formation occurs intensively in sediments with a high content of biogenic mineral components, such as opal diatom frustules and calcite plates of coccolithophores. Scanning electron microscopy SEM has demonstrated the presence of spherical pyrite framboids, as well as individual pyrite crystallites and their disordered clusters of various shapes. The diameter of the framboids varies from 3 to 10 microns. Pyrite crystallites have subspherical, complicated polyhedral, prismatic, octahedral, pentagonal dodekahedron shapes. The crystallite sizes range from 0.5 to 3 microns. Based on geochemical data, there is no direct correlation between the intensity of pyrite formation and the high content of such components as Fe and C$_\mathrmorg$ in bottom sediments. We assume that the formation of authigenic framboid pyrite in surface sediments including the fluffy layer is most likely due to the presence of hydrogen sulfide contamination in the near bottom water of the South Caspian Basin. Framboidal pyrite in the deeper layers up to a depth of 35~cm is the result of diagenetic processes that occur in accordance with the classical scheme of vertical zonality of diagenetic transformations in sediments.
Framboidal pyrite, the Caspian Sea, the South Caspian Basin, authigenic minerals, hydrogen sulfide contamination, sulfate-reducing bacteria
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