THE METAX,LOGRAiPHY OF METEORIC IROiN' 55 



percent nickel, show no structure due to phase changes. At higher 

 temperatures they necessarily had a transitory octahedral structure — 

 from the point where the gamma-alpha range began to the point 

 where it ended; but the latter point was passed at a fairly high tem- 

 perature where diffusion was still active, and therefore the octahedral 

 structure disappeared. 



In octahedrites, which contain more than 6 percent nickel, the 

 gamma-alpha transformation was not completed at 450° or 500° 

 where diffusion practically ceases, and therefore the octahedral 

 structure persists. 



When an iron with more than 6 percent nickel m cooling reaches 

 the beginning of the gamma-alpha transformation, the gamma phase 

 (taenite) separates in lamellae on octahedral planes, the alpha phase 

 (kamacite) being rejected in plates between them. 



The taenite lamellae are not infrequently referred to as having 

 been precipitated, but in reality it is the alpha phase (kamacite) 

 which is precipitated from the original high-temperature gamma 

 solid solution in the form of plates on octahedral planes. Both 

 taenite and kamacite are cubic, but the kamacite plates form along 

 the octahedral planes of the taenite because of the lattice relation- 

 ship between the two. The interaction of two participating lattices 

 was established by Young, whose conclusions are summarized by 

 Melil and Barrett (1931). If any phase other than kamacite 

 had precipitated, it would have been on planes other than octahedral. 



The separation of the octahedral bands having begun, as trans- 

 formation proceeds, the alpha plates grow at the expense of the 

 diminishing gamma phase, which becomes progressively richer in 

 nickel. This gamma phase is taenite, which is of variable nickel 

 content depending on the temperature at which it was produced. It 

 may have been produced at a higher temperature and preserved by 

 quick cooling, and thus be low in nickel; or at a lower temperature 

 with slower cooling, when the nickel content would be greater. 



After the kamacite has separated it has reached its final alpha 

 state and can have no further structural change. Its nickel content, 

 however, gradually increases, as the temperature falls, up to 6 per- 

 cent, which is the Imiit of saturation of iron with respect to nickel; 

 but this involves no change of structure because below 6 percent the 

 nickel-iron solid solution remains unsaturated and homogeneous. 



The gamma phase, however, continues to transform into alpha, 

 to an extent determined by thermal conditions. If by reason of 

 low nickel content the transformation takes place in a high thermal 

 range where diffusion is rapid, the transformation is relatively com- 

 plete; the result is the production of clear taenite segregations, with 

 little or no "spotted" taenite or black plessite. 



If a high nickel content retards the transformation to a thermal 



