THE METALLOGKlAlPHY OF METEORIC IRO-N 67 



A two-phase structure. — The conclusion seems inescapable that 

 fine and dense plessite, and nickel-rich ataxites, all represent varia- 

 tions in a two-phase equilibrium stnicture. 



With the nickel content of coarse octahedrites the transformation 

 would be completed at a relatively high temperature, at which the 

 atomic mobOity of the mass would permit a comparatively rapid 

 and perfect phase separation — that is, ligiit or "normal" plessite 

 with a coarse structure, the taenite clear because transformation is 

 complete and an equilibrium structure has been attained. 



In a high-nickel iron, however, the process would be much de- 

 pressed, and the gamma-alpha transformation thus retarded to a 

 temperature at which there would be greatly reduced atomic mobility. 

 In a mass of such rigidity the transformation would remain incom- 

 plete, and instead of a complete separation of the two phases (as in 

 the fine kamacite bands and the lamellae and skeletal growths of 

 taenite in light plessite fields) the product would be a fine ag-gregate 

 of kamacite and taenite. 



This aggregate first takes the form of a thick dispersion of taenite 

 dots in kamacite, forming the darker but easily resolvable plessite, 

 often banded. With a higher nickel content there is a more refined 

 structure, the dots bemg so minute and thickly crowded that we have 

 dense plessite, resolvable only with high magnification. With nickel 

 above 15 percent (sometimes less) a more or less perfect paraeutec- 

 toid structure is produced, that of the nickel-rich ataxites, when 

 there has been a sufficiently long sojourn at lower temperatures. 



Transformation is retarded by rapid cooling to a low temperature 

 when diffusion is slight and a dense black structure resembling sorbite 

 or troostite in artificial irons is produced. It ordinarily is unresolved, 

 but it is more or less resolved at high magnification (pis. 12, 14, 16, 43). 

 It may show a coarse acicular oriented pattern, but the black needles 

 themselves are composed of an extremely fine ga nma-alpha aggregate. 

 Such retarded or arrested transformation also accounts for the dark- 

 ened spots and cores so often observed in taenite bodies ('spotted" 

 taenite). When the transformation is nearly, but not absolutely 

 complete, a slight graying of taenite may appear. 



Although dark plessite does not depend upon the presence of 

 phosphorus, all plessite fields are remnants of the gamma solid solu- 

 tion after the separation of the kamacite bands and taenite lamellae, 

 and if the mass contained phosphorus it must have been more largely 

 concentrated in these mterstitial remnants. That would tend to 

 promote the formation of paraeutectoid structures by depressing 

 the gamma-alpha transformation. The same structures, however, 

 form in nearly and quite phosphorus-free irons. 



