METEORITES — HENDERSON AND PERRY 



245 



can expel the taenite — thin hith areas of kamacite should be produced. 

 Such a pattern is known as a fine octahedrite. The rate of cooling is 

 important and may be responsible for many structures in iron 

 meteorites. 



Table 2. — Composition of octahedrites 

 (E. P. Henderson, analyst) 



Note.— Nos. 1-5 are coarse octahedrites, Nos. 6-8 are medium to fine octahedrites. 



Although it is impossible to define exactly the boundary between 

 hexahedrites and octahedrites, it is obvious that the two tables indicate 

 the approximate position of this boundary. The accuracy with which 

 nickel and especially cobalt can be determined is limited by certain 

 sources of error ; hence it is increasingly difficult to fix boundaries for 

 the chemical composition of either of these groups. The behavior of 

 cobalt and nickel is practically identical in these alloys, and in discuss- 

 ing the compositions of these groups, therefore, the cobalt and nickel 

 together are considered as a unit. 



There are octahedrites that iiave large areas of kamacite separated 

 from adjoining areas of the same alloy by only a fine line. Careful 

 search along these boundaries usually will detect some taenite ; but as 

 the compositions approach the limits of solubility for cobalt and nickel 

 in iron at the different temperatures, a time comes when there will be 

 formed only a trace of taenite, which can be very easily overlooked. 

 Thin, discontinuous plates of taenite were found in the El Burro 

 specimen, and none at all in the Mount Joy and Sandia Mountains 

 specimen. Hence the composition of these three meteorites is near 

 the border line between the octahedrite and hexahedrite groups. 



A simplified equilibrium diagram of the iron-nickel system given 

 by E. A. Owen and A. H. Sully ^ traces the composition and tem- 

 perature range for the boundary of kamacite (hexahedrite) and kama- 

 cite-taenite (octahedrite) structures. It shows (fig. 1) that at 400° C. 

 kamacite contains its maximum nickel content, or slightly over 6 

 percent ; at 300° C. the solubility had decreased to 5 percent nickel. 



Phil. Mag. and Journ. Sci., vol. 27, No. 184, p. 614, 1939. 



