THE METALLOGRAPHY OF METEiORIC IRONl 49 



Iron meteorites are generally regarded as single or unigrain crys- 

 tals, their structure running uninterruptedly throughout the mass. 

 This may be due to one of two mechanisms. The liquid metal 

 may have frozen with such extreme slowness that the whole mass 

 crystallized upon one nucleus, producing a single crystal; or if the 

 mass in the gamma range was an aggregate of grains, with cooling 

 of secular slowless, the aggregate may have changed to a single 

 crystal by simple grain growth — smaller grains merging into larger 

 ones until finally only one was left. 



The unigrain character is shown in "normal" hexahedrites, which 

 have a distinct cubic cleavage. In granular hexahedrites, the planes 

 may be diversely oriented in different grains and the cleavage is 

 interrupted. In octahedrites the unigrain character is reflected in 

 the contmuity of the octahedral pattern, which normally runs through 

 the entire mass without change of directions. 



Octahedrites, however, although they are practically unigrain in 

 character, are nevertheless not single octahedral crystals. Slow 

 cooling in the gamma-alpha range produced an apparent unigrain 

 octahedral structure in the Widmanstatten pattern. The kamacite 

 of the bands, however, in body-centered alpha, the taenite lamellae 

 remaining face-centered gamma, so that the Widmanstatten structure 

 is only a remnant pattern or pseudomorph of a former octahedral 

 crystalhzation. Thus an octahedrite, though showing an apparent 

 unigrain character, has neither cubic nor octahedral cleavage — the 

 normal cubic cleavage of its kamacite being suppressed by the inter- 

 ference of the octahedrally arranged taenite lamellae, and probably 

 also in many cases by the acicular transformation structure often 

 found in the kamacite itself (see Chapter XI) . 



Exceptionally an apparent, though false, octahedral cleavage 

 may result from mechanical separation along the planes of the bands 

 because of the invasion of oxide. In the unique case of Horse Creek 

 such a cleavage is caused by lamellae of phosp'iide, but is not on 

 octahedral planes (see pis. 60, 61). 



The dendrites referred to, marking the first stage of crystallization 

 from the melt, may be preserved in artificial irons by quick cooling. 

 Their outlines also may be preserved by the segregation of impurities 

 even after the dendrites themselves have been wholly changed by 

 granulation. In meteoric iron dendrites apparently do not exist 

 except in rare instances as a result of a slight superficial invasion of 

 melted oxide during atmospheric flight (see Chapter XIII). In 

 two instances the author observed a dendritic structure in inclu- 

 sions containing an iron-carbon-phosphide eutectic (pis. 65, 66). 



Granulation. — Kamacite may be homogeneous, as in many hexa- 

 hedrites and in the bands of many octahedrites, or it may be gran- 



