80 BTJLILETIN 18 4, UNITED STATES NATIONIAL MXHSEUM 



others, may perhaps be explainable on the analogy of artificial iron- 

 phosphorus alloys. 



Data on the transformations of iron-phosphorus allojs are sum- 

 marized by Jordan (1939), who reproduces the diagram of Haughton 

 (1927), stating that it is consistent with the earlier views of Stead. 



According to Stead (1900) in carbon-free iron with less than about 

 1.7 percent of phosphorus (in the form of phosphide) a homogeneous 

 solid solution is formed. As the content of phosphorus rises, the 

 iron-phosphide eutectic begins to segregate, as the iron-carbon 

 eutectoid pearlite does when carbon exceeds 0.80 percent. An 

 example of such segregation in meteoric iron is seen in Soper (pi. 11). 

 Similar segregations in Zacatecas (pi. 54) appear clear with ordinary 

 etching, but might be eutectic in structure. 



At about 10.2 percent phosphorus a true eutectic is formed, solidi- 

 fying at 980° and resembling the structure of eutectic cast iron. 

 Compare plate C, figure 1, with the phosphide eutectic in Cincinnati, 

 plate 59. In the latter, and similar examples, the excess of iron 

 (kamacite) has been rejected in droplets, the remaining groundmass 

 being presumably about 10.2 percent phosphorus. 



When phosphorus exceeds 10.2 percent in artificial alloys the 

 excess of Fe^V begins to be rejected in crystals resembling rhabdites. 

 This structure has not, to the writer's knowledge, been observed in 

 meteoric irons. At about 15.58 percent phosphorus, the composition 

 of FesP, the phosphide becomes homogeneous. 



In the author's experience, with the employment of neutral sodium 

 picrate on many irons a eutectic structure apparently was developed 

 only once in rhabdites (CedartowTi, pi. 55) and in only one or two 

 other kinds of schreibersite inclusions (Helt Township, pi, 55). In 

 other cases such inclusions darkened uniformly. The structure of 

 the phosphide in Soper (pi. 11) has already been referred to. How- 

 ever, eutectic structures might have been revealed in other cases 

 if the picrate etching had been done with especial care and with 

 that object in view. 



Allusion has been made to the droplike particles of phosphide 

 appearing as inclusions within minute kamacite bodies in numerous 

 nickel-rich ataxites (e. g., lOoiidike, San Cristobal, Freda; pis. 27, 

 28, 29). They may possibly have acted as nonmetallic nuclei 

 in initiating the gamma-alpha transformation; or perhaps, by de- 

 creasing the solubility of nickel in their vicinity, their presence 

 permitted the transformation to take place at a higher temperature. 

 There appears, however, to be no visible structural difference between 

 kamacite bodies with such nuclei and those without them. 



Effects on phase transformations. — The presence of phosphorus in 

 some cases probably influences the transformations and the re- 



