Why oxides and sulfides?
Some oxides and most sulfides are semiconducting and, therefore, the highest-occupied (HOMO) and lowest-unoccupied (LUMO) orbitals are delocalized. Rosso and Becker (Geochimica et Cosmochimica Acta, March 2003) demonstrated the extent of such a LUMO orbital around a lead vacancy on a galena lead-sulfide surface. Using quantum mechanical calculations, we are now able to show the electronic relationship between two species that are some distance apart and explain the thermodynamics of co-adsorption on mineral surfaces and/or co-incorporation into their bulk. Surprisingly, the adsorbed oxygen atom is attracted by the lead vacancy because of its peculiar electronic interaction through the near-surface region of the semiconducting galena even though the laedvacancy (charge = -2) and the negatively charged oxygen atom on the surface would repel each other if just Coulomb interactions are considered. Delocalization of orbitals also plays a role when iron as an oxidant can charge-polarize and
