THE METALLOGKiAPHY OF METEORIC IRON 65 



medium to fine octahedral pattern. No analysis or microscopic 

 examination was made of this grain, but obviously its nickel content 

 is higher than that of the general mass. 



In the author's opinion the criterion of an octahedrite should be 

 the presence of a clear and dominant octahedral pattern. Compo- 

 sition alone is not an exact criterion; for the proportions of Ni-Co 

 in the two groups overlap, taenite and plessite may be lacking in a 

 clearly marked octahedrite (e.g., Osseo, Ni-Co 6.62 percent), and 

 again both may be present in a typical hexahedrite (e.g., Otumpa, 

 Ni-Co 5.68 percent). It must remain a matter of opinion in any 

 given case whether an iron showing signs of an indistinct, subsidiary 

 or localized octahedral structure should be termed a hexahedrite or 

 a transitional coarsest octahedrite. 



A similar uncertainty attaches to the classification of high -nick el 

 transitional types. Here again the criterion should be the presence 

 of what the observer regards as a definite octahedral pattern of 

 kamacite bands. It is not enough that spindles or needles of kama- 

 cite should show octahedral orientation; that appearance is often 

 found in very high-nickel ataxites — even including San Cristobal 

 (Ni-Co 26.60 percent) and Limestone Creek (Ni-Co 30.47 percent). 

 Indeed if kamacite particles in a high-nickel iron (above 10 percent) 

 show any traces of crystalline orientation it would necessarily be 

 octahedral; for in such irons the atoms become "frozen in" and dif- 

 fusion practically ceased around 500°, and thus any orientation 

 remaining at room temperature would be in conformity with the 

 octahedral planes of the gamma phase. 



Thus Cowra (Ni-Co 14.25 percent), which is classed as a finest 

 octahedrite, seems to the author clearly to be an ataxite, differing 

 in no essential respect from Wiley and Monahans. Laurens County 

 (Ni-Co 14.21 percent), on the other hand, though exhibiting some of 

 the same structural features as the above three, is clearly an octa- 

 hedrite because it has a striking Widmanstatten pattern. It is pos- 

 sible that the new analyses of these irons would show a higher 

 nickel-cobalt content in the former than in the latter. 



XII. PLESSITE; NICKEL-RICH ATAXITES 



Genesis of plessite. — Plessite may be regarded as a supersaturated 

 solid solution of taenite with respect to kamacite, its form depending 

 upon the conditions of temperature and rate of cooling. Though 

 occurring characteristically in octahedrites, it is not necessarily a 

 part of the Widmanstatten structure ; for, as mentioned in Chapters 

 III and X, it is sometimes found in hexahedrites. 



In octahedrites the most convincing explanation is that plessite 



