Gold deposition on pyrite and the common sulfide minerals: An STM/STS and SR-XPS study of surface reactions and Au nanoparticles

Abstract

Gold species spontaneously deposited on pyrite and chalcopyrite, pyrrhotite, galena, sphalerite from HAuCl 4 solutions at room temperature, as well as the state of the reacted mineral surfaces have been characterized using synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS), scanning tunneling microscopy and tunneling spectroscopy (STM/STS). The deposition of silver from 10 −4 M AgNO 3 has been examined for comparison. Gold precipitates as metallic nanoparticles (NPs) from about 3 nm to 30 nm in diameter, which tends to aggregate forming larger particles, especially on pyrite. The Au 4f binding energies increase up to 1 eV with decreasing size of individual Au 0 NPs, probably due to the temporal charging in the final state. Concurrently, a positive correlation between the tunneling current and the particle size was found in STS. Both these size effects were observed for unusually large, up to 20 nm Au particles. In contrast, silver deposited on the minerals as nanoparticles of semiconducting sulfide showed no shifts of photoelectron lines and different tunneling spectra. The quantity of gold deposited on pyrite and other minerals increased with time; it was lower for fracture surfaces and it grew if minerals were moderately pre-oxidized, while the preliminary leaching in Fe(III)-bearing media inhibited the following Au deposition. After the contact of polished minerals with 10 −4 M AuCl 4 - solution (pH 1.5) for 10 min, the gold uptake changed in the order CuFeS 2 > ZnS > PbS > FeAsS > FeS 2 > Fe 7S 8. It was noticed that the open circuit (mixed) potentials of the minerals varied in approximately the same order, excepting chalcopyrite. We concluded that the potentials of minerals were largely determined by Fe(II)/Fe(III) couple, whereas the reduction of gold complexes had a minor effect. As a result, the deposition of gold, although it proceeded via the electrochemical mechanism, increased with decreasing potential. This suggests, in particular, that the accumulation of “invisible” gold in arsenian pyrites and arsenopyrite under hydrothermal conditions may be explained by the low electrochemical potentials but not structural relationships between As and Au in solids.