THE' METAliLOGRAPHY OF METEORIC IRON 45 



meteoric irons. Thus, a medium or fine octahedrite with 7 or 8 

 percent of nickel could have developed its elaborate structure within 

 a range of only about 30° around 600° and must have retained it 

 intact while cooling from 600° to 500°, within which range there 

 still is considerable atomic mobility and therefore more or less active 

 diffusion. Such conditions of production are not indicated by the 

 observed structures. 



Below 350° the diagram indicates a gamma phase (taenite) with 

 about 35 percent nickel, and at about 25 percent an ordered gamma- 

 alpha structure designated as ai, corresponding with the eutectoid 

 ratio assumed by the earlier diagrams of Pfann and others. From 

 10 to 25 percent there is alpha (kamacite) with increasing proportions 

 of ai] above 25 percent ai with increasing gamma. 



IX. METEORIC NICKEL-IRON 



Although the structures of meteoric irons show variations due to 

 the presence of other substances (especially phosphorus), the iron- 

 nickel diagram is the basis for their study and interpretation. An 

 artificial iron-nickel alloy is commonly homogeneous, regardless of 

 composition or temperature. In the natural alloy, however, a 

 strongly marked heterogeneous structure may develop, caused by 

 the separation of one phase from another during transformation — 

 an alpha nickel-poor phase (kamacite) and a gamma nickel-rich 

 phase (taenite). 



Structural changes. — The structures resulting from the phase 

 changes parallel in a degree those in the artificial iron-carbon aUoy. 



A. When nickel is below 6 percent (corresponding with the satu- 

 ration point of taenite with respect to kamacite) the final product 

 is a homogeneous solid solution corresponding with the solid solu- 

 tion of cementite in ferrite when the percentage of carbon is below 

 0.01 percent. This is kamacite, the substance of hexahedrites. 



Assuming the nickel content to be 5 percent, the alloy cooling 

 through the gamma range, the gamma-alpha transformation will 

 begin at the point L (fig. 5). The transformation consists of the 

 separation of the two phases — a nickel-rich gamma of the composi- 

 tion L, and a nickel-poor (alpha) of the composition K- — starting 

 the formation of the Widmanstatten structure. The separation 

 takes place along octahedral planes. 



As transformation proceeds the nickel-rich and nickel-poor phases 

 must remain in equilibrium; therefore their respective compositions 

 change to M and N, and finally to F and Q at around 550° where 

 the transformation is completed, the alloy now consisting wholly 

 of alpha kamacite. This corresponds with the solid solution of 

 cementite in ferrite when the percentage of carbon is below 0.01 



