20 BULLEMN 149, UNITED STATES NATIONAL MUSEUM 



accompanying metal. At times the kamacite plates assume broad and 

 irregular forms as in the iron of Ainsworth and New Baltimore (lower 

 figure, pi. 10; upper figure, pi. 29), predominating over all other con- 

 stituents; in such octahedral structure is wholly undiscernible except 

 on large surfaces.*' In some instances taenite and plessite are almost 

 wholly lacking, the entire mass of the iron being composed of the 

 coarse kamacite granules. An interesting varietal phase of octahedral 

 structure is shown in Plate 6 from an iron found in Dungannon, 

 Va.,*^ some years ago. It will be noticed that this has undergone the 

 partial granulation described though traces of the original octahedral 

 structure are still discernible. The dark areas are of graphite with 

 metallic inclosures. 



Not all irons are octahedral. In some the metal occurs in the 

 form of granules so fine as to escape easy notice, and thus to appear of 

 a noncrystalline structure or amorphous. These irons will often 

 show on etching certain faint parallel lines traversing the etched 

 surface, which are due, according to Neuman of Vienna, after whom 

 they are named, to the union of crystals in definite opposed relations 

 technically known as twinning, and in this case parallel with the faces 

 of a cube. (Uppei figure, pi. 8.) Still other irons are distinctly gran- 

 ular throughout, a structure which as shown later, may be secondary 

 and due to the action of heat (lower figure of the same plate). The 

 systematic regularity of arrangement of the taenite and kamacite 

 plates which form the chief constituents of an octahedral iron is often 

 interrupted by the presence in minor quantities of various accessory 

 minerals as cohenite, schreibersite, and troilite or carbon nodules 

 which last are as a rule distributed without order or, it may be, lying 

 parallel with the kamacite bands. 



Nearly all irons carry varying, though minute quantities of olivine 

 and other silicates, quartz, and occasionally diamonds in quantities 

 so small as to be detected only in the residues left when the metal is 

 dissolved away by dilute acid. Graphite when present is left as a 

 black amorphous mud. The ubiquitous lawrencite makes its pres- 

 ence known through exudation of greenish drops on a polished surface 

 which quickly absorbs moisture and oxidizes to a rusty red color. 

 It is this mineral which brings about the rapid destruction of many 

 an iron meteorite and which perhaps explains the fact that no meteor- 

 ites are found in any but the most recent of formations. 



Closely related to the wholly metallic forms are those of a somewhat 

 limited group represented to advantage in Plate 9 by a cross section 

 of a mass found near the bounding corners of Arizona, Colorado, 

 New Mexico, and Utah in the United States and hence known as the 



*' Farrington's tabulations of analyses seems to show that the texture varies with the nickel content, the 

 finest crystallization being found in irons richest in nickel. The ratio is, however, by no means constant. 

 Field Museum publication, No. 120, 1907. 



» Proc. V. S. Nat. Mus., vol. 62, 1923, art. 18. 



