Plate 14 



1. Tazewell, Tennessee; finest octahedrite. An old analysis (1855) gives Ni-Co 15.28 

 percent, but this is almost certainly too high, as no other iron with even 14 percent shows a 

 typical octahedral structure. Apparently the Ni-Co content should be less than that of 

 Cowra (pis. 17, 54). 



The photograph shows the general pattern at moderate magnification and very light etch- 

 ing, which reveals only the kamacite bands. The graying of the bands is probably due to 

 cold working, the dragging of the abrasive having produced a thin film of worked metal, 

 which is dissolved away by stronger etching; see figure 2. The same efi^ect may be seen in 

 San Cristobal (pis. 29, 30). Picral 5 seconds; X 30. 



2. Tazewell. An area similar to the preceding, strongly etched. The kamacite bands 

 have now become clear. The very abundant taenite lamellae, now Aisible, are slightly gray 

 because their transformation could not be fully completed by reason of the rigidity of the 

 mass at the low transformation temperature. All plessite is fine to dense. In the upper- 

 most fields the unresolved black needles of the gamma-alpha mixture in a lighter ground are 

 oriented. Picral IS seconds; X 30. 



3. Tazewell. .\t higher magnification the graying of taenite and the acicular pattern 

 in plessite fields are more apparent. Picral 20 seconds; X 60. 



4. Tazewell. Plessite with oriented acicular structure. The black needles are partly 

 resolved. The black gamma-alpha mixture is also finely disseminated in the lighter ground. 

 Picral 20 seconds; X 300. 



134 



