The iron−nickel sulfide mineral violarite is relevant in origin of life theories and has potential catalytic activity. Here we present density functional theory simulations with a correction for the long-range interactions (DFT-D2) of the bulk and surfaces of violarite, and subsequent adsorption and dissociation of water. Relaxation of the low-Miller index surfaces shows the (001) surface to be energetically the most favorable, while the (111) is the least stable surface, but also the least reactive with respect to water. Calculation of the reaction energy pathways reveals that water adsorption is most likely to occur on the (011), as this stabilizes the low-coordinated atoms. Stronger adsorption energies are found for Fe octahedral sites, over tetrahedral Ni sites, with the dissociation of an adsorbed water molecule on the surface facilitating the adsorption of a subsequent molecule. Dissociation of water is also most likely to occur on the (011) surface, as reflected by negative values calculated for the overall reaction energies. The results ultimately show that associative adsorption of water is preferred over dissociative adsorption.

The structural analogy between Ni-doped greigite minerals (Fe 3 S 4 ) and the (Fe, Ni)S clusters present in biological enzymes has led to suggestions that these minerals could have acted as catalysts for the origin of life. However, little is known about the distribution and stability of Ni dopants in the greigite structure. We present here a theoretical investigation of mixed thiospinels (Fe 1Àx Ni x ) 3 S 4 , using a combination of density functional theory (DFT) calculations and Monte Carlo simulations. We find that the equilibrium distribution of the cations deviates significantly from a random distribution: at low Ni concentrations, Ni dopants are preferably located in octahedral sites, while at higher Ni concentrations the tetrahedral sites become much more favourable. The thermodynamic mixing behaviour between greigite and polydymite (Ni 3 S 4 ) is dominated by the stability field of violarite (FeNi 2 S 4 ), for which the mixing enthalpy exhibits a deep negative minimum. The analysis of the free energy of mixing shows that Ni doping of greigite is very unstable with respect to the formation of a separate violarite phase. The calculated variation of the cubic cell parameter with composition is found to be non-linear, exhibiting significant deviation from Vegard's law, but in agreement with experiment.

Density Functional Theory-based calculations have been employed to investigate the structure, stability and dynamics of iron sulphide clusters, FexSy (x, y ≤ 4), in water. Car-Parrinello molecular dynamics simulations of the building unit FeS in explicit water show that the iron is only four-coordinated, which indicates that the effect of sulphur is to significantly reduce the coordination shell of iron compared with the typical octahedral arrangement of hexa-aqua iron complexes in water. The molecular dynamics simulations of FexSy particles (x, y ≥ 2) in explicit water reveal that these clusters are highly unstable as they dissociate after a few picoseconds. The Gibbs free energies to form the FeS and Fe2S2 species have been evaluated in a simulated aqueous environment, using the mPW1B95 density functional theory level for the gas-phase component and the UAHF-CPCM solvation model for the hydration contribution, and the results indicate that while FeS is thermodynamically stable in aqueous solution, the formation of a Fe2S2 cluster is endergonic, and dissociation is preferred under natural water conditions.