Sulphide mineral surfaces: theory and experiment

The bulk crystal structures of the industrially important metal sulfide minerals are well established and problems concerning oxidation states and site occupancies in complex phases such as the tetrahedrites can now be resolved using spectroscopic methods (particularly X-ray absorption spectrometry). Chemical bonding in sulphides can also be studies spectroscopically and using a wide range of theoretical models and methods, ranging from isolated cluster to full lattice, and from atomistic to fully quantum mechanical. In considering the mineral surfaces, experimental evidence suggests that whereas more 'ionic' sulphides such as galena have surface geometric structures that closely resemble the truncated bulk solid, more 'covalent' sulphides such as sphalerite and chalcopyrite undergo significant distortions in the surface region. Atomistic calculations of surface distortions and surface energies reproduce general trends in these reconstructions, and suggest an important role for defects in stabilizing particular surfaces. Molecular theories can be used to explain the driving force for such distortions, and quantum mechanical modelling can indicate individual atom positions on a surface. Since a range of oxidation states is generally for metals and sulphur, sulphide minerals are quite reactive; when in contact with aqueous solutions, their surface chemistry is controlled largely by pH and redox potential. Surface mineral chemistry and reactivity of phases such as the Cu-Fe sulphides can be studies using a variety of electrochemical and spectroscopic methods (notably X-ray photoelectron and X-ray absorption spectroscopies) to determine reaction rates and mechanisms. Such studies reveal the importance of stoichiometry as a rate controlling factor in such sulphide systems. Detailed understanding of the initial stages of reactions such as oxidation can be achieved by combining quantum mechanical modelling of data such as scanning tunnelling microscope (STM) images and scanning tunnelling spectra, for example in the oxidation of galena and of pyrite.