72 BUI/LETESr 18 4, UNITED STATES NATIOKIAL MUSETJM 



Natural reheating is of two types — terrestrial and cosmic. The 

 former is brief, due to heat generated during the flight of the meteor 

 through the atmosphere, the structural changes being superficial 

 and usually only incipient. Cosmic reheating, reflected in altera- 

 tions of the general structure throughout the mass, must have been 

 higher and of longer duration, with probably slow cooling. 



Terrestrial heating, producing the familiar "zone of alteration," 

 will be discussed first because the results were recognized and inter- 

 preted before improved methods of research revealed the effects of 

 cosmic heating. 



Zones of alteration.- — Iron meteorites often show on etched slices 

 a peripheral band of varying width, up to a centimeter or more, in 

 which ordinary etching develops a dull granular surface. In octa- 

 hedrites the contrast with the inner Widmanstatten structure is 

 conspicuous to the eye. This is called a zone of alteration, the 

 result of superficial heating during flight through the atmosphere 



(pl. 71). 



Though described in numerous octahedrites, zones of alteration 

 were observed by the older v/riters in but few hexahedrites or ataxites. 

 Cohen mentions onlj^ half a dozen of the former and three of the 

 latter. Such zones, however, in the absence of metallographic 

 methods, would be likely to escape observation except in octahedrites. 

 The author has found them in many hexahedrites and ataxites, and 

 there is no reason to assume that they are not equally common in 

 all types of meteoric irons. 



Within the zone are found the usual indices of reheating — disap- 

 pearance of Neumann lines, granulation of kamacite, partial diffu- 

 sion of schreibersite, and more or less alteration of dense plessite 

 due to the partial diffusion of taenite particles. These changes in 

 various types of iron are shown in plates 71-74. 



In hexahedrites the zone of alteration usually shows fine granula- 

 tion, strongly marked, and diffusion of phosphide inclusions. The 

 resulting structure is in cases identical with that of nickel-poor 

 ataxites (cf. plates 9 and 58). Rhabdites lose their angular outlines 

 and become shapeless droplike particles, or may disappear entirely, 

 leaving only spots of phosphide enrichment which darken with 

 picrate etching. Masses and the larger needles of phosphide com- 

 monly show a thorny or prickly outline, caused by the diffused 

 phosphide invading the surrounding mass along grain boundaries 

 (pis. 58, 59). Such inclusions often show a phosphide eutectic 

 structure, as described in the following chapter. 



In nickel-poor ataxites the author has not observed a distinct 

 zone of alteration. This might be expected because, as explained 

 later in this chapter, such irons are a product of alteration by cosmic 



