Plate 20 



1. Tucson, Arizona; nickel-rich ataxite; Ni-Co 10.19 percent. For this class of ataxites 

 the nickel content of Tucson is low. The photograph shows the general structure of the 

 plessitic groundmass with irregular kamacite bodies. The black, rounded areas are for- 

 sterite, a silicate that is abundantly disseminated through the mass and that is reported to 

 have been found in no other meteoric iron. This is from the mass of Tucson known as the 

 Ainsa iron. Picral; X 60. 



2. Tucson. Groundmass with parts of two kamacite spindles. The groundmass shows 

 an imperfect paraeutectold structure of taenite particles In kamacite. The large kamacite 

 areas are surrounded by taenite borders, but not completely; at the upper left the kamacite 

 of the spindle merges with the corresponding component of the groundmass. The taenite 

 border also merges in places with Irregular taenite particles in the groundmass. Some of the 

 particles are darkened because of supersaturation, but their sharp dark outlines are due to 

 the strong attack of the etchant along their interfaces. Picral 15 seconds; X 600. 



3. NoRDHEiM, Texas; nickel-rich ataxite; Ni-Co 12.20 percent. General structure of the 

 Iron, a uniform gamma-alpha paraeutectold. The nickel content is higher than In the 

 preceding Irons and the ataxite structure Is more fully developed. A needle of schreibersite. 

 Picral 50 seconds; X 60. 



4. NoRDHEiM. Part of the area shown in figure 3 at higher magnification. The taenite 

 particles are slightly supersaturated, evidenced by a slight graying with fairly strong etching. 

 Orientation is evident. Picral 30 seconds; X 300. 



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