Pyrite formation

Iron sulfides represent the most important minerals that form in association with both organoclastic and methanotrophic snlfate reduction, or - more precisely - as a resnlt of the hydrogen snlfide prodnced by these processes. The different pathways of pyrite formation via intermediate iron snlfides will be described in more detail in Section 8.4.2. The first step in all pyrite forming seqnences involves a reaction of hydrogen snlfide with either dissolved Fe or solid-state iron (oxyhydr)oxides. The reactivity of oxidized iron minerals towards snlfide varies 

Iron-sulfide minerals are important sinks for iron and sulfur as well as for trace metals and play an important role in the global cycles of these elements. Over the past 30 years extensive studies - 

All pyrite forming pathways identified so far involve several reaction steps. First, hydrogen sulfide, produced during sulfate reduction (Eq. 

2 and 8.8), reacts with dissolved iron or reactive (towards sulfide) iron minerals to form amorphous iron sulfides, such as mackinawite (FeS). The precipitated amorphous iron sulfide is highly unstable and rapidly transforms to metastable iron sulfide phases such as pyrrhotite (Fe jS) or greigite (Fc3S ), both of which represent intermediates in the reaction pathways to pyrite (FeS ). The conversion of amorphous FeS to pyrite requires an electron acceptor and a change in the molar Fe S ratio from about 1 1 to 1 2. The electron acceptor is needed to oxidize the S(-II) component in FeS to the mean oxidation state of -I in FeS. Concurrently, the Fe S ratio has to decrease either via the addition of sulfur or the loss of iron. Three 

As reviewed in detail by Schoonen (2004) these different conversion mechanisms - and in particular the H,S pathway - have received controversial discussion. However, field studies have shown that hydrogen sulfide can indeed sulfidize amorphous FeS and form pyrite. Rickard (1997) found that the H,S process is by far the most rapid of the pyrite-forming reactions hitherto identified and suggested that it represents the dominant pyrite forming pathway in strictly anoxic systems. In addition, Morse (2002) discussed that the oxidation of FeS by hydrogen sulfide is the faster process compared with the oxidation by elemental sulfur. Berner (1970) suggested that, in the presence of zero-valent sulfur, a complete transformation of FeS to pyrite should be possible on a time scale of years. An incomplete conversion of FeS to pyrite, as often observed, e g. in 

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Berner, R. A. (1984). Sedimentary pyrite formation An update. Geochim. Cosmoehim. Acta 48,605-615. 

Various workers have estimated the rate of pyrite formation. Berner (1972) summed the... 

Berner, R. A. (1972). Sulfate reduction, pyrite formation and the oceanic sulfur budget. In "The changing chemistry of the oceans" (D. Dyrssen and D. Jagner, eds). Wiley-Interscience, Stockholm. 

Arsenic removal from seawater to sediments is mainly governed by pyrite formation in the seafloor sediments. Production rate of sedimentary pyrite is 2.5 x 10 g S/year (Holland, 1978). Therefore, As removal by pyrite from seawater is (1.3-2.9) x lO g/year. This is the same order of magnitude as As input to ocean by river which is equal to 0.7 x 10 g/year. 

Wachtcrshauser s prime candidate for a carbon-fixing process driven by pyrite formation is the reductive citrate cycle (RCC) mentioned above. Expressed simply, the RCC is the reversal of the normal Krebs cycle (tricarboxylic acid cycle TCA cycle), which is referred to as the turntable of metabolism because of its vital importance for metabolism in living cells. The Krebs cycle, in simplified form, can be summarized as follows ... 

Raiswell R, Canfield DE (1998) Sources of iron for pyrite formation in marine sediments. Am J Sci 298 219-245... 

Rickard D. T. (1975). Kinetics and mechanism of pyrite formation at low temperatures. Amer. Jour. Set, 275 636-652. 

Railsback LB, Anderson TF, Ackerly SC, Cisne JL (1989) Paleoceanic modeling of temperature-salinity profiles from stable isotope data, Paleoceanography 4 585-591 Raiswell R, Berner RA (1985) Pyrite formation in euxinic and semi-euxinic sediments. Am J Sci 285 710-724... 

Boero.V. Schwertmann, U. (1987) Occurrence and transformations of iron and manganese in a colluvial terra rossa toposequence of Northern Italy. Catena 14 519-531 Boesen, C. Postma, D. (1988) Pyrite formation in anoxic environments of the Baltic. 

Independently, there was the discovery in 1979 of the richness of organic compounds in hydrothermal hot vents (see for example Holm et al., 1992, and Chapter 3). The idea was fully developed by Wachtershauser (1988) and Cairns-Smith et al. (1992), and (of course) became another world. Life then began with the reduction of CO2 and N2 coupled with the reducing power of pyrite formation - and so was born the iron-sulfur-world hypothesis. Thus, the work of Wachtershauser also represents a link between the field of surface catalysis and the field of hydrothermal vents. 

