40 BUULETnsr 184, UNTITED STATES NlATIONlAL MUSEUM 



that typical meteoritic structures represent an equilibrium between 

 low and high nickel phases, the former kamacite with 6 percent, the 

 latter taenite with 26 to 30 percent nickel. 



The nickel-rich phase, corresponding with taenite, in the diagram 

 in International Critical Tables (1927), indicates a percentage of 

 34.5, corresponding with the formula Fe2Ni. That point also has 

 been found to be marked by a change in dilatation curves, which tends 

 to support the views of those who have favored the theory of an 

 iron-nickel intermetallic compound of definite composition. 



The foregoing views are cited to show that arguments still can be 

 adduced in support of the assumption of the existence of an inter- 

 metallic iron-nickel compound as one of the components of the 

 paraeutectoid. But with full deference to these authorities, the 

 prevailing view now seems to be that there is no such compound but 

 that in an alpha-gamma equilibrium structure the gamma phase is 

 variable in composition, depending upon the stage of the transfor- 

 mation. Thus whether taenite contains 13 or 55 percent nickel (the 

 extremes found by analysts) depends upon the temperature at which 

 it segregates and the rapidity of cooling. Therefore an ataxite 

 having the appearance of a true eutectoid of very fine and regular 

 pattern (e. g., Ternera, Linville, Morradal) is, after all, merely a 

 two-phase structure in which certainly the gamma component has 

 no fixed composition. 



An eclectic iron-nickel diagram. — In figure 5 an artificial iron- 

 nickel diagram is presented that, in the author's opinion, harmonizes 

 to a fair degree the more recently accepted views of metallographers. 

 It resembles the diagram presented by Marsh (1938, p. 53) as sum- 

 marizing the data of eight or nine recent investigators. 



The line AC, below about 500°, might well be dotted to indicate 

 that it is conjectural rather than exact; for the authorities are not 

 yet in agreement. Thus the recent diagrams of Owen and Sully 

 (1939) and of Bradley and Goldschmidt (1939) differ in important 

 respects from the diagram here presented. 



It is to be emphasized that this diagram, although reflecting the 

 prevailing views, is inconsistent with the transformation of meteoric 

 iron. Thus, according to the diagram, kamacite might contain 10 or 

 even 15 percent of nickel, whereas it actually contains not more than 

 about 6 percent. 



In the early diagrams the Ime AC drops almost vertically from a 

 point corresponding with 400° or 500° and less than 10 percent of 

 nickel, and in the diagram of Owen and Sully it drops from a point 

 corresponding with about 400° and 6 percent, approximately the point 

 E in figure 5. The diagram would be more nearly applicable to mete- 

 oric u'on if so drawn; for with less than about 6 percent nickel it is 



