THE METALLOGRiAPHY OF METEORIC IROK" 21 



inclusions were an iron-phosphide eutectic, that ratio would be 

 increased. It is also to be remembered that the phosphorus in 

 meteoric irons, especially in octahedrites, is often largely concen- 

 trated in local areas. 



Carbon. — Though carbon plays a subordinate role in the metal- 

 lography of meteoric iron, it is of interest because of its abun- 

 dance in some irons in the form of the carbide cohenite, and also 

 because in rare instances it produces iron-carbon structures identical 

 with some that are produced by carbon in artificial irons. Two 

 instances of such structures are described in Chapter XV. It also 

 occurs in the free form as graphite. 



Free carbon in the form of graphite is not uncommon. It may 

 occur as grains or lamellae, but most often as nodules, which some- 

 times reach dimensions of an inch or more (Cosby Creek, Magura, 

 Smith ville). It is often associated with troilite, adjoining or sur- 

 rounding it, or intergrown with it (pi. 62). In Dungannon (pi. 62), 

 which has the high carbon percentage of 0.532, the graphite is con- 

 fined to small areas where it appears in meandering arborescent 

 patterns. 



Other irons showing high percentages of carbon are Canyon Diablo 

 0.417 percent (in certain spots), Savannah 0.475 percent, Seelasgen 

 0.52 percent, and Cosby Creek 0.50 percent. Presumably the 

 material analyzed was free from the larger graphite inclusions found 

 in some of these irons. 



Graphite is easily recognized by its characteristic appearance and 

 softness and its visibility on an unetched surface. It does not show 

 in a sulphur print, a test that helps to recognize quickly the presence 

 of graphite in association with troilite, which it sometimes resembles 

 in appearance. 



Carbon in amorphous form is found along the boundaries of the 

 very large grains of some granular hexahedrites. 



Cohenite. — The carbon in steel and white cast iron does not occur 

 as free carbon but as the carbide called cementite, Fe^C In meteoric 

 iron the corresponding carbide is known as cohenite. It is nickel- 

 iron carbide (FeNi)3C; or, since some cobalt is usually present, 

 perhaps (FeNiCo)3C. 



Cohenite is comparatively rare, having been found chiefly in a 

 few coarse octahedral irons where it occurs in the form of imperfect, 

 rounded, elongated crystals, exceptionally reaching dimensions of 

 several millimeters. The rounded form of the crystals makes their 

 angles difficult to determine, but Hussak (Cohen, 1903, p. 222) stated 

 that they are isometric, and the same conclusion was reached by 

 Heide, Herschkowitsch, and Preuss (1932), who stated that they are 

 of the spinel type. That conclusion, however, seems open to question 