Compared with the surprising variety of biochemical compounds that can be readily synthesized in Miller-type one-pot simulation experiments, the suite of organics produced under the conditions proposed by Wachtershauser is quite limited. However, the impressive demonstration that the FeS/H2S combination can reduce nitrogen to ammonia shows that considerable attention should be given to the reducing power of pyrite formation. Primordial life may have not been autotrophic, but should we hesitate to accept the idea that the primitive soup was formed from both extraterrestrial sources and endogenous synthesis in which pyrite production played a role After all, a spicy, thick bouillon is always tastier than a bland, diluted broth. 

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 ... 

The importance of polysulfides in the pyrite formation process was outlined by several studies (37, 38). Schoonen and Barnes (37) showed that no precipitation from homogeneous solution can be observed within a reasonable time scale, even in solutions highly supersaturated with respect to pyrite, unless pyrite seeds are already existing. Therefore future studies should address the role of ferric oxide surfaces in promoting the nucleation of pyrite. 

As this short discussion shows, the kinetics of formation of the single parameters (Fe2+ and H2S) may control the extent and the pathway of pyrite formation. Oxidation of sulfide by elemental sulfur to form poly sulfides (pathway 1) should predominate at the oxygen-sulfide interface of very productive... 

The bulk solution becomes impoverished with respect to oxygen. However, the sulfate concentration remains constant as long as the recycling rate of reduced sulfur to sulfate is higher than the sulfate reduction rate (denoted SRR in Figure 6). In the opposite case the sulfate concentration of the bulk solution also decreases, and H2S is slowly enriched (point 6). The requirements for FeS precipitation and subsequent pyrite formation are then fulfilled (point 7). 

In addition to a better understanding of the reaction of sulfide with ferric oxides and its role in pyrite formation, a more exact definition of the term reactive iron is critical. Does reactive iron mean a different iron oxide fraction for bacterial dissolution (e.g., weathering products such as goethite or hematite) than for reaction with sulfide (e.g., reoxidized lepidocrocite) In other words, is there a predigestion of ferric oxides by bacteria that allows a subsequent rapid interaction of sulfide with ferric oxides ... 

Dellwig, O., Bottcher, M.E., Lipinski, M. and Brumsack, H.-J. (2002) Trace metals in Holocene coastal peats and their relation to pyrite formation (NW Germany). Chemical Geology, 182(2-4), 423-42. 

It has been shown by mineralogical, chemical and X-ray-diffraction analyses that the major part of reduced sulfur occurs in the form of pyrite in ancient sediments (Lein, 1978)81). It has been also established that pyrite may form rapidly in muds of recent sediments. In anoxic bottom waters, pyrite formation can take place before and after burial even during sedimentation (Berner, 1984)89). Also the geological occurrence and chemical stability relations indicate that authigenic pyrites can be synsedimentary or diagenetic (Kalliokosky, 1966)90). 

In the freshwater peat swamp, bacterial reduction of organic sulfur in plant tissues may be an important process in the formation of pyrite (93). Altschuler et al. (93) proposed that in the Everglades peat, pyrite precipitates directly by the reaction of HS or organic sulfide (produced by reduction of oxysulfur compounds in dissimilatory respiration) with ferrous iron in the degrading tissues. Pyrite formation in low-sulfur coal may be accounted for by this process. 

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... 

Organic sulfur is the dominant form in peats described in these studies. Pyrite, however, is abundant in brackish and marine peats, occurring in void spaces in or between plant debris (3). In a study of pyrite formation in freshwater peats, Altschuler et al. (5) determined parallel decline in ester sulfate with increases in pyrite as depth increased and concluded that pyrite formed at the expense of organic sulfur. In general, framboidal morphology is present at all salinities. Altschuler et al. (5) and Lowe and Bustin (10) found monosulfides to be minor in peats. 

In these clastic sediments the dominant form of sulfur is pyritic, while organic sulfur is usually present only in trace amounts. For this reason, much work on sulfur in these sediments focuses on pyrite formation and its crystallization has been studied in detail by Berner (IT), Sweeney and Kaplan (12). Rickard (13). Rickard (14) and others. Under saline and hypersaline conditions precipitation of monosulfides may be the initial step. Sulfur is then added to these precipitates, converting them to pyrite. Laboratory studies indicate that if griegite is present in the original precipitate, sulfurization may produce framboidal aggregates (12). Conversion may depend on chemical factors such as H2S concentrations (9). In contrast, in conditions that are undersaturated with respect to monosulfides, but supersaturated with respect to pyrite, pyrite may form directly and rapidly from... 

Berner R.A. (1974) Sedimentary pyrite formation an update. Geochim. Cosmochim. Acta 48, 605-615. 

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