THE METALLOGRAPHY OF METEORIC IRON 13 



does not change when the specimen is rotated; bright and dark areas 

 appear the same in any position. 



This is consistent with the foregoing explanation of the origin of 

 the sheen. If the atomic planes are cut obliquely much of the light 

 is reflected away regardless of the position of the specimen, and the 

 area therefore appears dark; while if the planes are normal to the 

 surface the effect of the etchant along the lines of such planes would 

 be slight and the light to a considerable extent would be reflected 

 upward in all positions, causing such an area to appear bright. A 

 very slight obliquity in the illumination may make a dark surface 

 turn bright. 



The sheen observed under vertical illumination is faint in com- 

 parison with the brilliant effects seen in oblique light on a macro- 

 etched specimen. 



Some areas show no sheen with oblique light. This presumably 

 is due to the fact that in such areas the surface is parallel with, or 

 normal to, the atomic planes; in which case the attack of the etchant 

 would be uniform and its effect upon the surface would appear the 

 same regardless of the direction of the light. 



Swathing kamacite. — Nodules of troilite in octahedral irons are 

 often enclosed by a narrow band of kamacite, unrelated to the sur- 

 rounding octahedral structure, which abuts against it unconformably. 

 This has been termed swathing kamacite. It differs little from the 

 adjoining kamacite, though it does not show the grain boundaries 

 that may be present elsewhere. 



Similar bands of kamacite, apparently originating in the same 

 manner, often surround troilite inclusions in hexahedrites and ataxites. 

 Owing to the absence of the strongly contrasting Widmanstatten 

 structure, they are often barely noticeable. They may often be 

 distinguished by the absence of minute inclusions that are abundant 

 in the surrounding mass. 



Swathing kamacite is also found around needles (lamellae) of 

 troilite, as well as around needles of schreibersite. 



The manner of production of swathing kamacite is discussed in 

 Chapter XV. Numerous examples are illustrated in the plates. 



Cold-working. — Although with microetching kamacite ordinarily 

 appears perfectly clear, very light etching (5 or 10 seconds) often 

 causes a grayness that disappears again with a longer application 

 of the etchant. This is probably due to " cold- working" — that is, 

 to a slight dragging of the surface of the soft metal by the abrasive. 



When kamacite appears in bands, lamellae, or particles associated 

 with harder structures, the softer metal naturally would be suscep- 

 tible to cold working, which often causes some diflSculty in obtaining 

 a perfect surface. The removal of the slight film of cold-worked 



