48 



BULLETIN 184, UNITED STATES NATION^AL MUSEUM 



An attempt has been made to indicate the composition ranges 

 of these structures, in connection with the iron-nickel diagram, in 

 figure 8. 



Ataxites are designated as nickel-rich to distinguish them from 

 certain low-nickel irons, usually hexahedrites, that have lost their 

 normal structure through reheating and which are designated as 

 nickel-poor ataxites. Such a structure is secondary and not a prod- 

 uct of the orderly transformations that have been discussed. 



900 



800 



700 



600 



500 



5 400 



300 



200 



100 



hexahedrites 

 m;-poor atavites , 



^WxWXX^v><y>^wS(^<^o<x^<xx>^ 



OCTAHEDRITES | 



COARSE FINE ] ' 



1 NICKEL-RICH 



ATAYITES 



PARAEUTECTOID + KAMACITE 



PARAEUTECTOID + TAENITE TAENITE 



5 10 15 20 25 30 



PERCENT NICKEL 

 Figure 8. — Composition ranges of meteoric irons. 



Crystallization in meteorites. — The crystallization of artificial irons 

 begins with the formation of dendrites from the melt, which change 

 into grains of body-centered delta iron. As atomic mobility is great, 

 the grains are large. In the gamma phase the grains become face- 

 centered and on further cooling transform into body-centered alpha 

 iron. This change is accompanied by recrystallization, the result 

 being an aggregate of grains that may be similar in size to the earlier 

 gamma grains, or smaller, depending upon the conditions of cooling. 

 The final product is an aggregate of grains (crystallites), but not 

 always uniformly oriented. 



