COMPOSITION AND STRUCTURE OF METEORITES 23 



out of a metallic magma; a condition difficult to realize. Both 

 Rose and Kokscharow have measured and determined crystal facets 

 on the olivines in the Krasnojarsk pallasite/^ That, however, the 

 olivines did not crystallize in all cases in the position they now occupy 

 is shown in the lower figure, Plate 13, where the silicate is in sharply 

 fragmental form, a condition thought by Brezina ^^ to be brought 

 about by movement in the plastic metal in which the olivines are 

 embedded. 



Plate 14 is from a pallasite found some years ago at a locality 

 known as Brenham, Kans. The light, net-like portion is composed 

 of nickel-iron alloys identical in composition, so far as now ascer- 

 tained, with those of the all-metal meteorites. The dark areas are 

 silicate minerals — in this case olivine (peridot). The structure has 

 been compared, not inaptly, to that of a sponge in which the original 

 sponge material is metal, the silicates filling the meshes. Meteorites 

 of this type are somewhat rare, only about 20 now being known. 

 It is to be noted that the metal, wherever surfaces of sufficient size 

 are exposed, shows a tripartite structure and is never granular. 

 Further, that the kamacite bands often surround the olivines in a 

 form known as "swathing" kamacite (pi. 7) or white iron, on account 

 of the color and brilliant reflection. Between the kamacite and 

 plessite is often a thin band of taenite as in the all-metal forms. 



3. STONY METEORITES, OR AEROLITES «o 



The structure of many stony meteorites is of so confused and 

 heterogeneous a character as to be at times almost indescribable in 

 words, and one must refer to the illustrations. In their study the 

 same devices are employed as are commonly used for terrestrial 

 rocks. The figures shown in the accompanying plates are from pho- 

 tomicrographs made from the thin sections prepared in the customary- 

 manner. 



The one great difficulty in the determination and description of 

 meteoric minerals and structures lies in the imperfect crystal develop- 

 ment of the individual constituents, their shattered condition and 

 discoloration caused by oxidation of the lawrencite. This is par- 

 ticularly the case in the chondritic varieties which often present in 

 the section but a confused aggregate of polarizing points so charged 

 with secondary iron oxides as to render indeterminable any but the 

 two or three prevailing constituents, and uncertain the true nature 



*8 It is stiU a question if these faces may not be due to compression by interference in process of crystalli- 

 zation of a granular olivine aggregate before the introduction of the metal. See, Concerning the Origin 

 of the Metal in Meteorites, Proc. U. S. Nat. Mus., vol. 73, 1928, no. 2742, pp. 1-7. Also, Calcite Oolites 

 with Pentagonal and dodecahedral form, by E. V. Shannon, Journ. Washington Acad, of Sci., October 4, 

 1927. 



*s Die Meteoriten Sammlungen, 1895. See also Merrill, Concerning the Origin of the Metal in Meteorites, 

 Proc. U. S. Nat. Mus., vol. 73, art. 21, 1928, p. 4. 



s" A most excellent series of photomicrographs show^ing structures of meteoric stones, with descriptive 

 matter, is given in Tschermak's Die Mikroskopische Beschafienheit der Meteoriten, Stuttgart, 1885. 

 Unfortunately this work is not generally available. 



