Plate 21 



1. Illinois Gulch, Montana; nickel-rich ataxite; Ni-Co 13.48 percent. General struc- 

 ture at moderate magnification, a paraeutectoid ground with oriented needles and spindles 

 of kamacite. In many places such inclusions are absent. Picral etching was ineffective 

 with this iron, bringing out no structure, as is the case with high-nickel artificial alloys. 

 The etching was done with nitric-acetic acid, which is used on high-nickel steels to develop 

 grain structure. In some places this iron shows faint gamma-grain boundaries at low 

 magnification. Nitric-acetic acid 10 seconds; X 60. 



2. Illinois Gulch. Part of area shown in figure 1, same etching, X 600. The very 

 irregular gamma-alpha structure reflects imperfect transformation. The outlines of gamma 

 grains appear strongly. 



3. Deep Springs. North Carolina; nickel-rich ataxite; Ni-Co 14.14 percent. Typical 

 structure, a fine, somewhat oriented, groundmass with an occasional needle (lamella) of 

 kamacite. The dark area at the left is due to invasion of h\'droxide. The iron darkens 

 quickly in etching. Light picral; X 60. 



4. Deep Springs. Part of the area shown in figure 3. A paraeutectoid gamma-alpha 

 structure, the phases in about equal proportions. The graying of the taenite indicates slight 

 supersaturation. The kamacite needle (lamella) is bordered by clear taenite, growing gray 

 at the outer edges, then merging with the taenite of the groundmass. Light picral; X 600. 



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