Article,
Experimental studies of magnetite formation in the solar nebula
METEORITICS & PLANETARY Science, 33 (5): 1101--1112 (1998)
Abstract
Oxidation of Fe metal and Gibeon meteorite metal to magnetite via
the net reaction 3 Fe (metal) + 4 H2O (gas)= Fe3O4 (magnetite)+ 4
H-2 (gas) was experimentally studied at ambient atmospheric pressure
at 91-442 degrees C in H-2 and H-2-He gas mixtures with H-2/H2O molar
ratios of similar to 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: GRAPHICS These rate
constants are significantly smaller than the parabolic rate constants
for FeS growth on Fe metal in H2S-H-2 gas mixtures containing 1000
or 10 000 ppmv H2S (Lauretta ef 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 mu m 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 H-2/H2O ratios,
at lower temperatures and pressures, and as a function of metal alloy
composition is needed for further modeling of nebular magnetite formation.
