NO. 5 HEXAHEDRITES — HENDERSON 3 



occupy the area to the right of AC and chiefly consist of taenite, but 

 usually also contain an appreciable quantity of alpha iron, kamacite. 



Meteorites lying along the borderlines of groups are difficult to 

 classify. Thus, any list of hexahedrites compiled by one investi- 

 gator will contain irons that another compiler might regard as coarse 

 octahedrites. 



The Ni-Fe diagram (fig. 1) shows the conditions under which 

 kamacite (alpha iron), the sole constituent of a hexahedrite, forms. 

 The two dotted vertical lines at 5.5 and 6.0 percent Ni define the 

 range within which hexahedrites grade into coarse octahedrites. 

 Thus, no hexahedrite has a nickel value to the right of these dotted 

 lines, and most hexahedrites lie either on or slightly to the left of the 

 5.5 line. 



The different classes of meteorites — hexahedrites, nickel-poor 

 ataxites, octahedrites, and nickel-rich ataxites — do not correspond 

 to the diagram for 1 atmosphere of pressure, as stated by Uhlig 

 (1954) and by Henderson and Perry (1954). According to Uhlig, 

 this is due to the fact that meteorites form under pressures greater 

 than 1 atmosphere. 



If a horizontal line is drawn in figure 1 at 490° C, and if it is as- 

 sumed that no changes take place below this temperature, the modified 

 diagram more faithfully depicts the structures in iron meteorites. 

 Following a 490° line from left to right, the hexahedrites, which 

 consist of one component, lie to the left of the line AB while the 

 octahedrites and nickel-rich ataxites lie between AB and AC. 



The chemical composition reported for the hexahedrites and 

 nickel-poor ataxites only represents an average composition of the 

 area selected for analysis and not the entire meteorite. Since the 

 Ni values of hexahedrites range from about 4 to 6 percent, there is 

 no valid chemical reason for continuing the use of the term nickel- 

 poor ataxite. This term should be dropped. 



The available analyses of the nickel-rich ataxites, hereafter referred 

 to as ataxites, show nickel values which can be located on the 490° 

 line to the left of where it crosses the AC line. Actually most of the 

 Ni percentages lie nearer to the AB than the AC line. Thus, 

 kamacite should be a common constituent in ataxites, and indeed, 

 kamacite spindles are recognizable in these meteorites. The struc- 

 tures of ataxites are more confused than the structures in the 

 octahedrites and hexahedrites. 



An interesting relationship was noted in the range of Ni percentages 

 of the three groups of meteorites. The difference between the 



