4 BULLETIK 18 4, UNITED STATES NATIONAl, MX'iSEXJM 



II. GENERAL TYPES OF METEORIC IRON 



The classification of meteorites by the older writers and their 

 descriptive terminology have long been accepted; it is therefore 

 necessary at the outset to summarize briefly the recognized types 

 and the more familiar structural features. 



Meteorites are of three classes — those composed of stone (aero- 

 lites) ; those composed of a mixture of stone and iron (siderolites) ; 

 and those consisting wholly of metal (siderites). The present work 

 treats only of siderites. It does not cover the structures of the iron 

 found in meteorites of the transitional types between stones and 

 irons, or in the metallic grains found in stony meteorites; though in 

 such of these as were examined the metallic structures were found to 

 be similar to those of siderites of similar composition. 



Siderites are classified in three groups — hexahedrites, octahe- 

 drites, and ataxites. 



Hexahedrites. — Hexahedrites (designated by the symbol H) are 

 composed of a single structural component, kamacite,^ and have a 

 nickel content of less than 6 percent. The proportion of nickel is 

 quite uniform at around 5.5 percent, although a few analyses have 

 indicated a somewhat lower percentage. Cobalt apparently always 

 is present, the combined percentage ranging from about 6 to 6.50 

 percent. 



Kamacite is cubic in crystallization and often shows a distinct 

 cubic cleavage on broken surfaces. It may be structurally homo- 

 geneous, when the iron is called a normal hexahedrite (H) ; or it 

 may show irregularly rounded or polygonal grains, sometimes of 

 large size, when it is called a granular or grained (often inaptly 

 "brecciated") hexahedrite (Hb). See plate 1. 



Hexahedrites generally show one or more systems of long, fine, 

 parallel lines known as Neumann lines, which may be conspicuous 

 on a lightly etched surface when the light falls at certain angles. 

 These lines, however, are not an invariable characteristic; some 

 hexahedrites do not show them, while they are abundant in octa- 

 hedrites and are sometimes found in ataxites. 



The metallic surface with ordinary etching shows no other struc- 

 tural features, although often there are inclusions of iron phosphide 

 (schreibersite) or iron sulphide (troilite), which sometimes are of 

 considerable size. 



Octahedrites. — Octahedral irons are characterized by the Widman- 

 statten structure— a network of bands crossing one another in two, 

 three, or four directions (pis. 2, 3). The bands are composed of 

 kamacite, usually bordered by thin lamellae of an iron-nickel alloy 



' In the language of metallography kamacite is alpha nickel-iron, and taenite (presently to be mentioned) 

 is nickel-rich gamma. The iron-nickel system is discussed at length in Chapters VIII and IX. 



