Mackinawite iron sulfides

To run the simulation, we decouple acetate from carbonate, and sulfide from sulfate, and suppress the iron sulfide minerals pyrite and troilite (FeS), which are more stable than mackinawite, but unlikely to form. We set the fluid composition, including an amount of HS small enough to avoid significantly supersaturating mackinawite, and define the rate law for the sulfate reducers. The procedure in REACT is... 

To set up the simulation, we use the thermodynamic dataset from the calculation in Section 18.5, which was expanded to include mackinawite (FeS). As before, we suppress the iron sulfide minerals pyrite and troilite, and decouple acetate and methane from carbonate, and sulfide from sulfate. We set the aquifer to include a small amount of siderite, which serves as a sink for aqueous sulfide,... 

Slowly reacts with water forming HCl (NIOSH, 1997). 1,1,1-Trichloroethane also was shown to react with a form of iron sulfide mineral, namely mackinawite (FeS(i x)). Complete removal of... 

In addition to pyrite, there are two additional iron-sulfide minerals commonly found in recent sediments. These phases have been termed metastable iron sulfides, because they are thermodynamically unstable with respect to transformation to pyrite, or to a mixture of pyrite plus pyrrhotite (Berner, 1967). They are also known as acid-volatile iron sulfides, because in contrast to pyrite they are soluble in nonoxidizing mineral acids such as HCl. The dominant acid-volatile sulfides found in both natural and experimental systems are mackinawite and greigite. 

The term hydrotroilite has been historically applied to the hydrated form of ferrous sulfide and it has been identified for some time as a common constituent of reducing sediments (Galliher, 1933). It is now believed that the initial iron sulfide precipitate is poorly crystallized mackinawite, FeSo 94 (Ward, 1970). 

Dissolved sulfides (H2S and HS ), formed from sulfate reduction, can react with Fe(ll) and precipitate as iron sulfides (Berner, 1970). Under conditions of early diagenesis of amorphous iron sulfides (FeS), mackinawite (FeSg 9) is formed. One of the first minerals formed is hydrotrolite (FeS H20), which is labile under acidic conditions. During diagenesis the hydrotrolite converts to pyrite, which is a more stable mineral. The reaction can be simply represented as... 

Six iron-sulfur minerals are stable enough to exist in nature all contain the ferrous (Fe +) form of iron. Those minerals with iron-to-sulfur ratios of 1 1 (FeS) are mackinawite and pyrrhotite. Those with iron-to-sulfur ratios of 3 4 0 0384) are greigite and smythite. All the iron sulfides with ratios of 1 1 and 3 4 are soluble in mild acids with formation of H2S. Those minerals with iron-to-sulfur ratios of 1 2 (FeSj) are pyrite and marcasite. Pyrite and marcasite are distinguished from the other four iron sulfides by their insolubility in concentrated HCl. Pyrite is highly pH- and temperature-stable. Due to this, it is found often in nature. This inertness also makes pyrite a desirable reaction product for sulfide removal using an iron-based scavenger. Various iron sulfides can be formed chemically from iron compounds reacting with soluble sulfides at ambient conditions in aqueous systems. The specific reaction conditions control both the products formed and the rate of reaction. 

Mackinawite, Fei+xS, with (x = 0.01-0.07) is a tetragonal iron sulfide with excess of iron. Morice et al. [153] reported a complex Mossbauer spectrum consisting of at least three sextets with hyperfine fields 29.8, 26.2 and 22.8 T and small quadrupole shifts of about 0.09, 0.06 and 0.09 mm/s respectively. On the other hand, only a singlet spectrum, even down to 4 K, has been observed by Vaughan and Ridout [154]. Probably the concentration of Co and Ni found to be present in the involved natural samples is decisive for the different magnetic behavior. 

