NO. 20 HELT TOWNSHIP METEORITE PERRY 5 



mation of granulated areas in which the Neumann lines are partly or 

 wholly obliterated. In the central part of the polished surface these 

 areas are surrounded by unaltered kamacite in which the lines show no 

 change, but near the edges the alteration is uniform. 



Experiments by several investigators referred to by the writer in 

 "The San Francisco Mountains Meteorite," ' have established that 

 kamacite is completely altered by heating for the equivalent of about 

 i,ooo° for 1 or 2 seconds. In this case the incomplete alteration indi- 

 cates a minimum degree of heating — probably brief and not far above 

 the alpha-gamma inversion, which for kamacite (with rising tempera- 

 ture) would be about 700 . 



Incipient alteration is observable around all inclusions. Wherever 

 taenite or schreibersite appears it is surrounded by granulations. They 

 also appear in stringlike form along the invisible, or barely visible, 

 lines connecting such inclusions (pi. 8, fig. 1 ) , and along the boundaries 

 of kamacite bands where no taenite is visible. In such places a plane 

 of nickel or phosphorus enrichment, although not producing a visible 

 line on the etched surface, was evidently sufficient to initiate the process 

 of recrystallization. 



In the aureoles surrounding rhabdite crystals (pi. 8, fig. 2) the grains 

 are noticeably lighter and more homogeneous than the surrounding 

 unaltered kamacite, which is more or less darkened by a profusion of 

 extremely minute particles. 



Assuming that such minute particles consist of phosphide, its migra- 

 tion to the rhabdite could account for such a clearing up of the adjacent 

 newly formed grains. Because of the lower melting point of the 

 phosphide the alteration of the kamacite started at the rhabdite 

 crystals, and as it proceeded outward, the newly formed grains gave 

 up their phosphide and became practically pure kamacite. As such 

 migration would take place far below the melting point of kamacite, 

 the process would be relatively slow and could extend only a short 

 distance during the brief period of heating. That the same process 

 should take place around taenite inclusions is not inconsistent, because 

 taenite probably always contains some phosphide and is often dis- 

 tinctly bordered with it, as sodium picrate etching reveals. Plate 4 

 shows a phosphide eutectoid area in taenite. 



Yogel 3 holds that the larger crystals and masses of schreibersite 



2 The San Francisco Mountains Meteorite. Amer. Journ. Sci., vol. 28, p. 216, 

 Sept. 1934. 



3 Eine umfassendere Deutung der Gefugeerscheinungen des Meteoreisens . . . 

 Abh. Ges. Wiss. Gottingen, Math.-Phys. Klasse, III Folge, Heft 6, 1932. 



liber die Strukturformen des Meteoreisens . . . Ibid., Neue Folge, Band XII, 

 2, 1927. 



