12 BULLETIN 184, UNITED STATES NATIOTslAL MUSEUM 



A somewhat more acceptable explanation is the formation of 

 iiltramicroscopic etching pits, cubic in their angles and oriented 

 according to the cubic planes of the iron, thus causing light to be 

 reflected more strongly in three directions corresponding with the 

 axes of a cubic crystal, the whole mass being unigrain in character. 

 The influence of such etching pits upon the sheen, however, is hypo- 

 thetical. The characteristic angular etching pits observed in arti- 

 ficial iron and other metals are so difficult to pioduce in meteoric 

 irons that the author has never observed them. In some cases 

 etching pits were quickly produced, but they were shapeless. 



Vogel (1927) attributed sheen to the presence of phosphorus, 

 stating that it is strongest in phosphorus-rich irons and weak or 

 absent in those containing little phosphorus, thus constituting an 

 index of the phosphorus content. The author cannot concur in 

 that view. 



Efect of atomic planes. — It would seem a reasonable explanation 

 that the sheen arises chiefly, if not whoUy, from the influence of 

 atomic planes revealed by the etching process. For example, if the 

 etched surface were parallel with a set of such planes it would be uni- 

 formly attacked. But if the surface intersected the same planes 

 obliquely, the differential attack of the etchant along the edges of the 

 slightly overlapping planes might produce a surface comparable with 

 that of overlapping clapboards, from which light would be reflected 

 more strongly in one direction. Further etching might conceivably 

 develop along such lines submicroscopic etching pits with cubic orien- 

 tation. It is to be noted, however, that the maximum sheen is devel- 

 oped by strong (macro) etching, which in the author's experience 

 never produces angular oriented etching pits. 



In the so-called "hatched" bands of many octahedrites the sheen 

 usually is particularly marked. The hatching is due to a transfor- 

 mation structure described in Chapter XI, which gives rise to many 

 planes along which the unequal attack of the etchant would be strong. 



In granular kamacite the orientation of such transformation 

 structures may vary in different grains because of the diverse orien- 

 tation of the atomic planes of the original grains previous to trans- 

 formation. Since the grains have a random orientation, the acicular 

 transformation structure may vary from grain to grain, resulting in 

 different etching effects and variations in sheen. 



Sheen in vertical illumination. — The reason why sheen is not 

 usually observed on microetched surfaces with vertical illumination 

 is primarily that the etching is too light to make an appreciable 

 attack along the atomic planes. With strong microetching certain 

 kamacite areas or grains are likely to exhibit a sheen, which, however, 



