THE METALLOGRAPHY OF METEORIC IROOST 47 



F. At somewhat more than 30 percent nickel the iron becomes 

 wholly taenite, the only example being Santa Catharina, with 34.37 

 percent nickel (35.87 percent Ni-Co). This, the homogeneous 

 unsaturated solution of kamacite with respect to taenite, is the 

 counterpart of A, which is the homogeneous unsaturated solution 

 of taenite with respect to kamacite. The two states are not infre- 

 quently referred to respectively as unsaturated solutions of kamacite 

 in taenite, and of taenite in kamacite, which is a convenient descrip- 

 tion although it represents a reversal of the actual mechanism. 



G. from this point onward the alloy is taenite with a rising nickel 

 content — that is, an increasingly nickel-rich gamma phase — passing 

 finally (in theory) into pure nickel. Actually no meteoric iron is 

 known with more than 62 percent nickel, the content of Octibbeha. 

 This iron is wholly in the gamma phase, as is shown by the charac- 

 teristic twinning of its grains (pi. 34). 



The trias summarized. — The foregoing outline traces the changes 

 that produce the three members of the "trias" — kamacite, plessite, 

 and taenite — substantially as stated by Pfann (1917) who summa- 

 rized their occurrence as follows: 



Kamacite: As (a) the sole component of irons with less than 6 

 percent nickel (hexahedrities) ; {h) as a segregate with octahedral 

 orientation, forming the Widmanstatten structure in irons with 

 more than about 6 percent nickel; (c) as a second segregate in 

 the coarse or light plessite fields in octahedral irons; and {d) inti- 

 mately mingled with taenite as a component of dense or eutectoid 

 (paraeutectoid) plessite. 



Taenite: As (a) the sole component in irons with more than 27 

 percent nickel; (6) as a segregate in irons with 18 to 27 percent nickel; 

 (c) as lamellae bounding the kamacite bands in the Widmanstatten 

 structure; and {d) as a component of eutectoid (paraeutectoid) 

 plessite, associated with the fourth form of kamacite. 



Plessite: As (a) noneutectoid fields with coarse structure in octa- 

 hedral irons; {h) as eutectoid (paraeutectoid) plessite, more or less 

 perfectly developed, in plessite-rich octahedrites and nickel-rich 

 ataxites. 



There are differences of opinion as to the formation of the Wid- 

 manstatten structure, which is discussed in Chapter XI. The fore- 

 going, however, seems to the author a satisfactory summary of the 

 essential mechanism. 



The demarcation in classifying the different types of structure is 

 not definite. Thus there is a narrow "twilight zone" between hexa- 

 hedrites and octahedrites, and a wider one between octahedrites 

 and ataxites. 



