16 BUL(LETIN 18 4, UNITED STATES NATIONIAL MUSEUM 



with one or more of the surrounding octahedral planes. Sometimes 

 fields are occupied by minute bands of kamacite, reproducing in 

 miniature the general octahedral structure. 



Another familiar type consists of relatively coarse lamellae, roughly 

 parallel though wavy and irregular. The taenite lamellae are often 

 thickened at their ends. Still another type consists of coarse or fine 

 rounded or tubular kamacite particles, resembling a wall of masonry. 



The various types of plessite are illustrated in the plates. 



In some fine octahedrites extensive plessite fields consist of kama- 

 cite or dense plessite filled with minute parallel threads of taenite. 

 Such fields glisten brilliantly with macroetching when the light falls 

 at right angles to the threads. 



A finer structure is seen in fields wherein the taenite appears in 

 minute particles. In clear fields of granular kamacite the taenite 

 often segregates in droplike form at the junction of grain boundaries. 

 Often fields are strewn with elongated particles of taenite, whose 

 direction may be oriented, and if the kamacite is granular, oriented 

 diversely in different grains. In such fields the kamacite often shows 

 Neumann lines and an oriented sheen. In granular fields the orien- 

 tation of the lines and of the sheen usually varies in different grains. 



When the minute rounded particles of taenite are very numerous 

 the plessite grows darker, often showing lighter and darker bands. 



Dense plessite. — Wliile light or normal plessite fields show an 

 endless variety of structure, the dense plessite is of one general type — 

 minute taenite particles in a matrix of kamacite, forming a dark or 

 black groundmass resolvable only at fairly high magnification. The 

 particles are often more or less oriented, frequently producing a 

 banded effect, and there are many examples transitional between 

 the darker banded plessite and the dense type. Often a light field 

 encloses one or more areas of dense plessite, with boundaries par- 

 allel with those of the field. 



Dense plessite appears black with macroetching, the fine division 

 of its components producing the dark effect characteristic of metallic 

 powders. The fact that with microetching and vertical illumination 

 such areas may still appear black, and that minute kamacite particles 

 in taenite also appear black although kamacite in larger areas is clear, 

 is explainable in this way: An area of clear kamacite etches uni- 

 formly, presenting no visible contrasts, whereas in a field filled with 

 finely divided kamacite and taenite the etchant attacks more strongly 

 along the interfaces of the two components. Thus the interface of 

 any inclusion is darkened, whatever its nature may be, and if a 

 particle is minute the darkening produces the effect of a black dot. 



Occurrence of plessite. — Coarse plessite is characteristic of the 

 coarser octahedrites. Exceptionally it occurs in octahedrites that 



