Amorphous iron sulfides

Below are experimentally determined data by various authors (see Davison, 1991) plotted as —log [Fe ] versus pH for amorphous iron sulfide at 298 K and 0.1 MPa H2S (= 10 atm). The data with numbers refer to various aged (hours) iron sulfide. What solubility constant for FeS (amorphous) + = Fe + HS can be derived from these data ... 

Paschin YV, Kozachenko VL, Sal nikova LE (1983) Differential mutagenic response at the HGPRT locus in V-79 and CHO cells after treatment with chromate. Mutat Res 122 362-365 Patterson RR, Fendorf S, Fendorf M (1997) Reduction of hexavalent chromium by amorphous iron sulfide. Environ Sci Technol 31 2039-2044... 

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... 

The form of nickel in particles from different industries varies. The mineralogical composition, chemical content, and form of dusts from nine industries in Cracow, Poland, were examined (Rybicka 1989). The chemical form of a particle-associated heavy metal that was assessed by a five-step extraction scheme classified the metal as exchangeable, easily reducible (manganese oxides, partly amorphous iron oxyhydrates and carbonates), moderately reducible (amorphous and poorly crystallized iron oxyhydrates), organically bound or sulfidic, and residual. Dusts from power plants had a silicate characteristic with quartz and mullite predominant. Approximately 90% of the nickel from these... 

Reactions 2a and 2b only yield a negative free energy (AG) in the neutral pH range if a crystalline Fe(III) oxohydroxide (such as goethite) is formed as the reaction product. Inversely, the oxidation of As(III) is thermodynamically favored by reaction with an amorphous iron hydroxide, but not with goethite. The most suitable reductant under the conditions of the anoxic lake hypolimnion is certainly sulfide, which is thermodynamically favored and has been shown to react with As(V) (68, 16) according to reaction 3. 

To make things even more complicated, species without detectable emission features, like metallic iron, iron sulfide, or amorphous carbon, can dominate the radiative transfer in protoplanetary disks and thereby set the temperature structure. However, the abundances and size distributions of such grains are extremely difficult to constrain. 

Direct precipitation of pyrite without intermediate iron sulfide precursors was reported for salt marsh sediments, where pore waters were undersaturated with respect to amorphous FeS but oversaturated with respect to pyrite (Howarth 1979 Giblin and Howarth 1984). In these sediments, the oxidizing activity of the roots favored the formation of elemental sulfur and polysulfides which were thought to react directly with Fe +. The direct reaction pathway may proceed within hours, resulting in the formation of single small, euhedral pyrite crystals (Rickard 1975 Luther et al. 1982). Framboidal pyrite, apart from that formed by the mechanism presented by Rickard (1997), is formed slowly (over years) via intermediate iron sulfides (Sweeney and Kaplan 1973 Raiswell... 

Iron sulfide forms initially as precursor (amorphous FeS) that changes to pyrite during diagenesis. The mechanism of formation of pyrite is not well understood. 

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