Pyrite in sediments

The formation of pyrite in sediments depends on the availability of three parameters iron, sulfate, and organic matter (48). Although organic matter content controls the formation rate under marine sulfate-rich conditions, sulfate concentration is usually regarded as the limiting factor under fresh-... 

Eurther work may be required to define the full spectmm of reactions forming pyrite in sediments. The pathways involving oxidation such as in Equation (13) seem appropriate in the upper portions of marine sediments, where most pyrite... 

Strauss H, Beukes NJ (1996) Carbon and sulfur isotopic compositions of organic carbon and pyrite in sediments from the Transvaal Supergroup, South Africa. Precamb Res 79 57-71 Strauss H, Des Marais DJ, Sutmnons RE, Hayes JM (1992a) The eatbon isotopic record. In Schopf JW, Klein C (eds) The Porterozoic Biosphere A Multidisciplinary Study. Cambridge University Press, New York, p 117-127... 

AVS is not the sole partitioning phase for predicting the acute toxicity, especially for Cu, which may be strongly associated with sediment organic matter (e.g. Ankley et al., 1993). Although Huerta-Diaz et al. (1993) have presented experiments to measure the quantity of trace metals associated with AVS and pyrite in sediments, selective extraction techniques for prediction of solid phases controlling pore-water concentrations of trace metals in (partly) anaerobic sediments solid phases are still absent (Wallmann et al., 1993). 

Going back before 2.8 billion years ago and back to the end of a good sedimentary rock record 3.5 billion years ago, there were only trace amounts of O2 in the water and there may have been significant amounts of methane in the air. (Trace O2 does not persist in air with methane because of photolysis [34].) Macroscopic evidence includes mobile iron in paleosols and survival of easily oxidizable detrital minerals like pyrite in sediments. 

Arsenic(As) in ocean is mainly removed by formation of pyrite in marine sediments. The production rate of sulfur in pyrite is 3.3 X 10 mol my (2.5 X 10 ° g my ) (Holland 1978). As/S ratio of pyrite in sediments previously reported is (8.7 3) x 10" (Huerta-Diaz and Morse 1992). Thus, As sink by pyrite is (1.7-3.9) x 10 mol my . This flux seems to be not different from As input to ocean ((1.6-8.1) x lO mol my (Table 5.3). As concentration of ocean is considered to be controlled by hydrothermal input, riverine input and pyrite output. Fluxes by volcanic gas from atmosphere and by weathering of ocean-floor basalt are small, compared with hydrothermal, riverine and pyrite As fluxes. Residence time of As in seawater is estimated as the amount of As in seawater (4.2 x 10 g) divided by As input to seawater (1.6-8.1) X 10 mol my which is equal to (1.7-3.8) x 10" year. This is shorter than previously estimated one (10 year by Holland 1978). Subducting sulfur flux is estimated to be 6.1 x 10 g my from S contents of altered basalt and sediments ( 0.1 wt%) (Kawahata and Shikazono 1988) and subducting rates of... 

Six sulphide species were observed in the non-ferromagnetic heavy mineral concentrates (NFM-HMCs) of bedrock samples arsenopyrite pyrite > chalcopyrite > bismuthinite = molybdenite = cobaltite. Chalcopyrite, pyrite and bismuthinite do survive in near-surface till but only in minor amounts (<8 grains/sample). Although the Co-rich composition of arsenopyrite is possibly the strongest vector to Au-rich polymetallic mineralization in the study area, sandsized arsenopyrite is absent in C-horizon tills, suggesting that arsenopyrite more readily oxidizes than chalcopyrite and pyrite in till, and therefore is an impractical indicator mineral to detect mineralization using surficial sediments at NICO. 

Iron sulphides are ubiquitous in marine and freshwater sediments. They are usually present either as pyrite or as monosulphides, which can be liberated by hydrochloric acid. These acid volatile sulphides give rise to an intense black colour that is characteristic of anoxic sediments. They play an important role in recent diagenetic processes in sediments and the ratio of pyrite to acid volatile sulphides has been used as an historical indicator to determine whether sediments were formed in marine or freshwater conditions. They can be present over a wide range of... 

Morse and Cornwell [112] investigated methods for determining acid volatile sulphides and pyrites in marine sediments from several typical... 

Acid sulfate soils are an especially difficult class of acid soil formed in former marine sediments that have been drained. The acidity is generated from the oxidation of pyrite in the soil resulting in acute aluminium toxicity, iron toxicity, and deficiencies of most nutrients, especially phosphate which becomes immobilized in ferric oxide. The development and management of acid sulfate soils are discussed in detail in Dost and van Breemen (1983) and Dent (1986). 

