This project deals with a new type of reaction mechanism: the co-reaction of different species on semiconducting mineral surfaces or within a semiconducting mineral. Hereby, the reactants can be some distance apart from each other and, nonetheless, enhance or inhibit the interaction of the other reactant with the mineral. We first described this reaction mechanism, which we call the proximity effect, a year ago and have begun to evaluate the distance dependence between the reaction partners. In this project, a systematic study is proposed on such proximity effects, mainly on sulfides but also on some important reactions on oxides. The quantum mechanical evaluation of the reaction partners such as As/Au or Bi/Ag in galena and pyrite/arsenopyrite will help to understand previously described processes such as coupled substitutions and the preferred incorporation of gold into arsenopyrite and arsenian pyrite (compared with pyrite). Furthermore, the detailed description of surface diffusion processes will elucidate the mechanism of cluster or nanoparticle formation on sulfides surfaces or within the bulk. Understanding these processes is important to develop a consistent theory on the formation of gold and silver-containing ore deposits. Furthermore, the proximity effect may play an important role in the oxidation and weathering of sulfides and is, therefore, instrumental for the evaluation of environmentally important processes in acid mine drainage.
In addition, environmentally and technically important reactions on oxides will be evaluated. It was previously observed that the electronic structure on hematite surface steps is significantly different from the valence band structure of flat surfaces. Therefore, the directed proximity effect will be examined along steps, which enhances electron transfer along steps on Fe2O3 surfaces and thus, the adsorption and oxidation of Mn at these steps. Furthermore, the combined attack of water and oxygen on different UO2 surfaces will be compared with the previously observed formation of oxidation patches on pyrite in light of the proximity effect. This effect helped resolve the complicated oxidation and weathering mechanism on a FeS2 (001) surface and may resolve the reaction path of uraninite corrosion, which is an unwanted effect in storing radioactive materials.
Even though the proposed studies aim at a basic understanding of the proximity effect, it will have a broader impact on a wide variety of applications in environmental geochemistry, in the evaluation of ore deposits, in future options for metal extraction techniques, and for other technical applications such as the purification of drinking water using hematite as a filter material, or the evaluation of potential hazards due to the weathering of uranium oxide minerals. Due to the general character of these enhanced co-reactivity processes, the theory and findings can be applied to other fields such as physics, chemical engineering, materials science, and nuclear engineering. Early stages and planning of this project have already sparked collaborations across campus and with other universities. Finally, the new teaching program on minerals and materials surfaces at the University of Michigan can use these processes as a practical application of the interface of quantum mechanics of semiconductors and more classical approaches to mineral surface reactivity.
