THE GROWTH OF THE WIDMANSTATTEN PATTERN IN METALLIC METEORITES,

The effects of pressure, temperature and time on the formation of the Widmanstatten pattern found in metallic meteorites have been established. A means of analysis, using the method of finite differences, was developed for the study of the diffusion-controlled growth of the Widmanstatten pattern. The growth analysis accounted for 1 the change of D with Ni concentration 2 the change of D and the Fe-Ni phase diagram with pressure 3 the average meteorite composition 4 the radius of the parent body and the pressure within it 5 the degree of undercooling before precipitation took place and 6 the distance between precipitates. The individual effect of each of these factors was determined and displayed in the form of parametric curves. Two cooling models with low internal pressure and two with high internal pressure were examined for the formation of the Widmanstatten pattern. These models were used to determine the temperature-time relationship in the growth analysis. Excellent agreement between the measured and the calculated taenite composition gradients was found for the low-pressure 12 kb models. The formation of plessite, the development of the two phase structures in ataxites, and the decrease in Ni concentration in the kamacite near the alphagamma boundary are all explained in terms of a low-pressure model. Author