THE METAI^LOGRiASPHY OF METEORIC IRODST 89 



kamacite containing minute phosphide particles but with no structure 

 that would interrupt the formation of Neumann lines. As pointed 

 out in Chapter XIII, there is no definite demarcation between hexa- 

 hedrites and nickel-poor ataxites. 



In artificial irons Neumann lines occur only in ferrite, and in 

 meteoric irons only in kamacite. They are absent in nickel-rich 

 ataxites, which are composed of an exceedingly fine mixture of 

 kamacite and taenite. 



Sometimes there is only a single set of parallel lines, but, espe- 

 cially in hexahedrites, there often are two or more sets. It has been 

 established that the lines are fonned along the (211) alpha, a 12- 

 family plane, or in other words are parallel with the planes of the 

 trapezohedron, so the lines might develop in any number of directions 

 up to twelve. Three or four sets are common, and occasionally 

 five or six are observed. In Scottsville (pi. 4) traces of a seventh 

 can be recognized. The lines are usually straight, often of the most 

 perfect regularity, but somethnes they are irregular or distorted. 



Neumann lines and slip bands. — In artificial irons, where the effects 

 of deformation have been mmutely studied, a distinction is drawn 

 between Neumann lines and slip bands. The latter (termed by 

 some writers, perhaps more aptly, slip lines) appear as very fine 

 black lines on the polished surface of metal that has been subjected 

 to strain, and which disappear with repolishing and etching, whUe 

 Neumann lines persist. Slip bands are caused by a slight movement 

 along gliding planes. Neumann lines are produced in artificial iron 

 most readily by shock or sudden stress, though they may also result 

 from strain. 



Origin. — The most generally accepted view is that slip bands are 

 produced by gliding, but that Neumann lines are mechanical twins; 

 though Rosenhain and McMinn (1925) take the view that they are 

 not twins but the result of local deformation from rapid heavy 

 stress. In New Baltimore (pi. 76), where there is abundant evidence 

 of shearing displacements and other movements, the lines follow 

 the planes of such deformation. They are very irregular in places, 

 conforming with the movements of the metal. 



Neumann lines produced in artificial iron by the action of explo- 

 sives are illustrated in a large number of photographs by Foley and 

 Howell (1923) and Foley and Crawshaw (1926). As in meteoric 

 irons, sets of fine and coarse lines appear in the same grain, each set 

 being uniform in character. Lines were produced in three directions 

 within one grain, and in a few instances in four directions. 



We may therefore assume that in meteoric irons Neumann lines 

 may be the result either of stress or shock. Whether they are faint 

 or strong, sparing or abundant, and whether there is one set or sev- 



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