THE METALLOGRtAPHY OF METEORIC IRON 15 



bands being in contact. Conversely, in very fine octahedrites the 

 kamacite bands become narrow and the taenite lamellae broad and 

 thickly crowded. Taenite also occm-s as needles, as fine or coarse 

 skeletal growths, in large irregular masses, and in minute particles. 



Taenite lamellae very often enclose rounded or elongated areas 

 of darker color, the darkness increasing toward thoir centers, which 

 is observable both with macroetching and v/ith microetching (pi. 8, 

 fig. 2; pi. 13, fig. 4; pi. 35, fig. 1). Such taenite is termed by German 

 writers "fleckig," which is commonly translated as "spotted," though 

 darkened or shaded would better describe it. The appearance is 

 due to incomplete transformation, resulting in an imperfect separation 

 of taenite and kamacite. (See chapter XI.) 



Composition of taenite. — Taenite is rarely homogeneous in compo- 

 sition. The shaded areas referred to, which are very common, are 

 from the manner of their formation poorer in nickel than the clear 

 peripheral portions. This is definitely proved by the process of 

 quantitative coloration described in Chapter XVII. Sodium picrate 

 etching also reveals that taenite lamellae, though appearing homo- 

 geneous with ordinary etching, often have borders rich in phosphide 

 and even may be phosphide-rich throughout. 



Because of such variations and the mechanical impossibility of 

 isolating a hypothetically "pure" taenite, it is impossible to establish 

 any definite composition by analysis. The nickel content has been 

 fixed by various analysts at percentages all the way from 13 to 48, 

 corresponding with formulas ranging from FeNi to FcyNi (Farrington, 

 1907). 



In view of the fact that taenite lamellae show lighter and darker 

 zones, and from a study of 26 analyses, Buddhue (1938) suggests 

 two forms, a lighter nickel-richer and a darker nickel-poorer, the 

 former contaming about 34 percent nickel and corresponding with 

 Fe2Ni, while the latter is about FceNi. It seems unsound to the 

 author, however, to postulate any definite composition for taenite 

 on the basis of chemical analysis, because from the mamier of its 

 formation the nickel content necessarily varies widely. Taenite 

 must be regarded as a nickel-rich alloy of variable composition. 

 This subject is discussed more fully in Chapter VIII. 



Plessite. — Plessite is the structural component of the fields in 

 octahedral irons. Such fields usually are bordered by taenite, and 

 their structure is infinitely varied. Plessite is of two types, commonly 

 designated as light or normal and dense or dark (termed by Pfann 

 microplessite) . The former is a relatively coarse, often macroscopic, 

 combination of kamacite and taenite; the latter a very fine structure 

 appearing black and homogeneous with low magnification. 



The coarsest types of light plessite often show skeletal growths of 

 taenite, referred to by German writers as combs (Kamme), parallel 



