Modeling the Surface Structure and Reactivity of Pyrite:  Introducing a Potential Model for FeS₂

Abstract

Atomistic simulation techniques are used to investigate the surface structure, stability and reactivity of pyrite. We introduce a potential model for FeS2 which reproduces experimental structural parameters, elastic constants and hydration energies of pyrite. We modeled the {100}, {110}, and {111} surfaces of pyrite and calculated the {100} surface to be the most stable and to show little surface relaxation, in agreement with experiment. The surfaces were hydrated by associative adsorption of water molecules which stabilized all three surfaces, especially the unstable {111} surface. The calculated adsorption energy of −47 kJ mol-1 for water on the {100} surface agrees well with an adsorption energy of −42 kJ mol-1, determined for the stoichiometric (100) surface by temperature-programmed desorption. Adsorption of water molecules at surface sites of lower coordination (four- or three- coordinated) showed increased reactivity of these sites. We calculated an increase in adsorption energy of 50−60 kJ mol-1 per loss of bond. We next created stepped {100} planes in order to model a more realistic {100} surface with one-dimensional defects. Four different steps were investigated in two orthogonal directions. Because of the asymmetry of the sulfur dimers, the geometry of the dimers on the edge showed the dimers either leaning forward (F-steps) or backward (B-steps) with respect to the {100} terrace. We used water molecules as a probe of the reactivity of the different surface sites. Corresponding adsorption sites (terrace, edge or below the step) on the F- and B-steps were found to have different reactivities toward water due to the different adsorption modes of the probe molecule. On the B-steps the increased reactivity of the low-coordinated edge iron atom toward water (approximately −70 kJ mol-1) was outweighed by the network of interactions of the water molecule to atoms on terrace and step wall in the position below the step (−91 kJ mol-1). On the F-steps the four-coordinated edge site was calculated to be the most reactive adsorption site.

Density functional theory study of the relative stability of the iron disulfide polymorphs pyrite and marcasite

“…[16][17][18] Pyrite itself has received much attention from computational methods, including DFT [19][20][21] and interatomic potential based studies. 22 In contrast, theoretical studies involving the marcasite phase are very limited, with one study comparing the thermodynamics of arsenic incorporation into pyrite and marcasite. 23 The majority of DFT studies to date have only focused on one polymorph of this mineral, thereby avoiding the difficult issue of the relative phase stability of two very similar structures.…”

Molecular dynamics simulation of water in a contact with an iron pyrite FeS2 surface

“…12 As the Buckingham potential for Fe-O interaction is purely repulsive and at the same time the code required the presence of an attractive term, the fit was performed in such a way as to correctly reproduce the Buckingham curve in the region of the onset of repulsion in order to be able to determine the distance of the water molecules from the iron atoms at the pyrite surface. The attractive part of the Lennard-Jones potential was made as small as possible, which resulted in the increase of parameter for iron.…”

“…12 A crystal slab of 200ϫ200ϫ100 lattice periods was used for the summation of the Madelung-type electrostatic contribution to the energy. The Buckingham potentials are softer then the Lennard-Jones types in the region of repulsion.…”

AB INITIO Simulations of Desorption and Reactivity of Glycine at a Water-Pyrite Interface at “Iron-Sulfur World” Prebiotic Conditions

“…Molecular dynamics computer simulations of the interface between water molecules and a (001) pyrite surface (Philpott et al, 2004) showed that water molecules do not desorb in the temperature range 225-425 • C. In addition, the molecules are found to be attached via their oxygen atom, O wat , to an iron atom and via only one (Philpott et al, 2004) or both (de Leeuw et al, 2000) of the hydrogen atoms, H wat , to sulfur atoms of this pyrite surface. The formation of a shorter and a longer hydrogen bond with surface sulfur atoms was also confirmed by ab initio molecular dynamics of monolayer systems (Stirling et al, 2003b), which in addition established the existence of hydrogen bonds between the adsorbed water molecules themselves, see also Ref.…”