Plate 28 



1. MoRRADAL, Norway; nickel-rich ataxite; Ni-Co 19.95 percent. The general structure 

 of the iron is a very fine gamma-alpha paraeutectoid. Groups of kamacite needles, as 

 shown here, are exceptional; the groundmass usually is uniform with no inclusions. Picral 

 20 seconds; X 60. U. S. National Museum. 



2. MoRRADAL. Typical area of groundmass at high magnification. A regular para- 

 eutectoid structure is fully developed, the gamma phase predominating. The taenite is 

 slightly darkened because of supersaturation. Picral 10 seconds; X 600. 



.1. Freda, North Dakota; nickel-rich ataxite; Ni-Co 24.15 percent. General structure, a 

 very fine acicular groundmass In which are scattered abundant spindles and irregular 

 inclusions of kamacite, many of which enclose rounded particles of iron-phosphide eutectic. 



The acicular groundmass is analogous to the transformation structures in numerous 

 octahedrites. A tentative explanation of its origin might be the following: Owing to the 

 high-nickel content the gamma-alpha transformation was depressed to the neighborhood 

 of 350°, the rate of cooling being very slow. At that point the transformation began, 

 the primary separation of kamacite occurring in many places around previously separated 

 phosphide bodies. After the initial transformation the remaining gamma solid solution 

 underwent only a partial transformation, the kamacite being very sparingly precipitated 

 on octahedral planes, producing the acicular structure. This may have been due to, or pre- 

 served by, more rapid cooling from that point downward. Such an origin, as well as the 

 final structure, would be analogous to that of iron-carbon martensite in artificial irons. 



The kamacite bodies are without the taenite borders common in high-nickel ataxites, 

 but are surrounded by considerable areas of taenite, which appear clear. These might be 

 termed areas of kamacite impoverishment, the kamacite segregations having absorbed in 

 their growth all the alpha phase in the vicinity. 



Picral 25 seconds; X 60. U. S. National Museum. 



4. Freda. The structure shown in figure 3; very light picral and sodium picrate; X 150. 

 The minute inclusions are shown to be phosphide eutectic, the excess of iron having been 

 rejected in minute clear drops. The kamacite bodies themsehes show a considerable 

 phosphide content. 



148, 



