Phase Decomposition in the Fe-rich Fe-Ni-S System from 900 Degrees C to 300 Degrees C--Application to Meteoritic Metal
It has been observed that metal particles in ordinary chondrites contain essentially no P and that the tetrataenite rim of the metal particles is much wider than that in other types of meteorites, especially when the taenite rim abuts troilites (FeS) . It is possible that S plays an important role in the formation of the zoned tetrataenite at low cooling temperatures. Most of the studies of the Fe-Ni-S system have concentrated on high temperature and high Ni-high S part of the ternary diagram . In this study we have systematically investigated the microstructure and microchemistry of the Fe-rich Fe-Ni-S system in regions where meteoritic metal forms from 900 degrees C down to 300 degrees C. High spatial resolution electron probe microanalysis (EPMA) and analytical electron microscopy (AEM) techniques were employed. The two and three phase boundaries at high temperatures (900 degrees C to 600 degrees C) are consistent with previous studies. However, at 500 degrees C, an Fe-Ni phase with 51.6 +/- 1.4 wt.% Ni was observed to form along some of the g/g and g/FeS boundaries. The size of this Fe-Ni phase is as large as 10 micrometers in width. AEM analysis indicates that this Fe-Ni phase may have even higher Ni content, 56 wt.%. In addition, the phase has a FCC structure and is disordered. Because the composition of this phase is very close to the stoichiometric composition of FeNi, it is very likely that the phase is tetrataenite. High Ni precipitates with similar morphology were also observed in the Fe-Ni-S alloy aged at 400 degrees C. However, the Ni content is 60.9 +/- 4.0 wt.% measured with EPMA, which is much higher than that in the corresponding 500 degree C sample. The fact that all the high Ni precipitates formed at boundaries of g/g or g/FeS indicates the boundaries are favorable energy nucleation sites. Such a high Ni phase with a Ni content over 60 wt.% has not been observed in the Fe-Ni and Fe-Ni (P) systems above 400 degrees C. The tetrataenite phase forms in the Fe-Ni-S system at higher temperatures (500 degrees C) than in the Fe-Ni and Fe-Ni (P) systems (<400 degrees C) . Because the tetrataenite phase forms in the Fe-Ni-S system at higher temperatures, the tetrataenite rim in chondrites should be much wider than that in other meteorites where phase growth is controlled by the presence of P. It is possible that the Ni distribution in the metal phases of chondrites is dictated by the g/FeS and g/FeS/g' tie-line variations rather than by a/g tie-lines in the case of iron or stony iron meteorites. References:  Holland-Duffield C. E. et al. (1991) Meteoritics, 26, 97-103.  Kullerud G. (1963) Carnegie Inst. Wash. Yearb., 62, 175-189.  Clark L. A. and Kullerud G. (1963) Econ. Geol., 58, 853-885.  Yang C. W. (1994) Ph.D. dissertation, Lehigh Univ.
- Pub Date:
- September 1995
- X-RAY MICROANALYSIS