THE METALLOGRiAPHY OF METEORIC IROOST 69 



separated from the groundmass by a dark confused zone, and no such 

 coalescence can be observed. Indeed the problem would not be 

 simple even with metallographic methods, if its solution depended 

 upon the study of that iron alone. 



Recapitulation. — The following conclusions therefore can be satis- 

 factorily drawn in the case of nickel-rich ataxites: 



They are not a secondary product of alteration from a primary 

 Widmanstatten structure but represent an original two-phase equi- 

 librium structure produced by transformation at a low temperature, 

 perhaps completed at as low a point as 350° or 360°. The transfor- 

 mation was retarded to such a low range by the high proportion of 

 nickel, the retardation being perhaps sometimes increased by the 

 presence of phosphorus. 



The structure is more or less characteristically eutectoid in appear- 

 ance, though the areas of gamma and of alpha phases are not of 

 definite composition. One component appears as islands witliin 

 the other when the structure is typically developed (Morradal, pi. 

 28; Linville, pi. 24; Ternera, pi. 25). 



Needles or spmdles of kamacite, when present, represent an incip- 

 ient octahedral structure, which developed imperfectly because of 

 the low temperature of transformation. They may be oriented but 

 are less often so in very high -nickel irons. By attracting to them- 

 selves the alpha phase (kamacite) of the immediately surrounding 

 mass, a border of pure taenite is left around them, which coalesces 

 with the same component in the groundmass. The paraeutectoid 

 is essentially plessitic, and its structure sometimes appears identical 

 with that of some plessites found in octahedral irons. 



With very light etching the two components may both appear 

 perfectly clear and indistinguishable; or even the kamacite may 

 darken slightly because of oold-working, as explained in Chapter III. 

 With stronger etching the kamacite is clear, but the taenite usually 

 becomes more or less gray, or develops darker areas, because of 

 supersaturation with respect to kamacite. 



Nickel-poor ataxites do not present a primary structure, but are 

 a product of alteration by reheating. This is discussed in the 

 following chapter. 



Spheroidized taenite in plessite. — Mention has been made of a 

 peculiar type of plessite in which minute spheroids of taenite are 

 dispersed in a matrix of clear kamacite, observed in Otumpa, 

 Seelasgen, and Leeds (pi. 46). 



This peculiar structure suggests the spheroidizing of cementite 

 (iron carbide) in steel (pi. C) as the result of annealing in a certain 

 thermal range, and it may have been produced in an analogous 

 mamier. Taenite not infrequently shows some tendency to spheroid- 

 ization, though on a smaller scale and less perfectly, in the minute 



