Framboidal pyrite formation in the bottom sediments of the South Caspian Basin under conditions of hydrogen sulfide contamination

Abstract

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 org 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.

Authigenic Minerals of the Derbent and South Caspian Basins (Caspian Sea): Features of Forms, Distribution and Genesis under Conditions of Hydrogen Sulfide Contamination

“…This research continues a series of studies devoted to the issue of authigenic mineral formation in the bottom sediments in areas of hydrogen sulfide contamination of the enclosed and marginal seas of Russia [8,9,[49][50][51][52]. Here, we aim to study the features of the processes of authigenic mineral formation in the bottom sediments of the deep-water basins of the Caspian Sea under conditions of hydrogen sulfide contamination.…”

“…Authigenesis occurs in the place of burial of sediments as a result of chemical, physicochemical, and biological (vital activity of organisms) processes [4][5][6]. Therefore, authigenic minerals are the main indicators of the environment, in which the formation of sedimentary deposits takes place, as well as of the processes of sediment transformation [7][8][9][10][11][12]. Authigenic minerals in the mineral composition of bottom sediments are an important genetic sign.…”

“…The role of clusters, including fine-grained ones, coming from the mountain structures of the Caucasus and the Elburs, is already very small near the coasts, which is largely determined by both the influence of the marginal filter and the large-scale current cycles existing in the Caspian Sea [42,43]. As a result, the mineral composition of surface bottom sediments has already been described [8,9,[44][45][46]. In the works of Kozina [8,9,[44][45][46], the mineral composition of heavy and light subfractions of modern bottom sediments of the Caspian Sea have been studied.…”

“…As a result, the mineral composition of surface bottom sediments has already been described [8,9,[44][45][46]. In the works of Kozina [8,9,[44][45][46], the mineral composition of heavy and light subfractions of modern bottom sediments of the Caspian Sea have been studied. Terrigenous-mineralogical provinces have been identified.…”

Morphology dominated rapid oxidation of framboidal pyrite

“…The oxidation of framboidal pyrite is considered to be fast due to its high specific surface area (SSA) (Pugh et al, 1984;Weber et al, 2004), and the influence of the crystal shape is often neglected. Several studies have shown that framboidal pyrite grains are mostly euhedral microcrystals with specific shapes (mainly with {100} and {111} facets) (Ohfuji and Rickard, 2005;Rigby et al, 2006;Kozina et al, 2018). Poorly understood issues regarding the rapid oxidation of framboidal pyrite are what controls the oxidation processes; the unique crystal shape or the extremely fine crystal size?…”