THE METALLOGRAPHY OF METEORIC IROuS" 11 



a needle, such as a very mild steel. It etches qiiickl}^, being soluble 

 in dilute acids. 



In the literature of meteorites kamacite is commonly designated 

 as granular, hatched, and spotted. The granular form, as the name 

 indicates, shows a structure of minute polyhedral grains. Hatched 

 kamacite, appearing onty in octahedrites, is marked by a prolusion 

 of fine transverse or oblique lines, giving a band (with macroetching) 

 somewhat the appearance of a file. This structure, formerly attrib- 

 uted to Neumann lines, is a product of phase transformation, which 

 is explained in Chapter XL Spotted kamacite (mottled or darkened 

 might be more apt), showing shaded or darkened areas, is less often 

 observed. Like the similar structure frequently found in taenite, it 

 is a transformation product. 



Composition of kamacite. — The most satisfactory analyses of 

 kamacite have been made from hexahedrites, which consist wholly 

 of that material. They show a rather uniform nickel content of 

 aromid 5.5 percent, with a few slightly below 5 percent. Some 

 analyses have been made of the kamacite in octahedrites which 

 show nickel-cobalt percentages around 7. Such figures cannot be 

 regarded as a reliable index of its composition because of the diffi- 

 culty in obtaining samples that are free from taenite. 



As will be explained in chapters IX and XIII, about 6 percent 

 is the upper possible limit of the nickel content of kamacite. Below 

 that percentage the nickel content is indefinite; but, as previously 

 pointed out, very few recent analyses show less than 5 percent of 

 nickel in any meteoric iron. 



Oriented sheen. — A characteristic property of kamacite is a dis- 

 tinct, often brilliant, oriented sheen, which changes as the specimen 

 is rotated. This appearance, which is conspicuous only with macro- 

 etching, is also observable in many hexahedrites; but it is most 

 striking in octahedrites, various bands becoming bright or dark in 

 changes of light. 



A number of explanations have been offered for the sheen — that 

 it is due to Neumann lines, or to polysynthetic twinning; Vogel's 

 suggestion that (at least in some cases) it is due to oriented sub- 

 microscopic particles of schi*eibersite; and the older view of Tschermak 

 that it is due to minute or submicroscopic etching pits, cubic in form 

 and oriented. 



The first explanation is clearly imtenable. A prolusion of Neumann 

 lines in one direction m some hexahedrites may give somewhat of 

 the effect of an oriented sheen; but hexahedrites exhibit such a sheen 

 in the absence of Neiunann lines, and the same is true of the sheen 

 of kamacite plates in octahedrites. A sheen is also found in the 

 grains of granular ataxites, in which there are no lines. 



