Plate 66 



1. Locust Gro\e. Part of the acicular border zone shown In plate 65, figure 1, at higher 

 magnification. The structure is due to a precipitation of carbide along crystallographic 

 planes. These planes probably are not octahedral, as the carbide in artificial irons has been 

 found to precipitate along planes with indices of approximately (722). Nital 4 percent, 

 12 seconds; X 120. 



2. Locust Grove. An area similar to that shown in figure 1, with additional etching 3 

 minutes with alkaline sodium picrate. The picrate darkens the precipitate, showing that 

 it is carbide (cohenite, cementite), a characteristic test for carbide in artifical irons. The 

 structure resembles that of a 3 to 5 percent nickel carburized steel. 



The conclusions, both as to this structure and as to the Fe-P-C eutectic structure in the 

 preceding figures, are consistent with the composition of this iron, which contains 0.02 per- 

 cent of carbon and 0.18 percent of phosphorus. As the inclusion is a ^ot of carbon en- 

 richment, the percentage of carbon locally might be much higher than the average for the 

 whole mass as reflected in the analysis. 



3. Chesterville, South Carolina; nickel-poor ataxite; Ni-Co 6.25 percent. .\ spot of 

 carbon enrichment. The upper rounded light area is essentially a white phosphoretic cast 

 iron. The dendrites, probably pearlitic, appear in a white matrix which may be an Fe-FcaC 

 eutectic, or perhaps the Fe-C-P eutectic known in artificial irons as steadite. The dark 

 coarsely granular area surrounding it is the equivalent of about a 0.55 percent carbon steel; 

 dark pearlitic grains with white ferrite (kamacite) along their boundaries which increases in 

 proportion outwards. The outermost light granular area (below) is the equivalent of about 

 a 0.25 percent carbon steel in which ferrite predominates; it contains a few small inclusions 

 of phosphide (not steadite). 



Of three analyses by Sjostrom (in Cohen, 1898) two showed 0.34 phosphorus and two 

 0.02 percent carbon. One of the three showed no phosphorus, and one no carbon. The 

 variations apparently were due to the presence or absence of cohenite and schreibersite 

 inclusions in the material analyzed. Picral 80 seconds; X 30. \j. S. National Museum. 



4. Chesterville. Part of the rounded area shown in figure 3 at higher magnification. 

 Picral 40 seconds; X 150. 



186 



