54 BULILETIN 18 4, UNITED STATE'S NATIONlAl. MUBEXJM 



is not and could not be present. The author, however, has observed 

 at least two hexahedrites with plessite fields in which taenite was 

 clearly developed (Otumpa, Sierra Gorda, pis. 44, 46, 78). In both 

 these irons the Ni-Co content is even below the average of hexa- 

 hedrites, it bemg, respectively, 5.68 and 5.83 percent. 



The existence of such fields might be explamed by rapid coolmg 

 from a relatively high range. The separation of gamma and alpha 

 in such a range, in an iron of such low nickel content, would normally 

 be transitory and the taenite would disappear as the mass became 

 wholly alpha. With quick cooling, however, the mass might have 

 become rigid so rapidly that some of the taenite would be preserved. 

 In Sierra Gorda the structure was not visible with light etching, 

 indicating little difference in the solubility of the taenite and kama- 

 cite. This would naturally be the case if the taenite was a product 

 of a normally transitory separation at a high temperature. 



These instances are altogether exceptional. In a typical hexa- 

 hedrite, well below the borderlme of nickel content for such irons, 

 it may be assumed that there is no segregation of taenite, and there- 

 fore no plessite. 



Transitional types. — With a rising nickel content hexahedrites 

 graduate into coarsest octahedrites. In such transitional types 

 traces of the octahedral pattern may be discernible in one place and 

 only a uniform hexahedral structure in another part. Even after 

 the banded pattern is clearly established no taenite may be present, 

 the bands not being separated by visible taenite lamellae (e.g., 

 Osseo). It may be in such cases that there are invisible mcipient 

 segregations of taenite between the bands. 



Hexahedrites may be altered by heat into nickel-poor ataxites, 

 as described in Chapter XIII. Here again there are transitional 

 forms which might be assigned to either class. 



A number of all-kamacite irons with very large grains, more or 

 less suggesting a banded arrangement, are classed by some writers 

 as hexahedrites and by others as coarsest octahedrites. Examples 

 of such irons are discussed in Chapter II. 



XL OCTAHEDRAL STRUCTURES 



Octahedrites have a macroscopic structure consisting of mter- 

 secting bands corresponding with the planes of the gamma phase, 

 to which the name Widmanstatten structure was given in honor of 

 Alois Widmanstatten of Vienna who discovered it in 1808, first by 

 heat tinting and later by etching. It begins with about 6 percent 

 nickel and disappears in the neighborhood of 13 percent. 



Genesis of the Widmanstatten structure. — A nickel-uon alloy with 

 less than 6 percent nickel is in the alpha phase at room temperature. 

 Therefore, hexahedrites, which contain an average of about 5.5 



