Synthetic Fe²⁺ monosulfide, FeS_am, 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 FeS_am 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 FeS_am 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 FeS_am 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.

Studies of phosphorus (P) dynamics in surface sediments of lakes and coastal seas typically emphasize the role of coupled iron (Fe), sulfur (S) and P cycling for sediment P burial and release. Here, we show that anaerobic oxidation of methane (AOM) also may impact sediment P cycling in such systems. Using porewater and sediment profiles for sites in an oligotrophic coastal basin (Bothnian Sea), we provide evidence for the formation of Fe-bound P (possibly vivianite; Fe₃(PO₄)₂^.8H₂O) below the zone of AOM with sulfate. Here, dissolved Fe²⁺ released from oxides is no longer scavenged by sulfide and high concentrations of both dissolved Fe²⁺ (>1 mM) and PO₄ in the porewater allow supersaturation with respect to vivianite to be reached. Besides formation of Fe(II)-P, preservation of Fe-oxide bound P likely also contributes to permanent burial of P in Bothnian Sea sediments. Preliminary budget calculations suggest that the burial of Fe-bound P allows these sediments to act as a major sink for P from the adjacent eutrophic Baltic Proper.

The Charge Distribution MUltiSite Ion Complexation or CD–MUSIC modeling approach is used to describe the chemical structure of carbonate mineral-aqueous solution interfaces. The new model extends existing surface complexation models of carbonate minerals, by including atomic scale information on the surface lattice and the adsorbed water layer. In principle, the model can account for variable proportions of face, edge and kink sites exposed at the mineral surface, and for the formation of inner- and outer-sphere surface complexes. The model is used to simulate the development of surface charges and surface potentials on divalent carbonate minerals as a function of the aqueous solution composition. A comparison of experimental data and model output indicates that the large variability in the observed pH trends of the surface potential for calcite may in part reflect variable degrees of thermodynamic disequilibrium between mineral, solution and, when present, gas phase during the experiments. Sample preparation and non-stoichiometric surfaces may introduce further artifacts that complicate the interpretation of electrokinetic and surface titration measurements carried out with carbonate mineral suspensions. The experimental artifacts, together with the high sensitivity of the model toward parameters describing hydrogen bridging and bond lengths at the mineral-water interface, currently limit the predictive application of the proposed CD–MUSIC model. The results of this study emphasize the need for internally consistent experimental data sets obtained with well-characterized mineral surfaces and in situ aqueous solution compositions (that is, determined during the charge or potential measurements), as well as for further molecular dynamic simulations of the carbonate mineral-water interface to better constrain the bond lengths and the number plus valence contribution of hydrogen bridges associated with different structural surface sites.

Until recently the influence of solution stoichiometry on calcite crystal growth kinetics has attracted little attention, despite the fact that in most aqueous environments calcite precipitates from non-stoichiometric solution. In order to account for the dependence of the calcite crystal growth rate on the cation to anion ratio in solution, we extend the growth model for binary symmetrical electrolyte crystals of Zhang and Nancollas (1998) by combining it with the surface complexation model for the chemical structure of the calcite-aqueous solution interface of Wolthers et al. (2008). To maintain crystal stoichiometry, the rate of attachment of calcium ions to step edges is assumed to equal the rate of attachment of carbonate plus bicarbonate ions. The model parameters are optimized by fitting the model to the step velocities obtained previously by atomic force microscopy (AFM, Teng et al., 2000; Stack and Grantham, 2010). A variable surface roughness factor is introduced in order to reconcile the new process-based growth model with bulk precipitation rates measured in seeded calcite growth experiments. For practical applications, we further present empirical parabolic rate equations fitted to bulk growth rates of calcite in common background electrolytes and in artificial seawater-type solutions. Both the process-based and empirical growth rate equations agree with measured calcite growth rates over broad ranges of ionic strength, pH, solution stoichiometry and degree of supersaturation