In brief, the steps in the formation of pyrite in marine sediments are ... 

Reductive dissolution of Fe oxyhydroxides holding sorbed As appears to explain the very large concentrations of As in water from wells drilled into alluvial sediments of the Brahmaputra and Ganges Rivers in Bangladesh and West Begal (Nickson et al 1998, 2000). Dissolved As has accumulated from the reduction of As-rich Fe oxyhydroxides formed upstream of the contaminated areas by weathering of As-rich base metal sulfides. The reduction is driven by sedimentary organic matter in the deposits. Release of As from oxidation of pyrite in shallow wells contributes little to the water contamination because any As(IV) released would be re-sorbed on Fe oxides formed in pyrite oxidation. 

Postma, D. (1983) Pyrite and siderite oxidation in swamp sediments. J. Soil Sci. 34 163-182 Postma, D. (1993) The reactivity of iron oxides in sediments A kinetic approach. Geochim. Cosmochim. Acta 57 5027-5034 Pourbaix, M. (1963) Atlas of equilibrium diagrams. Gauthier-Villars, republished by Per-gamon, London in 1966. 

Oxidation of Reduced S. Indirect evidence suggests that microbial oxidation of sulfide is important in sediments. If it is assumed that loss of organic S from sediments occurs via formation of H2S and subsequent oxidation of sulfide to sulfate (with the exception of pyrite, no intermediate oxidation states accumulate in sediments cf. 120, 121), the stated estimates of organic S mineralization suggest that sulfide production and oxidation rates of 3.6-124 mmol/m2 per year occur in lake sediments. Similar estimates were made by Cook and Schindler (1.5 mmol/m2 per year 122) and Nriagu (11 mmol/m2 per year 25). A comparison of sulfate reduction rates (Table I) and rates of reduced S accumulation in sediments (Table III) indicates that most sulfide produced by sulfate reduction also must be reoxidized but at rates of 716-8700 mmol/m2 per year. Comparison of abiotic and microbial oxidation rates suggests that such high rates of sulfide oxidation are possible only via microbial mediation. 

Pyrite is formed by two mechanisms in freshwater sediments. Fram-boidal pyrite results from reaction of iron monosulfides with S° (15), a slow reaction leading to gradual conversion of iron monosulfides to pyrite. In contrast, single crystals of pyrite are formed rapidly through reaction of Fe2+ and poly sulfides (161). Framboidal pyrite has been reported in lake sediments (37, 189), where it appears to form in microenvironments of plant or animal skeletons (cf. 35, 36). Rapid formation of pyrite has been observed in short-term measurements of sulfate reduction with SO/-. Up to 90% of reduced has been observed in pyrite after incubations of 1-24 h (72, 79, 98). A large fraction of inorganic S in the form of pyrite in surface sediments also has been interpreted to indicate rapid formation (112, 190). As discussed later, there is little evidence for extensive conversion of monosulfides to pyrite. 

Figure 6. The degree of pyritization, defined as the fraction of reactive iron present as pyrite, is a measure of the extent to which available iron has reacted with sulfur (226). In lake sediments, iron monosulfides frequently are as abundant as pyrite and hence were included with pyrite in the values calculated for surface sediments from 13 lakes and presented here. Even this correction neglects Fe(II) that may have been reduced by sulfide but may be present as siderite. Availability of iron appears to be more important than bottom-water oxygenation in determining the degree of pyritization. In the right-hand graph, darkened squares represent sediments known to experience seasonal anoxia only the uppermost point experiences permanent anoxia. (Data are from references 30, 34, 56, and 61.)...

The strong coupling between sulfur and iron chemistry becomes obvious in this example. Conservation of alkalinity within the system is achieved only if the sulfide formed is prevented from reoxidation, a process that would restore the acidity. Prevention of reoxidation occurs through the ultimate storage of sulfide in sediments, either as organic sulfur or as iron sulfides (12, 13). The overall reaction of pyrite formation proceeds via formation of FeS ... 

Pore-water profiles are frequently interpreted according to this concept. For example, White et ah (35) described a conceptual model of biogeo-chemical processes of sediments in an acidic lake (cf. Figure 4). They discussed the numbered points in Figure 4 as follows Diffusion of dissolved oxygen across the sediment-water interface leads to oxidation of ferrous iron and to an enrichment of ferric oxide (point 1). Bacterial reductive dissolution of the ferric oxides in the deeper zones releases ferrous iron (point 2). The decrease in sulfate concentration stems from sulfate reduction, which produces H2S to react with ferrous iron to form mostly pyrite in the zone below the ferric oxide accumulation (point 3). 

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