14 BUX.LETIN 184, UNIT'ED STATES NATIO'KiAL MOSETJM 



metal by further etching would account for the disappearance of the 

 gray color. These effects are well shown in plates 14, 30. 



Occasionally in granular irons certain grains will be darkened by 

 cold-working, while adjoining grains are not; which might seem 

 paradoxical, inasmuch as the grains are identical in composition 

 and hardness. It may be explained, however, by the fact that the 

 atomic planes of the grains are variously oriented, and in certain 

 of them the planes are in such positions as to be less resistant to the 

 action of the abrasive. The same effect is often observed in pure 

 artificial iron and in certain steels, 



Taenite. — Taenite (gamma nickel-iron), composed of iron and 

 nickel in varying proportions, is tin-white, elastic, and usually mag- 

 netic. It is easily distinguished from kamacite by its white color 

 and its resistance to etchants, it being practically insoluble in cold 

 dilute acids. It is soluble in concentrated hydrochloric and nitric 

 acids, and like kamacite it reduces copper from solutions of copper 

 salts. 



Although in the literature of meteorites taenite is alwaj s described 

 as strongly magnetic, that property would depend upon its compo- 

 sition. A nickel-iron alloy with less than about 26 percent nickel 

 is magnetic at room temperature, and the same is true if the nickel 

 content is more than about 30 percent; but between those limits 

 such an alloy is nonmagnetic. Taenite being of widely varying 

 composition, depending on the conditions of the phase transforma- 

 tions that produce it, there naturally must be many cases where a 

 given sample would fall within the nonmagnetic range of composition. 



In hardness taenite usually differs little from kamacite, a needle 

 scratch appearing the same across both. Its hardness, however, 

 also varies according to its composition. The Brumell hardness 

 curve for nickel-iron indicates that when the nickel content is about 

 5 to 15 percent the hardness of the alloy is only 5 to 10 percent 

 greater than that of iron, and the same is true when the propor- 

 tion of nickel exceeds about 20 percent; but between these per- 

 centages the hardness becomes about 18 percent greater than that 

 of iron. Thus, except for a narrow range of composition, the differ- 

 ence in hardness between taenite and kamacite would hardly be 

 distinguishable by any ordinary hardness test. Kase (1925) veri- 

 fied experimentally that the hardness of kamacite and taenite is 

 virtually identical. 



Occurrence oj taenite. — In octahedral irons taenite appears typi- 

 cally as lamellae, from fairly thick and visible to the eye, to very 

 tenuous. In a regular octahedral structure they are prevailingly 

 fairly straight and uniform, but they are often irregular. Not infre- 

 quently in the coarsest octahedrites they are absent, the kamacite 



