COMPOSITION AND STRUCTURE OF METEORITES 19 



etched that chief reliance must be placed. As with terrestrial rocks, 

 it would be very difficult to give by written description alone a clear 

 impression of some of the curious features as revealed by this method, 

 and the reader is advised to consult carefully the numerous illustra- 

 tions which are reproductions from photographs made in part through 

 the microscope. 



1. ALL-METAL METEORITES: SIDERITES 



The structural peculiarities of the all-metal meteorites have been 

 well described and beautifidly illustrated photographically by Cohen 

 and Brezina in their work Die Struktur und Zusammensetzung der 

 Meteoreisen (Stuttgart, 1906). As, however, this work is quite inac- 

 cessible to the majority of students, space may well be given here to 

 a description of a few of the more typical forms.^^ 



Metallic meteorites, as has been said, are composed almost wholly 

 of iron with small and variable percentages of nickel and cobalt. 

 In the main, these metals are combined to form the two alloys named, 

 respectively, kamacite and taenite as already described. Each of 

 these alloys in the large majority of cases occurs in the form of thin 

 plates with intervening areas of a third alloy or properly eutectic 

 called plessite which fills the interstices. A characteristic feature 

 of the kamacite and taenite is a tendency to arrange themselves 

 in the form of thin plates lying parallel to the faces of a possible 

 octahedron. To reveal this structure clearly, as shown in Figure 1, 

 Plate 5 and Plate 7, it is necessary to polish a flat surface and etch 

 it with dilute acid.*° Owing to the differential solubility of the three 

 alloys mentioned, they will be acted upon unequally and stand out 

 each Avith its own relief. Such markings are called Widmanstatten 

 figures, after their discoverer. In thickness the plates vary from 

 the fraction of one to several millimeters, which fact forms the basis 

 of separation into fine odaJiedrites (Of), medium odahedrites (Om), 

 and coarse odahedrites (Og), etc. The kamacite presents several 

 varietal forms, dependent upon position and internal peculiarities 

 brought out by magnification. In the octahedral irons the bands are 

 often swollen in the middle and constricted at the ends, as shown in 

 Plate 7. In the pallasites a band of white kamacite a few millimeters 

 in diameter often incloses the silicates and is known as swathing or 

 "wickel" kamacite. (See upper figure of pi. 7.) A thin band of 

 taenite may or may not lie parallel with this and between it and the 



59 The all-metal meteorites furnish a problem for the metallurgist which needs scarcely be touched upon 

 here, and the reader is referred to standard treatises on the subject. See especially Osmond and Cartaud 

 in the Metallurgist, vol. 4, 1891, and also the Comptes Rendus, vol. 137, 1903, p. 1057; Stahl u. Meteoreisen 

 by F. Berwerth, Metallurgie, vol. 4, 1907, p. 722, reprinted under the title Steel and Meteoric Iron in the 

 Iron and Steel Institute, September, 1907, Ein Naturlisches System der Eisen Meteoriten, Sitz d. Kais. 

 Akad. der Wiss, vol. 123, Abt. 1914, p. 1047; and finally Borgstrom's paper: On the Composition of the Nickel 

 Iron Alloys and on Magnetic Lines on Sections of Meteoric Irons, Fennia, Helsingfors, vol. 45, No. 2, 

 1925, pp. 1-18. * 



*" Details of the etching process are given by Farrington in his Meteorites, pp. 127-130. 



