THE METALLOGRAPHY OF METEORIC lEON 5 



rich in nickel, termed taenite. Filling the angular interstices of the 

 network are areas called fields, which are composed of plessite— a 

 more or less fine mLxture of kamacite and taenite — which assume a 

 great diversity of form and structure. 



These three structural components — kamacite, taenite, and 

 plessite — are often referred to in descriptions as the trias. 



Octahedrites are classified according to the width of the kamacite 

 bands as follows, the letters in parentheses being the commonly- 

 accepted symbols (abbreviations of German words) for the various 

 types: 



Coarsest (Ogg), bands 2.5 mm. or wider. 



Coarse (Og), bands 1.5 to 2 mm. wide. 



Medium (Om), bands 0.5 to 1 mm. wide. 



Fine (Of), bands 0.2 to 0.4 mm. wide. 



Finest (Off), bands narrower than 0.2 mm. 



The nickel cod tent of the coarsest and coarse octahedrites is 

 roughly 6 to 8 percent; of medium octahedrites 7 to 9 percent; of 

 fine and finest 8 to 13 percent. It is impossible to generalize more 

 exactly, because analyses show variations from these averages, and 

 many of the older analyses cannot be accepted as accurate. 



The lines of demarcation between classes also are not always 

 clear. Some of the coarsest are practically hexahedrites, while the 

 finest graduate into nickel-rich ataxites. 



Cleavage in octahedrites. — As has been noted, hexahedrites often 

 have a well-marked cubic cleavage. Occasionally octahedrites 

 appear to yield a definite octahedral cleavage. This, however, is 

 not a true cleavage. It does not take place along the cleavage 

 planes of tlie iron, which are cubic, but is a mechanical separation 

 along the octahedrally oriented bands, which are bounded by taenite 

 lamellae. 



Such a separation is commonly due to infiltrating rust and may 

 be seen near the surface of weathered specimens. In the unique 

 case of Horse Creek (Chapter XI) there is a highly perfect cleavage 

 due to the presence of brittle iron-nickel phosphide (schreibersite) 

 along crystallographic planes that are not octahedral. 



The Widmanstdtten structure. — While hexahedrites are featureless 

 with ordinary etching, except for Neumann lines and grain bound- 

 aries, octahedrites show the striking and beautiful Widmanstatten 

 structure. This structure, which is discussed at greater length in 

 Chapter XI, is due to the crystallization of the kamacite in thin 

 plates along octahedral planes and to the segregation of taenite in 

 lamellae between the plates. Owing to the lesser solubility of taenite 

 in ordinary etching reagents, the two components are left in strong 

 contrast, producing a pattern of gray bands separated by fine shining 

 lines of taenite. 



