Plate 18 



1. Bacubirito, Mexico; finest octaliedrite; Ni-Co 10.38 percent. Though clearly an 

 octahedrlte, and with a markedly lower nickel content than some other typical finest octa- 

 hedrites, this iron is close to the borderline between octahedrites and ataxites. In some 

 areas the structure is much more irregular and confused than is here shown, in other places 

 more regular. Picral 60 seconds; X 30. 



2. Ballinoo, Australia; finest octahedrite (or ataxite); Ni-Co 10.47 percent. Though 

 described as a finest octahedrite, this iron possesses only a vestigial octahedral structure 

 and may be regarded as transitional. Compare Monahans, below; also Wiley (pi. 19). 

 Picral 40 seconds; X 60. 



3. Monahans, Texas; nickel-rich ataxite; Xi-Co 11.51 percent. This iron is transitional 

 between finest octahedrites and ataxites. The taenite lamellae in the photograph show 

 vestigial octahedral orientation. The general structure of the iron, which in many places 

 has no taenite lamellae, consists of minute kamacite bodies in dense plessite ot in taenite. 

 Picral 60 seconds; X 60. 



4. Monahans. Part of the groundmass shown in figure 1. Plessitic areas show an 

 imperfectly transformed gamma structure. In other places, they are clear, composed of 

 homogeneous taenite. Practically the same structures are found in Bacubirito, Ballinoo, 

 and Cowra. Picral 20 seconds; X 300. 



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