Computer Science
Scientific paper
May 1964
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1964gecoa..28..699l&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 28, Issue 5, pp.699-708
Computer Science
4
Scientific paper
To evaluate the usefulness of cohenite as a pressure indicator in iron meteorites, the thermodynamics and kinetics of its decomposition have been re-examined. Laboratory experiments on the decomposition of Fe 3 C at high pressures ( and , 1961b) failed to show the stabilization predicted by ringwood (1960), and it has therefore been necessary to revise the high-pressure portion of the Fe-C diagram. The stability field of Fe 3 C becomes greatly constricted, and the stabilisation of cohenite by high pressures no longer seems feasible. If iron meteorites had cooled slowly at pressures of 30-60 kb, and , 1962), cohenite should have decomposed in a matter of hours to graphite or diamond, at temperatures below 565-640°C. The times actually available for decomposition were longer by factors of 10 10 -10 11 , and the survival of cohenite therefore seems to require the assumption that nucleation was inhibited by a factor of this order. If the iron meteorites had originated at low pressures, precisely the same assumption (nucleation inhibited by a factor of 10 10 -10 11 ) must be made. Hence there is no longer any reason to attribute the survival of cohenite to high pressures. Laboratory experiments show that the decomposition of cementite at 680° and 1 atm can indeed be inhibited by factors > 10 5 . It does not seem unreasonable that meteoritic cohenite, having a much higher degree of crystal perfection, should be inhibited in its decomposition to an even greater extent.
Anders Edward
Lipschutz Michael E.
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