Iron-carbon interactions at high pressures and temperatures

Details Iron-carbon interactions at high pressures and temperatures Iron-carbon interactions at high pressures and temperatures

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmmr51c..03r&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #MR51C-03

Other

3611 Thermodynamics (0766, 1011, 8411), 3924 High-Pressure Behavior, 3929 Nmr, Mossbauer Spectroscopy, And Other Magnetic Techniques, 3934 Optical, Infrared, And Raman Spectroscopy, 3954 X-Ray, Neutron, And Electron Spectroscopy And Diffraction

Scientific paper

Phase relations in the iron-carbon system at one atmosphere have been the subject of a number of investigations for metallurgical applications. This system contains intermediate phases, e.g., Fe3C (cementite), that are metastable with respect to á-iron and graphite. In nature, Fe3C (cohenite) occurs as a minor constituent in iron meteorites. At pressures up to 25 GPa and temperatures to 1900 oC, iron and carbon react and produce several carbides (Fe3C, Fe7C3). Since Wood's suggestion [1] that iron carbide may be stable in the Earth's inner core, physical properties of Fe3C have been intensely studied. Recent experimental and theoretical investigations on Fe3C provide new constraints on its geochemical significance. The stability of cementite at high pressures implies incompatibility of the iron-diamond assemblage. On the other hand, the Fe3C phase has not yet been found in inclusions in diamonds from the deep mantle, whereas diamond and metallic iron have been observed to coexist. The cementite-diamond relationship could be viewed as an incongruent transformation, Fe3C = Fe-rich phase + C, that occurs at 1630 K and 5.3 GPa [2]. In this contribution, experimental and thermodynamic evidence is presented for the stability of Fe3C at high pressures from diamond anvil cell (DAC) and multi-anvil press investigations. Experiments in the laser-heated DAC using metallic iron as starting material yielded the iron carbide, by reaction between iron and the diamond enclosure. This unusual reactivity between metallic iron and the diamond form of carbon has a great importance for DAC experiments in general. This effect must be routinely considered in all experiments including those on iron-based compounds in the laser-heated DAC. These results point to additional factors in experimental methodology and/or natural observations that need to be investigated. [1] B.J. Wood, Earth and Planetary Science Letters 117, 593 (1993). [2] Y. Long-Wei et al., Journal of Crystal Growth 234, 1 (2002).

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