Abstract
The kinetics and mechanisms of kamacite sulfurization were studied
experimentally at temperatures and H2S/H-2 ratios relevant to the
solar nebula. Pieces of the Canyon Diablo meteorite were heated at
558 K, 613 K, and 643 K in 50 parts per million by volume (ppmv)
H2S-H-2 gas mixtures for up to one month. Optical microscopy and
x-ray diffraction analyses show that the morphology and crystal orientation
of the resulting sulfide layers vary with both time and temperature.
Electron microprobe analyses reveal three distinct phases in the
reaction products: monosulfide solid solution (mss), (Fe,Ni,Co)(1-x)S,
pentlandite (Fe,Ni,Co)(9-x)S-8, and a P-rich phase. The bulk composition
of the remnant metal was not significantly changed by sulfurization.
Kamacite sulfurization at 558 K followed parabolic kinetics for the
entire duration of the experiments. Sulfide layers that formed at
613 K grew linearly with time, while those that formed at 643 K initially
grew linearly with time then switched to parabolic kinetics upon
reaching a critical thickness. The experimental results suggest that
a variety of thermodynamic, kinetic, and physical processes control
the final composition and morphology of the sulfide layers. We combine
morphological, x-ray diffraction, electron microprobe, and kinetic
data to produce a comprehensive model of sulfide formation in the
solar nebula. Then, we present a set of criteria to assist in the
identification of solar nebula condensate sulfides in primitive meteorites.
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