78 BULLETIN 18 4, UNITED STATES NATIOKlAL MUSEUM 



structure in the phosphide, but is only a stain pattern. The picrate 

 does not actually etch the phosphide surface but merely deposits a 

 film of stain which tends to assume a reticulated form, finer or coarser 

 depending on whether the phosphide content in the area is higher 

 or lower. 



An interesting example of a phosphide-poor area is shown in plate 

 57. In this case the area showing traces of reticulation is so low in 

 phosphide that with ordinary etching it is not distinguishable from 

 the adjacent kamacite, but it is slightly darkened by the picrate. 

 Possibly there might be some darkening with an even more atten- 

 uated solution of phosphide. 



Phosphide in taenite. — By the application of sodium picrate to 

 taenite Vogel (1927) established that it often contains phosphide. 

 The latter usually shows a concentration gradient, being most abun- 

 dant along the interface of a taenite body, or of the taenite border 

 of a plcssite field, decreasing inward. Taenite thus etched often 

 shows a distinct dark border and in some cases is darkened throughout. 

 This is consistent with the fact that iron in the ganmaa phase can 

 hold much more phosphorus in solution than in the alpha phase. 

 Taenite, representing the gamma phase, therefore would naturally 

 contain more than the alpha kamacite. 



Schreibersite and troilite. — Schreibersite and troilite are often asso- 

 ciated though distmctly separate, the one either adjoining or sur- 

 rounding the other, or sometimes intergrown with it. The distinct 

 separation of the two is due to the fact that they are virtually insol- 

 uble with each other in the solid state and therefore segregate in 

 cooling. (See pi. 49.) 



While pure iron sulphide melts at 1,300° the melting point is 

 lowered if it holds iron in solution, and according to Treitschke and 

 Tammann (1906) the sulphide-iron eutectic (Fe 84 percent, S 16 

 percent) solidifies at 970°. According to Stead (1900) the melting 

 point of the iron-phosphide eutectic (Fe 89.8 percent, P 10.2 per- 

 cent) is placed at 980° C. From these points downward both eutectics 

 would progressively reject the iron in excess of the eutectic ratio, 

 the rejection of iron from the phosphide eutectic probably proceedmg 

 the more rapidly. In meteoric irons in which troilite and schreiber- 

 site are associated, the latter sometimes surrounds the former (pi. 49), 

 indicating that troilite solidified first. 



Diffusion of phosphide. — The reheating of a meteoric iron above the 

 meltmg point of schreibersite, or even to as low a point as 700° or 

 800°, results in diffusion of the phosphide into the surrounding area. 

 Rhabdites thus become rounded particles or may disappear entirely, 

 leaving only spots of phosphide enrichment, which are invisible 

 with ordinary etching but which darken by the application of sodium 

 picrate. 



