COMPOSITION AND STRUCTURE OF METEORITES 15 



pyrrhotite and in the metallic as troilite. The form assumed is 

 somewhat variable. In the irons it most commonly occurs in glob- 

 ular, spherical masses in sizes up to two or three centimeters or 

 more in diameter; sometimes in greatly elongated, conical forms, as 

 in the Santa Rosa iron. These may be wholly of sulphide, or of 

 sulphide admixed with amorphous carbon. Such nodules are often 

 surrounded by a shell of schreibersite. In the stony meteorites the 

 sulphide occurs in much smaller masses, rarely over two or three 

 millimeters in diameter, scattered through the ground or closely 

 associated with the metallic particles. In the stone of Holbrook, 

 Ariz., the sulphide occurs in such size and form as to be separable 

 from the matrix. Material thus obtained was found to be non- 

 magnetic and yielded on analysis: Iron, 63.62 per cent; sulphur, 

 36.50 per cent; with no nickel, cobalt, nor copper. The mineral in 

 this case is, therefore, evidently the monosulphide troilite. Ramsay 

 and Borgstrom arrived at similar conclusions in their investigation 

 of sulphide in the Bjurbole meteorite. The idea advanced by 

 Allen ^* to the effect that the mineral is not to be considered a true 

 species but rather as the end member of the pyrrhotite series of iron- 

 sulphur compounds is doubtless correct. The name is, however, 

 retained as a convenient term to distinguish the monosulphide so 

 characteristic of meteorites.^^ 



CHEMICAL COMPOSITION OF METEORITES 



Since meteorites are aggregates in varying proportions of the 

 minerals described it must necessarily follow that they are some- 

 what variable in ultimate chemical composition. The average of a 

 selected number of analyses of wholly metallic forms has been given 

 in the table on page 9. 



Whether or no the aU iron meteorites are to be considered inde- 

 pendent of the stony forms, and perhaps even from different sources,^^ 

 is an open question, though the presence of transitional forms like the 

 mesosiderites and pallasites, especially those of the Brenham and 

 Estherville type (pis. 13, 14, and 15) suggests that they may be but 

 residual segregations in large masses from which all of the siliceous 

 portions have been eliminated. The size of some of these metalHc 

 masses, as that of Cape York or Bacubirito, to be sure, puts a con- 

 siderable strain upon one's imagination, but there is nothing im- 

 possible, or improbable about it, and those who argue in favor of a 

 metallic nucleus for our own earth should certainly find no difficulty 

 in accepting the idea. 



" Amer. Jour. Sci., vol. 33, 1912. 



" Until recently this form of iron sulphide was regarded as of purely meteoric origin. It has of late, 

 however, been discovered in considerable quantity in a copper mine in Del Norte County, Calif. See 

 Amer. Mineralogist, May 1922, p. 77. 



2« Pickering, it will be recalled (Popular Astronomy, No. 165), regarded the cometary origin of iron 

 meteorites as plausible, but not so the stones which he felt were of terrestrial origin. 



