THE METALLOGRAPHY OF METEORIC IRON 17 



are classed as fine (e.g., Bear Creek) but not in the finest. In some 

 of the coarsest octahedrites it is wholly absent, Osseo, for example, 

 shows neither plessite nor taenite. 



Logically it would seem that plessite also should be absent in 

 hexahedrites, which contain even less nickel than the coarsest octa- 

 hedrites, and in fact that has hitherto been assumed. Plessite has 

 been regarded as exclusively an octahedral structure, and the absence 

 of both plessite and taenite as characteristic of hexahedrites. The 

 author, however, has found numerous coarse plessite fields, with 

 taenite borders, in Otumpa, which is a typical hexahedrite both as 

 to general structure and composition. In Sierra Gorda, a typical 

 "normal" hexahedrite with the low nickel-cobalt content of 5.83 per- 

 cent, a small area of plessitic structure was found in which taenite 

 is segregated both as a border and in oriented inclusions (pi. 78). 

 As a rule plessite fields in the coarsest octahedrites that approach 

 hexahedrites in composition and character are irregular and atypical. 



Dense plessite is characteristic of the finer octahedrites, and in 

 the finest it is the chief component. In the coarser types it occurs 

 sparmgly and in small areas. Its structure is substantially the 

 same as that of nickel-rich ataxites. The manner of production 

 of such structures is discussed in Chapter XII. 



IV. ACCESSORY CONSTITUENTS OF METEORIC IRON 



Nonmetallic inclusions are found abundantly in meteoric irons, 

 of which the commonest are troilite (FeS) ; daubreelite (FeS-CrgSs) ; 

 schreibersite, a nickel-iron phosphide of varying composition; and, 

 more rarely, the nickel-iron carbide known as cohenite (FeNi)3C, 

 iron oxides, and silicates. Only these will here be considered, as they 

 are the only ones having any important relationship with the struc- 

 ture of such irons. Their general character will first be noted, and 

 their metallographic aspects will be discussed more fully in later 

 chapters. 



Troilite. — Iron sulphide, the most familiar inclusion in meteoric 

 irons, was recognized by the oldest writers and was named in 1863 

 by Haidinger in honor of Domenico Troili, who in 1766 mentioned 

 an iron sulphide found in a stony meteorite. It is common in both 

 meteoric stones and irons, but it occurs in much larger masses in 

 the irons, sometimes in nodules 2 or 3 inches in diameter. It appears 

 commonly in droplike inclusions or in irregular rounded forms. It 

 shows a microstructure of polygonal grains (pi. 49). 



In octahedral irons troilite is occasionally observed in the form 

 of scattered plates or lamellae with a definite cubic orientation un- 

 related to the octahedral pattern. These are known as Reichenbach 

 lamellae (pi. 47). 



