244 AKNUAIi REPORT SMITHSONIAN INSTITUTION, 1942 



It will be further noted that these areas form rather long but narrow 

 bands and are separated from each other by delicate lines of an 

 alloy with a totally different appearance. Kamacite is the name 

 given to the metal in these wide lath-shaped areas and the metal in 

 the delicate lines bordering the kamacite is taenite. The chemical 

 analysis (table 2) shows that this group of meteorites always has a 

 higher percentage of nickel than hexahedrites. At this point the 

 explanation of structures begins to become apparent. The single 

 alloy, kamacite, of which hexahedrites are made up, is composed of 

 iron that is about saturated with respect to cobalt and nickel. Any 

 excess of these metals over that needed to make kamacite combines 

 with iron to make a second alloy, taenite, and the distribution 

 arrangement of these two alloys forms an octahedral pattern. 

 Taenite is slightly different in its acid-resisting properties, and on 

 the etched face of a meteorite it stands out in relief against the 

 kamacite. 



Wlien a series of etched faces of octahedrites is spread before the 

 observer, a difference in width is noted between these kamacite bands 

 in different specimens. Some are very narrow and surrounded with 

 a continuous thin, but well-developed, line of taenite; others have a 

 very wide, sometimes irregular-shaped, area of kamacite with only a 

 mere suggestion of taenite around it. In fact these octahedrite and 

 hexahedrite groups reach a point where there is little difference be- 

 tween them. 



The relationship between composition and structure is rather sim- 

 ple to this point, but as structures of more and more octahedrites are 

 examined it is noticeable that increasing narrowness or fineness of 

 the kamacite bands does not always guarantee progressively higher 

 nickel content. It is true that medium or narrow widths of the 

 kamacite bands will always imply definitely higher nickel content 

 than is found in the very coarse octahedrite structures, but occa- 

 sionally it is impossible to predict definitely which of two rather 

 similar octahedrite structures will have the higher percentage of 

 nickel. Consequently there is good reason to believe that another 

 factor should be considered besides the rate at which a melt cools 

 down, and that is the length of time it was held at a fixed tempera- 

 ture, because this may have something to do with the fineness or 

 coarseness of octahedrite structures. In the laboratory it is difficult 

 to retain for a long time a prepared nickel-iron mix at a temperature 

 where the metal is a solid but still at a temperature sufficiently high 

 for the molecules of these two alloys, kamacite and taenite, to migrate 

 and segregate from each other. 



Quick freezing can create a structure of thin areas of kamacite 

 separated by taenite. When the meteorite freezes before the taenite 

 can collect together — or, to express it differently, before the kamacite 



