Synthetic Fe2+ monosulfide, FeSam, displays a disordered tetragonal mackinawite structure. It is nanocrystalline, with an average primary particle size equivalent to a crystallite size of 4 nm and a corresponding specific surface area of 350 m2/g. It can be described in terms of a mixture of two end-member phases with different long-range ordering, which we refer to as MkA and MkB. MkA has an average primary particle size of 2.2 × 1.7 nm and lattice parameters a = b = 4.0 Å, c = 6.6 ±0.1 Å. MkB has an average primary particle size of 7.4 × 2.9 nm and lattice parameters a = b = 3.7 Å, c = 5.5 ± 0.2 Å. A typical disordered mackinawite precipitate consist of 30% MkA and 70% MkB and the proportion of MkA decreases with age. Lattice expansions relative to crystalline mackinawite (a = b = 3.7 Å, c = 5.0 Å) may be explained by intercalation of water molecules between the tetrahedral sheets and by lattice relaxation due to small crystallite size.

The formation of two phases of FeSam is consistent with competing pathways involved in its formation from aqueous solution. MkA may be equivalent to sheet-like precipitated aqueous FeS clusters. The reactivity of FeSam is dependent on the proportion of the two end-member phases. These in turn are dependent on the conditions of formation, especially pH, and the age of the precipitate. These observations partly explain the reported differences in FeSam reactivity in experimentation and in the environment. The structural model has implications for the behavior of natural acid volatile sulfides in scavenging elements from solution in natural environments.

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