52 BULLETIN 18 4, tHSTETED STATE'S NATIONAL MUSEUM 



of the maximum of the two in kamacite. Theoretically there would 

 seem to be no reason why that content might not sometimes be much 

 lower; but as previously observed, no recent analysis of any mete- 

 oric iron, to the author's knowledge, shows less than about 5 per- 

 cent nickel, and only two show slightly less than 6 percent of nickel 

 and cobalt. Old analyses showing less than 4 percent nickel can- 

 not be accepted unreservedly. 



X. HEXAHEDRAL STRUCTURES 



As is apparent from the diagram, the simplest structure of meteoric 

 iron is that of hexahedrites, which contain less than 6 percent of 

 nickel and therefore consist wholly of alpha kamacite. The hetero- 

 geneous gamma-alpha stage (AEF in fig. 5) was transitory, and 

 before diffusion was stopped by cooling the alpha phase was estab- 

 lished. As a result the structure of such irons is simple, lacking the 

 varied and interesting features in octahedrites and ataxites that 

 were produced during transformation. 



Classification. — Hexahedrites are classified as normal and granular 

 (also less aptly termed brecciated). 



A normal hexahedrite consists of homogeneous kamacite. A 

 macroetched surface shows no structure except for Neumann lines 

 and occasional visible inclusions. Under the microscope a typical 

 field shows only clear kamacite, usually marked by Neumann lines 

 and often strewn thickly with rhabdites or phosphide particles. 

 Remnant grain boundaries may be visible, but they are usually faint. 

 An outstanding characteristic of normal hexahedrites is a cubic 

 cleavage, which is often observable on a broken surface, due to the 

 cubic crystallization of the alpha phase. 



Granular hexahedrites are characterized by visible macrograins, 

 often of large size. When grain boundaries are not strongly marked, 

 the coarse granulation may still be recognized on a polished surface 

 because of the varying orientation of Neumann lines in the grains. 

 In some hexahedral irons (e. g.. Central Missouri, New Baltimore) 

 the grains may reach dimensions of several inches, the boundaries 

 strongly marked by the deposition of impurities. Each grain is a 

 unit, typically hexahedral in structure, with its own crystallographic 

 planes. 



Inclusions. — Hexahedrites as a class are rich in nonmetallic inclu- 

 sions^ — schreibersite, troilite, less frequently graphite, and daubreelite. 



The phosphide appears as schreibersite bodies, often numerous 

 and sometimes reaching dimensions of inches. They are of varied 

 irregular shape, often sprangling or meandering, sometimes in clear- 

 cut graphic forms. It also occurs as needles (lamellae) of various 

 dimensions. These are sometimes parallel and oriented; sometimes 



