6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



originate in the melted stage, citing the fact that they are often sur- 

 rounded by a granular area corresponding with a phosphorus-poor 

 zone, which would indicate a rapid separation of the schreibersite as 

 the cooling progressed ; but that nevertheless such areas can arise from 

 incipient solution of the crystals in a solid state, which process with a 

 rising temperature in an alloy of the composition of kamacite would 

 begin at around 700 °. The temperature named by Vogel is the alpha- 

 gamma inversion point on heating ; on cooling that point would be 

 about 200 ° lower, so the process would be correspondingly prolonged. 



Either of Vogel's explanations of the origin of such granular 

 kamacite areas involves phosphorus impoverishment near the schrei- 

 bersite masses. In the present instance, since the alteration which 

 produced the new grain boundaries was the result of reheating after 

 the Neumann lines were formed, it is possible that the migra- 

 tion of phosphide from the surrounding kamacite accompanied the 

 formation of the grains. 



An interesting analogy is presented by the process of recrystalliza- 

 tion in cast iron, as shown by a sample that had been subjected to 

 more and more extended annealing at a temperature of about 8oo°. 

 Graphite nuclei grew by accretion, carbon being absorbed from the 

 cementite in the surrounding areas, until finally granulation was com- 

 plete, the graphite nuclei had absorbed all the carbon, and the ground 

 mass consisted of grains of pure ferrite. The later stages of the 

 process produced granulated aureoles around the graphite nuclei much 

 resembling those surrounding the rhabdites in the iron here described. 



The more remote the rhabdite crystals are from the surface of the 

 mass, the less granulation appears around them. The surface left after 

 removing the slice, which is perhaps half an inch below the highest 

 part of the original surface of the mass, shows in its central portions 

 many rhabdite crystals with little or no surrounding granulation. 



Plate 7, figure 1, shows a very minute schreibersite inclusion of 

 unusual character, its edges having a prickly appearance due to a 

 slight extension of phosphide into the surrounding iron at grain 

 boundaries. The phosphide also appears segregated along certain grain 

 boundaries at some distance from the inclusion. 



This inclusion is close to the surface of the mass, where the effect 

 of the temporary reheating during flight was greatest, and the in- 

 vasion of the iron by the phosphide at grain boundaries is obviously 

 due to a brief melting of the schreibersite. As its melting point is 

 below 1,000°, a reheating sufficient to produce granulation in the 

 kamacite would also be sufficient for such fusion. Slight traces of a 



