Troilite Formation Kinetics and Growth Mechanism in the Solar Nebula
Abstract
Troilite formation via the reaction Fe(s) + H2S(g) = FeS(s) + H2(g) is the major mechanism for sulfur retention in grains in the solar nebula. Thermodynamic calculations predict that troilite condenses from a solar composition gas [1-3]. Here we present experimental results on the kinetics and growth of troilite crystals on iron metal at temperature (450-650 degrees C) and composition (50-1000 ppm H2S in H2) conditions similar to those in the solar nebula. Figure 1a plots the fraction of iron reacted (based on gravimetric data) at 450, 505, 575, and 650 degrees C and Fig. 1b plots the thickness change of unreacted iron (measured by optical microscopy) at 575 and 650 degrees C vs. time. The weight change per unit area varies as the square root of time at the lower temperatures and varies linearly with time at the highest temperature. The growth behavior along the lower isotherms is due to diffusion. This behavior was noted by previous investigators though [4] suggests sulfide diffusion to the metal-sulfide interface and [5] suggests Fe^2+ diffusion to the sulfide-gas interface. The reaction along the highest isotherm appears to be interface controlled. The formation of troilite crystals is a rapid process forming measurable layers in a few hours. However, the crystal growth is complicated. Initially, there are intergrowths of troilite into the pure iron metal. As the reaction progresses two distinct layers of troilite crystals form (see Fig. 2). One is in contact with the iron metal and consists of small randomly oriented crystals with pore space between them. The outermost layer contains large crystals that are all oriented in the same direction. The intergrowth layer is much smaller at 650 degrees C than at 575 degrees C. This suggests that FeS nucleation is inhibited at the higher temperature, accounting for the initially slower reaction rate. Once nucleated, the reaction kinetics are apparently controlled by the growth of the crystals at the interface. Acknowledgments: This work was supported by a grant from the NASA Origins of Solar Systems Program to Washington University (B. Fegley, Jr., P.I.). We acknowledge advice from K. Lodders and K. Kelton and assistance from D. Kremser and R. Poli. References: [1] Lewis J. S. (1972) Earth Planet. Sci. Lett., 15, 286-290. [2] Larimer J. W. (1967) GCA, 31, 1215-1238. [3] Lauretta D. and Fegley B. Jr. (1994) LPS XXV, 773-774. [4] Haycock E. W. (1959) J. Electrochem. Soc., 106, 764-771. [5] Worrel W. L. and Turkdogan E. T. (1968) Trans. Metal. Soc. of AIME, 242, 1673-167.
- Publication:
-
Meteoritics
- Pub Date:
- July 1994
- Bibcode:
- 1994Metic..29R.490L
- Keywords:
-
- Antarctic Regions;
- Chemical Composition;
- Chondrites;
- Meteoritic Composition;
- Petrology;
- Glass;
- Metamorphism (Geology);
- Olivine;
- Weathering;
- Lunar and Planetary Exploration;
- GAS-SOLID REACTIONS; KINETICS; NEBULAR CHEMISTRY; SOLAR NEBULA; TROILITE