Pyrite diagenesis

In summary, it seems meaningful to consider both the formation of pyrite from the reaction of H2S with reactive ferric oxides and sulfate recycling as a result of this process in any discussion of the early diagenesis of sulfur and iron in sediments. 

Diagenesis of Microbially Reduced Sulfur. Postdepositional transformations play an important role in controlling the extent of recycling of microbially reduced S. Pore water profiles from many freshwater systems clearly show that H2S is a short-lived intermediate in sulfate reduction which does not accumulate in sediments (14.16 41-431. However, the conventional paradigm for sulfur diagenesis, in which H2S is initially immobilized by iron monosulfides that later are diagenetically altered to pyrite and elemental S (e.g., 2Q)> does not apply to all freshwater systems. Instead, organic S and CRS (chromium reducible S, which is believed to represent pyrite + S° after preliminary acid distillation to remove AVS), are important initial endproducts of dissimilatoiy reduction. 

In marine and lacustrine muds, the initial sulfide phase precipitated during early diagenesis is mackinawite (FeS09) which is subsequently converted to greigite (Fe3S4) and pyrite (FeS2) (85-89). This reaction path leads to the formation of framboidal pyrite (88.90). However, in salt marsh sediments under low pH and low sulfide ion activity conditions, direct precipitation of pyrite by reaction of ferrous iron with elemental sulfur without the formation of iron monosulfides as intermediates has been reported (85-87.89.91.92). This reaction is one possible pathway for the precipitation of pyrite as single crystals (89). 

Formation of Pyrite. Iron is carried to the peat swamp, before seawater transgression, as ferric oxide and hydroxides adsorbed on fluvial clays (123). During early diagenesis in a reducing environment, ferric iron is reduced to ferrous, which reacts with hydrogen sulfide to form iron monosulfide. If the basic mechanism of pyrite formation is similar to that in marine sediments... 

Pyrite Morphology. Pyrite formed during early diagenesis in marsh sediments nas essentially two modes of occurrence, as small single crystals and as framboids (18). The processes that lead to one form or the other remain unclear. Euhedra may form by direct... 

Giblin, A.E. (1988) Pyrite formation in marshes during early diagenesis. Geomicrobiol. J. 6, 77-97. 

Recent sediments of water basins. In recent basins iron sediments consist mainly of the iron hydroxides Fe(OH)3 or Fe203-nH20, but in very rare cases silicates and carbonates of Fe ", pyrite, and hydrotroilite enter into the composition of the sediment all together they constitute reactive (mobile) iron, which actively takes part in the diagenetic processes. A mixture of clastic minerals, which decompose negligibly and take practically no part in the processes of diagenesis, constitute another group. 

The relationships between iron carbonate and sulfide minerals formed in the course of diagenesis are determined mainly by the concentrations of active forms of sulfur in the waters. The existence of a very narrow wedge of siderite field between the fields of pyrite and goethite in the region of sulfate-ion stabihty indicates that siderite can form in association with pyrite and goethite in environments with Eh values close to zero. 

Sweeney R. E. (1972) Pyritization during diagenesis of marine sediments. PhD, University of California, LA. 

Figure 18. Wavelength-dispersive electron microprobe image of a cluster of pyrite framboids within coal from the Black Warrior basin Alabama. Bright colored areas show the presence of arsenic, nickel, and sulfur. Arsenic is concentrated in epigenetic overgrowths and cement surrounding the framboids, while the diagenetic framboid interiors them.selves contain little or no arsenic. This indicates that the arsenic was added after earliest diagenesis.

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