SULFUR AND IRON SURFACE-STATES ON FRACTURED PYRITE SURFACES

Pyrite has a poor {001} cleavage. Unlike most other minerals with a ro cksalt-type structure, pyrite typically fractures conchoidally, demons trating that parting surfaces are not constrained to the {001} crystal lographic plane. Cleavage along {001} require rupture of only Fe-S bon ds, but pyrite consists of both Fe-S and S-S bonds. Analysis of bond e nergies indicates that S-S bonds are the weaker bonds and they are lik ely to be ruptured when pyrite is fractured. With each ruptured S-S bo nd, two mononuclear species (formally S1-) are produced, one bound to one fracture surface and the second to the opposite fracture surface. This monomer is reduced to S2- (monosulfide) during relaxation through oxidation of surface Fe2+ ions to Fe3+. These surface relaxation proc esses explain the surface states observed in S(2p) and Fe(2p(3/2)) X-r ay photoelectron spectra (XPS) of pyrite. The S(2p) XPS spectrum is in terpreted to include bulk disulfide contributions at 162.6 eV and two surface state contributions at 162.0 and 161.3 eV. The monosulfide (S2 -) emission is near 161.3 eV, as observed in S(2p) spectra of pyrrhoti te, and the 162 eV peak is interpreted to result from the surface-most sulfur atom of surface disulfide ions. The Fe(2p(3/2)) XPS spectrum i ncludes three contributions, a bulk Fe2+ emission near 707 eV and emis sions from two Fe surface states. One surface state is interpreted to be Fe2+ surface ions. Their coordination is changed from octahedral be fore fracture to square pyramidal after fracture. The consequent stabi lization of the antibonding Fe d(2)(z) orbital yields unpaired electro ns in the valence band resulting in multiplet peak structure in the Fe (2p(3/2)) spectrum. Similarly, each surface Fe3+ ion, having contribut ed a non-bonding 3d electron to the valence band (bonding orbital), co ntains unpaired 3d electrons, resulting in multiplet splitting of its Fe(2p(3/2)) signal. The high-energy tail observed in the Fe(2p(3/2)) s pectrum of pyrite is the product of emissions from both surface states with Fe2+ multiplet peaks centered near 708 eV and the surface Fe3+ m ultiplets spanning the binding energies from 708.75 to about 712 eV.