246 



ANTSrUAL REPORT SMITHSONIAN INSTITUTION, 1942 



111 figure 1, the location of the line AB, separating the a phase, 

 kamacite, and the a+y phase, kamacite and taenite, rather clearly 

 defines the chemical compositions for the two groups given in tables 

 1 and 2. Since the conditions of heating and rate of cooling of me- 

 teorites are not understood, they cannot be used to modify the nickel- 

 iron diagram prepared under a controlled environment. However, 

 their compositions and structures should be consistent with the com- 

 positions of their respective fields on the phase diagram. 



300 



SCO 



700 



600 



50O 



400 



300 



200 



o 



/o 



20 



30 



40 



SO 



60 



70 



SO 90 /OO 



FiGtTBE 1. — Equilibrium diagram of iron-nickel system below 1000° C. Composi- 

 tion expressed on horizontal line is the percentage of nickel in iron. 



Ataxites. — Ataxites lack the precise structural patterns of either 

 hexahedrites or octahedrites. Perhaps some day this group, which 

 varies widely in its nickel content, will be simplified ; however, at the 

 present time into this class fall all the irons that cannot readily be 

 classified as belonging to either of the other classes. It is significant 

 that the cobalt-nickel content for some ataxites closely parallels that 

 of hexahedrites. In certain other ataxites there is a vestige of octa- 

 hedrite pattern and also a chemical composition that lies within the 

 range for octahedrites. In addition to the above there are several 



