62 BU'LL'ETIN 18 4, UNITED OTATE'S NATIONAL MJUiSETJM 



high as to permit a full segregation of the two phases, and taenite 

 is usually clear. 



Kamacite may be similarly darkened by the presence of a ganmia- 

 alpha aggregate when transformation was incomplete. The author 

 has rarely observed it, but a good example is seen in Trenton (pi. 78) 

 where kamacite lamellae in a coarse plessite field are thus darkened. 

 The plessite field being richer in nickel than the surrounding mass, 

 transformation within it would naturally be retarded. 



The "ragged" structure. — A structure somewhat acicular in ap- 

 pearance, occurring in taenite bands and (it is said) less often in 

 kamacite, is referred to by some of the more recent German writers 

 as the "ragged" structure (Fetzenstruktur) and has been ascribed 

 to twinning in the gamma phase. 



Kamacite, like ferrite in artificial irons, develops a granular struc- 

 ture in cooling through the gamma range, and also on reheating 

 into the gamma range. The two types of grains differ in that those 

 produced within the alpha range are free from twinning; while those 

 produced in the ganama range are characterized by abundant twin- 

 ning, which disappears normally when the iron in cooling changes 

 to the alpha phase. Such twinning might be preserved in a high- 

 nickel taenite which remains in the gamma phase to room tempera- 

 ture (e.g., Octibbeha, pi. 34). 



This explanation is given by Vogel for the formation of the ragged 

 structure, of which he gives as a characteristic example a photomicro- 

 graph of a taenite lamella in the Burlington meteorite (Vogel, 1927, 

 fig. 11). He states that it is not due to a separation of nickel-rich 

 and nickel-poor components but is merely an evidence of gamma 

 twinning preserved by rapid cooling — especially when transformation 

 is retarded by a high-nickel content, which accounts for its being 

 more often observed in taenite than in kamacite. 



The author, however, has been unable to find in Burlington a 

 structure of the nature indicated by the published pliotomicrograph 

 referred to. Recent photographs of many such lamellae in Burling- 

 ton, at higher magnification and possibly more perfect than those 

 taken by Vogel 15 years ago, indicate a transfoimation structure 

 having only a faint resemblance to real gamma twinning, as shown 

 in Octibbeha (pi. 34) and in artificial irons. Hundreds of photo- 

 graphs of octahedral irons, at various magnifications, also have 

 failed to reveal an example of twinning in kamacite. The author 

 therefore is inclined to the view that the ragged structure is a form 

 of the gamma-alpha transformation structure already described, 

 which is of frequent occurrence and in a great variety of forms. 



Granulation already has been discussed in Chapter IX, and sec- 

 ondary granulation as the result of reheating is further considered 

 in Chapter XIII. 



