A photo-chemical method for the production of olivine nanoparticles as cosmic dust analogues

摘要

This paper describes a new experimental method to synthesise metal silicate particles in the laboratory with compositions and structures which reflect those likely to form in planetary atmospheres and in relatively cool regions of oxygen-rich stellar outflows. Fe-Mg-silicate nanoparticles of olivine composition were produced by the photo-oxidation of a mixture of Fe(CO)_5, Mg(OC_2H_5)_2 and Si(OC_2H_5)_4 vapours in the presence of O_3 at room temperature and atmospheric pressure. Transmission electron microscope X-ray and electron energy loss analysis of the particles from a number of experiments run with different precursor vapour mixture ratios show that Mg_2xFe_(2-2x)SiO_4 particles can be produced ranging from x=0 to 1, where x is linearly proportional to the ratio of Mg(OC_2H_5)_2/(Fe(CO)_5+Mg(OC_2H_5)_2). Electronic structure calculations with hybrid density functional/Hartree-Fock theory are then used to explore the pathways involved in producing olivine particles from the FeO_3, MgO_3 and SiO_2 produced from the photolysis of the organometallic precursors in O_3. These calculations indicate that highly exothermic reactions lead to the formation of Mg_2SiO_4, MgFeSiO_4 and Fe_2SiO_4 molecules, which then polymerize. An alternative pathway, also strongly favoured thermodynamically, is the polymerization of MgSiO_3 and FeSiO_3 to form pyroxenes, which then undergo structural rearrangement to olivine and silica phases. The implications for metal silicate formation in planetary atmosphere and stellar outflows are then discussed.