242 AKNUAL REPORT SMITHSONIAN INSTlTtJTIGN, 1942 



to believe that nine different meteorites of identical type would strike 

 the earth within such a limited area. 



It is well known that there is a definite relationship between the 

 structure and the composition of iron meteorites. Since the analytical 

 work on irons is not as simple as many chemists once believed, few of 

 the old analyses can be used in interpreting this relationship. Hence 

 it is very likely that the variation in nickel now reported in the litera- 

 ture may in large part be due to faulty methods or to improper sam- 

 pling. 



Search of the literature revealed a number of analyses which agree 

 perfectly with the results on the Chilean material. The finding of 

 so many identical analyses from all parts of the world suggests either 

 that (1) all meteorites of similar composition and structure have a 

 common origin and their distribution over the earth is due to their 

 having fallen at different times, or (2) meteorites should be considered 

 as forming from melts of iron and nickel, etc., and the resulting struc- 

 ture of the alloy will depend upon the composition of the original melt 

 as well as on the speed with which it cooled. Evidence definitely indi- 

 cates that the second supposition is the more logical one and that the 

 differences in meteorites are due not only to differences in the com- 

 position of the original melt, but also to temperatures and rates of 

 cooling. 



In other words, whenever the proportion of nickel and cobalt in 

 metallic meteorites is less than about 6 percent, only structures of one 

 class — hexahedrites — will form. When the nickel and cobalt content 

 in the original melt is higher than 6 percent, a second alloy will begin 

 to appear. When the large areas of simple structure are broken up by 

 inclusions of this second alloy arranged in a definite pattern, we have 

 a structure known as an octahedrite. The rate of cooling determines 

 the ease with which the different components migrate and increase in 

 size. When the percentage of nickel and cobalt is only slightly higher 

 than 6 percent, only a little of the second alloy, known as taenite, will 

 form, so that there are rather wide bands of one alloy (kamacite) 

 separated by specks or thin films of taenite. This combination pro- 

 duces what we call coarse octahedrites. As the nickel content in- 

 creases, finer and more delicate structures appear. It is not easy to say 

 whether composition or rate of cooling is the more important factor in 

 the production of a certain structure. 



The three classes of iron meteorites are hexahedrites, octahedrites, 

 and ataxites, each having distinctive general features that are most 

 noticeable after a polished surface is etched. Certain chemical dif- 

 ferences also exist, but there is no sharp boundary between the dif- 

 ferent groups and they intergrade with one another. 



