Experimental studies of magnetite formation in the solar nebula

Abstract

Oxidation of Fe metal and Gibeon meteorite metal to magnetite via the net reaction 3 Fe (metal) + 4 H2O (gas) = Fe3O4 (magnetite) + 4 H2 (gas) was experimentally studied at ambient atmospheric pressure at 91-442 °C in H2 and H2-He gas mixtures with H2/H2O molar ratios of ~4-41. The magnetite produced was identified by x-ray diffraction. Electron microprobe analyses showed 3.3 wt% NiO and 0.24 wt% CoO (presumably as NiFe2O4 and CoFe2O4) in magnetite formed from Gibeon metal. The NiO and CoO concentrations are higher than expected from equilibrium between metal and oxide under the experimental conditions. Elevated NiO contents in magnetite were also observed by metallurgists during initial stages of oxidation of Fe-Ni alloys. The rate constants for magnetite formation were calculated from the weight gain data using a constant surface area model and the Jander, Ginstling-Brounshtein, and Valensi-Carter models for powder reactions. Magnetite formation followed parabolic (i.e., diffusion-controlled) kinetics. The rate constants and apparent activation energies for Fe metal and Gibeon metal are: log10k = -4.14(±0.26) - 4825(±260)/T cm2h-1 E(act) = 92 ± 5(2σ) kJ mol-1 log10k = -5.41(±0.44) - 4942(±646)/T cm2h-1 E(act) = 95 ± 12(2σ) kJ mol-1 These rate constants are significantly smaller than the parabolic rate constants for FeS growth on Fe metal in H2S-H2 gas mixtures containing 1000 or 10 000 ppmv H2S (Lauretta et al., 1996a). The experimental data for Fe and Gibeon metal are used to model the reaction time of Fe alloy grains in the solar nebula as a function of grain size and temperature. The reaction times for 0.1-1 μm radius metal grains are generally within estimated lifetimes of the solar nebula (0.1-10 Ma). However, the calculated reaction times are probably lower limits, and further study of magnetite formation at larger H2/H2O ratios, at lower temperatures and pressures, and as a function of metal alloy composition is needed for further modeling of nebular magnetite formation.