Plate 15 



1. Laurens County, South Carolina; finest octahedrite; Ni-Co 14.21 percent, P 0.16 

 percent. It may be doubted whether any iron properly classified as an octahedrite has a 

 higher Ni-Co content than Laurens County and Cowra (14.25 percent). Though in Laurens 

 County (unlike Cowra) the octahedral structure is clear, the plessitic groundniass more 

 closely resembles that of ataxites. The photograph shows the general structure of very 

 fine kamacite bands, widely spaced in plessite, which mostly shows a uniform paraeutectoid 

 structure like that adjacent to the kamacite bands in the picture. In the centers of many 

 plessite fields are areas of oriented kamacite needles, of which two are shown. The plessite 

 within such areas is very dense (black), the small segregations of kamacite having left a 

 groundniass of higher nickel content, which therefore remained untransformed to a lower 

 temperature at which atomic mobility was so slight that the minute gamma-alpha particles 

 remained discrete. With strong etching the taenite lamellae show gray because of super- 

 saturation with respect to the alpha phase. Picral 60 seconds; X 30. \]. S. National 

 Museum. 



2. Laurens County. Part of a fine area of kamacite needles, similar to that in the left 

 portion of figure 1. Some interstices are filled with light paraeutectoid plessite; the dense 

 black areas are unresolved. Picral 25 seconds; X 60. 



3. Laurens County. Kamacite band with (upper left) lighter plessite; and (lower 

 right) denser plessite octahedrally oriented. The abundance of taenite in this iron is reflected 

 in the lamellae within the band and the wide taenite border along the interfaces. Small 

 areas of dense black plessite. Picral 25 seconds; X 60. 



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