THE METAI^LOGRIACPHY OF METEORIC IRON 91 



Neumann lines produced by explosions are usually in one direction. 

 In meteoric irons the numerous sets are consistent with deformation 

 by air pressure, which although rapid would be less so than by the 

 action of an explosion or a blow. 



Thermal alteration. — Neumann lines are very susceptible to altera- 

 tion by heat; in fact they afford the first visible evidence of change 

 upon heating within the higher alpha range. The lines begin to 

 widen and become distorted, merging into the developing grain bound- 

 aries, until they disappear m the completed secondary granulation. 



Experiments by Kase (1925) on samples of Sacramento Moun- 

 tains (nickel 8.7 percent) showed that with prolonged annealing 

 even at as low a temperature as 550° to 560° the lines are ultimately 

 obliterated. It has been stated by Tammann (1923) that they 

 disappear slowly with prolonged heating even at 400°.^ 



The presence of perfect Neumann lines therefore indicates that 

 the iron has not been much reheated, either during atmospheric 

 flight or artificially. It may, however, have been reheated cosmic- 

 ally, the lines having been produced after the alteration. In the 

 case of Forsyth County, previously mentioned, in which Neumann 

 lines are present, the diffusion of phosphide particles indicates cosmic 

 reheating. In Dungannon, an iron that has been markedly altered 

 by reheating, the presence of Neumann lines in some areas proves 

 that the alteration was cosmic and the lines were produced subse- 

 quently; for the degree of reheating that was required to produce 

 the alterations in structure would have obliterated any previously 

 existing Neumann lines. 



The fact that the lines are absent in zones of alteration would 

 indicate that when they are present in the interior of the mass they 

 were not produced by impact upon the earth. If they were pro- 

 duced by impact, there seems to be no reason why they should not 

 be formed within the zone of alteration, which in most cases would 

 be quite cool when the meteorite struck the earth. 



Wlien a sample of meteoric iron shows Neumann lines in some 

 areas but in others gives evidence of reheating by the diffusion of 

 phosphide, it is likely that the latter area is part of a zone of altera- 

 tion, even though the sample may not include any part of the original 

 surface of the mass. As schreibersite melts at 970°, and does not 

 even begin to diffuse below about 700°, no great change in phosphide 

 inclusions would be possible without the alteration or obhteration 

 of Neumann lines. 



» Two exceptional cases are mentioned by Foley and Howell in which Neumann lines proved resistant 

 to temperatures that have been regarded by all authorities as sufficient to obliterate them. One inves- 

 tigator, they report, heated a sample of artificial iron to 1,000 centigrade (sic) and cooled it slowly, and 

 another held a sample at that temperature for 30 days, both stating that the lines remained. As these 

 results are so greatly at variance with the experience of many investigators, it is hard to believe that some 

 unknown factor may not have influenced the results. 



