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 2 H 5 ) 2 and Si(OC 2 H 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 2x Fe 2ร2x SiO 4 particles can be produced ranging from x = 0 to 1, where x is linearly proportional to the ratio of Mg(OC 2 H 5 ) 2 /(Fe(CO) 5 + Mg(OC 2 H 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 2 SiO 4 , MgFeSiO 4 and Fe 2 SiO 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.