Behavior of Gold Nanoparticles at the Interphase Boundary of Quartz-Selenide Copper and Iron at a Temperature of 450 °C and Different Selenium Activity
V.V. Akimov, D.N. Babkin, O.Yu. Belozerova
Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia
Keywords: Experimental mineralogy, gold nanoparticles, aggregation mechanisms, “invisible” forms of gold, structure and composition of interphase boundaries, gold-quartz-selenide mineralization, quartz, copper and iron selenides
Abstract
In this work, based on structural and phase analysis data, we consider the behavior of Au nanoparticles (NP) during the formation of interphase boundaries between quartz and copper and iron selenides upon annealing at 450 °C and different selenium activities: lg f Se2 = -2.4 with a melted selenium buffer and lg f Se2 = -3.15 according to the indications of the indicator mineral mixture γ-Fe1- x Se and δ-Fe1- x Se. In two series of experiments, contrasting textural features (nano- and microstates) of the initial metal phases were used, which made it possible to construct 2D and 3D models of interphase boundaries. In the first case, thin layers of gold NPs (~30 nm thick), iron (~50 nm thick), and copper (~50 nm thick) were deposited onto quartz crystals less than 40 μm in size by magnetron sputtering, i.e., all metals were in a nanosized state. In the second case, iron, copper, and selenium (in eskebornite stoichiometry - CuFeSe2) were introduced into the system in the form of micron-sized powder materials (1-10 microns). Gold still remained in the form of NPs on the quartz surface. The thermal annealing products were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses, and X-ray photoelectron spectroscopy (XPS). The study showed that, regardless of the activity of selenium, annealing of Au NPs leads to partial enlargement of particles (up to submicron sizes) based on the self-assembly mechanism (the size distribution of Au particles corresponds to a lognormal law with a maximum shifted towards smaller sizes), and gold remains in the metallic state. The 2D model of the interphase boundary is a mineral mixture of copper selenides (Cu2Se), iron (γ-Fe1- x Se) and islands of submicron gold particles formed on the surface of quartz. The 3D model of the interphase boundary is characterized by the fact that Au particles are concentrated mainly in eskebornite, and not on the quartz surface. Moreover, NPs are localized in the pores of mesoporous eskebornite, and submicron particles with a size of ~ 200 nm or more are displaced to the surface of eskebornite particles. Based on the data obtained, typomorphic features of the presence of Au NPs in the ore-forming processes of the formation and development of gold ore deposits are formulated.
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