THE METALIiOGRlAPHY OF METEORIC IRON 63 



Granular octahedrites. — Certain forms of octahedrites have -been 

 designated as brecciated, or breccialike (Ob), and Brezina's clas- 

 sification divided these into several subgroups named after typical 

 examples. Altogether about half a dozen irons have been thus 

 classified. 



In a number of cases the author doubts whether this classification 

 is applicable. For example Zacatecas (Ni-Co 6.55 percent; Cohen, 

 1897) in the particular specimen studied shows no octahedral char- 

 acter, either macroscopic or microscopic, but resembles a typical 

 granular hexahedrite. The same is true of Barranca Blanca, the 

 specimen studied being apparently a granular hexahedrite somewhat 

 resembling Kendall County. The Ni-Co content of 8.66 percent 

 found by Fletcher (1889, p. 263) would seem to be high for such a 

 structure. 



Four Corners, however, would clearly fall within the classification, 

 although the term granular would be more appropriate than brec- 

 ciated. That unique iron shows a pattern of very large grains in 

 each of which a highly perfect fine octahedral structure has developed 

 on planes that are unrelated in the different grains. The analysis 

 by Whitfield (in Merrill, 1924) showed 10.16 percent Ni-Co, which 

 is consistent with the octahedral pattern. 



Santa Rosa, Colombia, also would come within the classification. 

 This iron (known also as Rasgata and Tocavita) shows unrelated 

 areas of fine octahedral pattern. Its Ni-Co content as found by 

 Manteuffel (Meteoreisen-Studien, pt. 3, p. 113, 1894) was 7.10 percent. 



An anomalous iron. — A meteoric iron of anomalous type is Horse 

 Creek (pis. 60, 61). In this extraordinary iron, which to the eye 

 resembles an octahedrite, there are no lamellae of taenite but in their 

 place lamellae of schreibersite have formed in a Widmanstatten struc- 

 ture, the mass consisting wholly of kamacite and phosphide. The 

 biittleness of the latter causes the mass to break very easily with a 

 highly perfect cleavage which might easily be mistaken for octahedral. 



The structure in general resembles that of a fine octahedrite, 

 although the phosphide lamellae are much finer, much more numer- 

 ous and closely spaced, and of extreme regularity. Some are fairly 

 thick, sufficiently so to reveal a eutectoid structure with picrate 

 etching; but for the most part they are very tenuous, appearing 

 even under high magnification as extremely fine and straight black 

 lines. Occasionally such fine lines thicken and coalesce with the 

 broader phosphide lamellae, thus dispelling any doubt that they 

 are phosphide. 



Minute phosphide needles and extremely fine particles are thickly 

 dissemmated in the angular interstices of the octahedral pattern, 

 being most abundant in the centers of such fields and sparse or 

 absent near the bounding phosphide lamellae. 



