The effect of phosphorus on the formation of the Widmanstätten pattern in iron meteorites

Details The effect of phosphorus on the formation of the Widmanstätten pattern in iron meteorites The effect of phosphorus on the formation of the Widmanstätten pattern in iron meteorites

Jan 1972

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1972gecoa..36...51g&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 36, Issue 1, pp.51-60

Computer Science

6

Scientific paper

A revised Fe-Ni-P phase diagram and laboratory cooling experiments on Fe-Ni-P alloys are combined to determine the effect of P on the formation of the Widmanstätten pattern. From the phase diagram results, two reaction paths were found for the formation of kamacite (1) (taenite) (kamacite) + (taenite) and (2) + Ph (phosphide, (FeNi) 3 P) + + Ph. The reaction path + is preferred at low P contents while at higher P contents and at Ni contents greater than 7·0 wt. %, the reaction path + Ph + + Ph controls the formation of kamacite. Above 7 wt. % Ni, the effect of P on the equilibrium nucleation temperature of kamacite is quite small, less than about ± 30°C with respect to the Fe-Ni binary diagram. We were able to produce kamacite during direct cooling of alloys of meteoritic composition containing as little as 0·1 wt. % P. This is the first report of a successful synthesis of the Widmanstätten pattern. Kamacite nucleates initially at prior taenite grain boundaries with little or no undercooling. A Widmanstätten pattern was produced in many of these same alloys only after undercooling 30 to more than 100°C below the equilibrium nucleation temperature. The addition of P (> 0·1 wt.%) to meteorites promotes nucleation of kamacite at higher temperatures and effectively lowers the amount of undercooling necessary to nucleate kamacite homogeneously. Ni has just the opposite effect, decreasing the temperature of nucleation and increasing the amount of undercooling. Our experimental results indicate that (1) significant amounts of undercooling, 50-100°C, are necessary for the nucleation of the Widmanstätten structure in meteorites, and (2) chemical equilibrium is maintained in the various phases of slowly cooled iron meteorites to 650°C and probably to 600°C. The effect of P on the cooling rate calculations for iron meteorites and pallasites appears to be small, that is within the factor of 2 error currently ascribed to the method. Since the amount of P determines the Ni content of kamacite in equilibrium with taenite, one can no longer use the binary phase diagram and the measured Ni contents at kamacite/taenite interfaces to obtain the lowest equilibrium temperature during cooling.

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