THE METALLOGRAPHY OF METEORIC IRON 61 



transformation — minute alpha needles (lamellae) developing in the 

 gamma iron, similar to the martensitic structure in very low-carbon 

 steels. 



The explanation is quite satisfying for this important structure, 

 which is seen in many octahedrites. It is common and often very 

 strongly pronounced (e.g., Trenton, Seneca Township, Canton, 

 Smith's Mountain, Thunda; pis. 35-38, 77). 



It often has a strongly oriented acicular pattern, producing the 

 effect termed "hatching"in the bands of octahedrites, as well as the 

 strong sheen that such bands exhibit, which formerly was attributed 

 to Neumann lines or other causes. 



Though characteristic of octahedrites, this structure occasionally 

 is found in irons of such low nickel content that presumably they 

 should be wholly in the alpha phase. For example, in Primitiva, 

 a nickel-poor ataxite (Ni-Co 5.61 percent), the structure is strongly 

 developed (pi. 10), and it is also found in New Baltimore (pis. 76, 77), 

 which has a Ni-Co content of 6.74 percent and is wholly hexa- 

 hedral in its microstructure. An analogous structure is found in 

 spots in San Francisco del Mezquital (Ni-Co 6.33), a nickel-poor 

 ataxite (pi. 78). 



In such cases the structure of gamma-alpha needles may have 

 been a result of rapid cooling, lowering the transformation to a range 

 of temperature in which the kamacitc and taenite could not agglomer- 

 ate because of slow diffusion, a process analagous to the formation of 

 martensite in artificial irons. 



"Spotted" taenite and kamacite. — Another product of imperfect 

 transformation is the so-called ''spotted" (more aptly "darkened") 

 taenite, which has previously been referred to, and the similar ap- 

 pearance less often observed in kamacite. Taenite bodies with 

 darkened cores are so common that they might be called the rule 

 rather than the exception; many examples appear in the plates. 

 They are found almost always in medium and fine octahedrites in 

 which the nickel content was high enough to retard transformation 

 to a low range where it could not be fully completed because of the 

 rigidity of the mass. Atomic mobility being insufficient to permit 

 complete coalescence of the taenite, an aggregate of very fine alpha 

 and gamma particles was left, appearing black because of the strong 

 attack of the etchant around the particles. 



Even apparently clear taenite often shows, with strong etching, 

 more or less grayish darkening, indicating that transformation was 

 not quite perfect. The taenite, being slightly supersaturated with 

 respect to kamacite, has traces of a gamma-alpha aggregate. In 

 coarse octahedrites transformation took place in a therm.al range so 



