Plate 16 



1. Laurens County, South Carolina, finest octahedrlte; Ni-Co 14.21 percent. Part of 

 the oriented plessite area in plate 15, figure 4. The structure is one of taenite particles, in 

 kamacite, the former often darkened because of supersaturation, with areas of clear kamacite. 

 Picral 15 seconds; X 300. U. S. National Museum. 



2. Laurens County. Plessite with paraeutectoid structure, taenite particles in a ground 

 of kamacite. The structure apparently formed at a temperature at which diff'usion was 

 sufficiently active to permit a very perfect segregation of the two phases. The taenite 

 shows no darkening, indicating no supersaturation. Picral 15 seconds; X 300. 



3. Laurens County. Paraeutectoid plessite, which, perhaps because of higher nickel 

 content, transformed at a lower temperature than that in figure 2. With a lower diffusion 

 rate segregation was impeded, resulting in a finer structure. The particles show strong 

 orientation. This type of plessite is more common than the former in Laurens Cotrnty. 

 Picral 15 seconds; X 300. 



4. Laurens County. Two kamacite spindles in paraeutectoid plessite, with taenite 

 borders, which merge with the taenite of the surrounding structure. In this photograph 

 it is easy to trace the two components of the groundmass, which is often difficult in such 

 structures. Some slightly darkened areas merge with the clear taenite bordering the 

 spindles. The taenite in the border is clear because the excess of the alpha phase (kama- 

 cite) was completely absorbed into the kamacite spindle; farther away some of the alpha 

 phase remained, leaving the taenite slightly supersaturated. At the tip of the upper spindle 

 the kamacite composing it extends as if by a narrow channel, widening into the clear ground- 

 mass in which small taenite bodies appear as islands. Picral 15 seconds; X 300. 



136 



