Iron is a geochemically abundant metal with a complex redox chemistry and a mineralogy dominated by oxide and sulphide species; copper is a geochemically scarce metal with a mineralogy dominated by sulphide species. The surface chemistries and reactivities of systems involving these two metals present interesting problems of relevance to environmental geochemistry and mineral technology, which we have approached using a range of experimental methods.
Studies of the breakdown of copper-iron sulphides in aqueous systems, leading to release of metals and acid, have involved electrochemical methods combined with X-ray photoelectron, Auger electron and X-ray absorption spectroscopies. In chalcopyrite for example, the rate of development of surface layers of hydrated ferric oxide and subsurface iron-depleted copper sulphide appears to be controlled by not only Eh and pH conditions, but also by metal sulphide stoichiometry.
Copper released into solution from sulphide breakdown may be transported or re-precipitated, or may be sorbed onto the surfaces of common minerals and fixed in soils or sediments (or transported with them as detrital phases). For example, we have studied the sorption of copper from aqueous solution onto the surface of muscovite (which also serves as a model for clay mineral species) using X-ray photoelectron and X-ray absorption spectroscopy. These studies indicate multinuclear clusters or ribbons with copper hydroxide-like character forming by chemisorption of copper on the surface.
The surface structures and chemistries of iron oxides (hematite, magnetite) are being probed at atomic resolution using
scanning tunneling microscopy, revealing various terminations of the bulk structure. These surfaces show both vacancies and adsorbates, and annealing under redox conditions on the magnetite-hematite phase boundary gives rise to complex two-dimensional surface-ordering phenomena.
The strengths and weaknesses of these different experimental approaches will be discussed along with applications to the geochemical behaviour of copper and iron.