Iron monosulfide, FeS, is produced in soils and sediments primarily through dissimilatory microbial reduction of sulfate to sulfide, which subsequently reacts with available iron to precipitate FeS (5, 4). The mineral mackinawite, often in poorly crystalline form (3, 23-25), is the initial FeS precipitate in the transformation of iron minerals by sulfate-reducing bacteria (3). For example, when the sulfate-reducing bacterium Desulfovibrio desulfuricans was grown at pH 8 in cultures containing a Fe(II)/Fe(III) oxyhydroxide and synthetic geothite (FeOOH), mackinawite was the predominant iron sulfide phase present after six and nine months, respectively (26). Even at lower pH values, mackinawite was the only iron sulfide phase detected after two weeks of microbial activity, and still a minor phase after that. 

We can expect the sulfide produced by the bacteria to react with the iron in solution to form mackinawite (FeS), a precursor to pyrite (FeS2). The mineral is not in the default thermodynamic database, so we add to the file the reaction... 

Many mineral species are known to be selectively crystallized by the presence of bacteria. Carbonate minerals, such as calcite, aragonite, hydroxycalcite, and siderite oxide minerals, such as magnetite and todorokite oxalate minerals, such as whewellite and weddellite sulfide minerals, such as pyrite, sphalerite, wurtzite, greigite, and mackinawite and other minerals, such as jarosite, iron-jarosite, and g3q>sum, are known to precipitate in the presence of bacteria. Therefore, investigations have been developed to analyze the formation of banded iron ore by the action of bacteria, and to analyze the ancient environmental conditions of the Earth through the study of fossilized bacteria. 

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). 

Reactions involving mackinawite and an oxidized sulfur species have been repeatedly shown to lead to pyrite formation (e.g., Bemer, 1969 Rickard, 1969, 1975). In addition, Wilkin and Bames (1996) and Penning et al. (2000) have shown that pyrite formation is exceptionally rapid when the mackinawite is pre-oxidized (e.g., exposed briefly to air) prior to the experiment. Based partly on X-ray photoelectron and Auger spectroscopy results of pyrrhotite oxidation (Mycroft et al., 1995), Wilkin and Bames (1996) hypothesized that this oxidative exposure initiates an iron-loss pathway similar to Equation (13). In sulfidic solutions, Fe(II) oxyhydroxides, shown as a product in this reaction, would not accumulate, but instead would undergo reductive dissolution by a reaction similar to Equation (14) ... 

This method by Sullivan etal. (2000) is based on the conversion of reduced inorganic sulfur to H2S by hot acidic chromium(II) chloride (CrCl2) solution. The hydrogen sulfide (H2S) evolved is trapped in a zinc acetate solution as ZnS, and subsequently quantified by iodometric titration (see Fig. 5.1). The reduced inorganic sulfur compounds measured are (i) pyrite and other iron disulfides (ii) elemental sulfur and (iii) acid volatile sulfides (e.g. greigite and mackinawite). The method can be made specific to the iron disulfide fractions if pre-treatments are used to remove the acid volatile sulfides and elemental S fractions. 

The sulfide produced during sulfate reduction reacts rapidly with iron-rich particles to form the acid-volatile sulfides mackinawite and greigite (Berner, 1967 Rickard, 1974). These labile sulfides represent transient intermediates which, if conditions are appropriate, commonly convert through several possible mechanisms to pyrite (Berner, 1970 Sweeney and Kaplan, 1973 Rickard, 1969,1975). Acid-volatile sulfides (FeS) readily oxidize back to Fe and if exposed to oxygen (e.g., Sato, 1960 ... 

Sulfur reaches the sediment essentially only by means of diagenetic processes. Here, the fixation of sulfate-bound sulfur in the form of barite in the sulfate-methane transition (SMT) zone produces an interesting signal, but is quantitatively irrelevant on account of the low barium concentrations in the pore water. Sulfur precipitates in much greater amounts in shape of sulfide (Mackinawite FeS, pyrite FeS ) wherever sulfide resulting from the reduction of sulfate comes into contact with divalent iron by means of diffusion. In the core shown in Figure 3.29 this obviously proceeded at a depth of about 7 m below the sediment surface over a relatively long period of time. 

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