6 BUXiLETES" 18 4, UNITED STATES NATIO'KIAL MfUSBUM 



Though the pattern is octahedral, the term octahedrite is in a 

 sense a misnomer, because the iron is actually cubic in crystalliza- 

 tion. The pattern originated in a higher temperature range in which 

 the kamacite bands developed on octahedral planes. As the mass 

 cooled it entered a new phase with a different space lattice, in which 

 the formation of bands on octahedral planes was impossible. The 

 macrostructure produced in the higher range remained, however, 

 the pattern thus being in effect a pseudomorph of a prior structure. 



The form of the pattern depends upon the direction in which the 

 section is made with reference to the Widmanstatten structure. If 

 it happens to be parallel with an octahedral plane there will be tliree 

 systems of bands forming equilateral triangles. If it is parallel with 

 a cubic plane there will be two sets intersecting at right angles. If 

 parallel with a dodecahedral plane there will be three sets, two mak- 

 ing angles of 109° 28' and another bisecting that angle. If made in 

 any other direction four systems of bands at various angles will 

 appear. 



As a mass of meteoric iron is generally in effect a single crystal, 

 when the structure is regularly developed the directions of the bands 

 are parallel throughout the mass. Individual bands can often be 

 traced for many inches without interruption. The structure, how- 

 ever, is often irregular and the bands are wavy, interrupted, and 

 sometimes barely distinguishable. 



The fineness of structure increases with the percentage of nickel. 

 The greater the nickel content the more taenite and the less kama- 

 cite appear; therefore the bands grow narrower and the taenite 

 lamellae thicker and more abundant. At the same time fields of 

 plessite become larger and more numerous. 



In the coarsest octahedrites taenite lamellae are barely observ- 

 able, or entirely lacking, and plessite is typically scanty or absent. 

 In medium octahedrites taenite lamellae are conspicuous and plessite 

 fields are abundant. In the finest octahedrites the greater part of 

 the surface examined is found to consist of a plessitic groundmass 

 in which the narrow kamacite bands are often widely separated. 

 Finally plessite prevails wholly, the kamacite becomes vestigial, 

 and with a nickel content aromid 14 percent the structure becomes 

 an ataxite. 



Ataxites. — The third group of irons, termed ataxites, appear to the 

 eye to be without structure and under the ordinary microscope show 

 only a fine mat textm*e. 



The name (from the Greek ara^la, disorder) was given to irons 

 that showed neither cleavage, Neumann lines, nor Widmanstatten 

 structure. Though high magnification with central illumination 

 reveals a rich microstructure, very little was observed by the older 



