1. Tremox, Wisconsin; medium octahedrite; Ni-Co 7.73 percent (1869). A coarse 

 plessite field in which kamacite lamellae are darkened by the presence of a fine gamma-alpha 

 aggregate, a result of incomplete transformation, an example of the so-called "spotted" 

 kamacite. In this iron the kamacite generally shows an acicular gamma-alpha structure 

 reflecting incomplete transformation; see plate 35. Picral 50 seconds; X 60. 



2. San Francisco del Mezquital, Mexico; nickel-poor ataxite; Ni-Co 6.33 percent. 

 Though having a nickel-cobalt content typical of hexahedrites, and in the alpha phase, 

 this iron in places shows an acicular gamma-alpha transformation structure similar to that 

 in a number of octahedrites (pi. 35). Compare the similar structure in New Baltimore (Ni- 

 Co 6.74 percent) and Primitiva (Nl-Co 5.61 percent) in which this structure also occurs 

 (pis. 76, 11, 10). Picral 40 seconds; X 150. 



3. Sierra Gorda, Chile; hexahedrite; Ni-Co 5.83 percent (Henderson, 1941). An 

 atypical plessite field. Although this iron is a "normal" hexahedrite, with a Ni-Co content 

 below the average even for that group, it contains a number of minute areas suggesting ples- 

 sitic structure of which this was the onh' one of definite character. It shows irregular 

 lamellae of taenite and a considerable amount of black gamma-alpha mixture. Although 

 the structure is irregular and confused, it shows fairly distinct orientation, both as to the 

 lamellae and, in places, the dark gamma-alpha aggregate. The dead black spots were 

 occupied by large schreibersite inclusions which were chipping out in polishing, only 

 remnants remaining. Three small schreibersite bodies also appear in the edges of the field. 



The field is mostly surrounded by a border of taenite, which is not darkened by reason 

 of supersaturation, although the etching was very strong. Light etching did not reveal the 

 structure at all. Such a structure would seem to be a product of incipient transformation 

 at a relatively high temperature, followed by rapid cooling; thus some taenite of low nickel 

 content formed in a high range, and which normally would have disappeared, was preserved 

 because the mass so quickly became rigid. In such a case there would be a relatively small 

 diff'erence in the nickel content of the taenite and of the kamacite, and therefore little 

 difference in their solubility in acid, which would explain the fact that the structure was 

 not visible with light or even moderate etching. 



While the presence of taenite in a typical hexahedrite is unusual, it is found also in Otumpa 

 (pi. 44), which has a Ni-Co content even lower than Sierra Gorda. Picral 200 seconds; 

 X 60. 



198 



