THE METALLOGRAPHY OF METEORIC IRON 7 



investigators by the methods available to them. Indeed, as late as 

 1915, O. L. Farrington wrote: "Some show a fine-granular structure, 

 but others even imder the microscope exhibit no division into grains 

 which can be detected. They are, therefore, except for accessory 

 minerals, quite structureless." 



Tv/o subgroups were ear\j recognized — nickel-poor and nickel- 

 rich ataxites. In the former the nickel content is approximately 

 that of hexahedrites; in the latter it commonly varies between about 

 12 and 20 percent, with a few showing much higher percentages. 



Ataxites were classified by the older writers into numerous sub- 

 groups, including the Cape (symbol Dc), Shingle Springs (Dsh), 

 Babb's Mill (Db), Linville (Dl), NedagoUa (Dn), Siratik (Ds), 

 Primitiva (Dp), and Muchachos (Dm). These designations or 

 symbols appear in many museum labels and catalogs. Such classi- 

 fication, however, was based upon superficial characteristics and has 

 little meaning from the standpoint of metallography. In the WTiter's 

 opinion only two classes need be recognized, nickel -poor and nickel- 

 rich ataxites. 



The peculiar fact that an ataxite may have either a very low or 

 a very high nickel content is explained in later chapters. 



Transitional forms. — A number of octahedrites with very low, or 

 very high, nickel content are barely distinguishable as such, their 

 structures being transitional respectively from those of hexahedrites 

 and those of nickel-rich ataxites. 



Several irons showing a very coarse and irregular pattern, the 

 width of the bands (or grains) reaching an inch or more, have been 

 generally classed as coarsest octahedrites. In some of these the 

 author can see no tendency to octahedral orientation, the large 

 grains being irregular in form and disposition. The nickel content 

 of these irons is as low as that of some hexahedrites, and their micro- 

 structure is characteristically hexahedral. 



On the other hand, in some high -nickel irons classed as finest 

 octahedrites the structure is almost or quite ataxitic. An example 

 is Cowra (Ni-Co 14.25 percent) in which there are no typical kama- 

 cite bands but only needles or spindles of kamacite in a dense ataxitic 

 groundmass, the only connecting linlv: with octahedrites being an 

 octahedral orientation of the needles, a feature found also in ataxites. 



In some cases the assignment of an iron to one or the other group 

 is more or less arbitrary. This matter is further discussed in 

 Chapter XI. 



Proportions of types. — According to the latest available figures, by 

 Watson (1941), the total number of meteorites of all types is 1,202. 

 Of these, 438, or a little more than one-third, are siderites. Of the 

 three types of siderites, octahedrites are by far the most numerous, 



