5-ii Kraus and Qoldsberry — Chemical Composition of 



elusions or admixtures. The surfaces showed absolute homo- 

 geneity, which was further confirmed by etching with nitro- 

 hydrochloric acid. 



Interpretation of the Chemical Data. 



It is now clearly evident that the material investigated con- 

 sisted, in the cases referred to, of crystallized bornite, was 

 homogeneous, possessed different specific gravities, and showed 

 marked differences in chemical composition. That the analyses 

 of crystallized bornite made by earlier investigators had also 

 led to the formula Cu 6 Fe„S 6 must be emphasized here. We, 

 thus, have three formulas which may be used to express the 

 composition of the mineral we usually call bornite, namely 

 Cu 6 Fe 2 S 6 , Cu 10 Fe„S 8 , and Cu 12 Fe„S 9 . It is, therefore, necessary 

 to determine how this great variation in composition can be 

 explained most satisfactorily. 



"With this in mind the various analyses of bornite and chal- 

 cocite, as listed by Hintze,* were calculated for the possible 

 formulas they might yield. The analyses of chalcocite were 

 included in this study because it was found that one of them 

 leads directly to the formula Cu 6 Fe„S 6 , the one very commonly 

 accepted for bornite. Another reason for including chalcocite 

 is the fact that this mineral usually contains iron in varying 

 amounts and, when massive, may resemble in many ways some 

 bornites. Of the various analyses considered, it was found 

 that thirty-eight lead quite directly to definite formulas. The 

 following table shows the various formulas which resulted, 

 arranged in order of increasing copper content. The number 

 of analyses yielding these formulas is indicated in each case, 

 and where possible the specific gravity is also given. It will 

 be noted that by adding to the list the compound Fe 2 S 3 , 

 not as yet found in nature, chalcopyrite, barnhardtite, and 

 chalcocite, a most interesting series from Fe„S 3 to Cu 2 S can be 

 arranged. 



Name of Specific Number of 



mineral gravity Formula analyses 



Fe 2 S 3 



Chalcopyrite 4-2 Cu a Fe 2 S 4 



Barnhardtite 4-52 1 Cu 4 Fe 2 S 5 



Bornite..- ..4-9 Cu„ Fe 2 S 6 4 



" Cu 8 Fe 2 S 7 f 3 



5-072 Cu 10 Fe 2 S 8 17 



" 5-086 Cu 19 Fe a S„ 4 



*Loc. cit., pp. 914-916 and 537-538. 



f This conforms to the formula generally accepted for tetrahedrite, see 

 page 549. Some cupriferous tetrahedrites possess specific gravities slightly 

 over 5 ; Dana, System of Mineralogy, 1892, 6th edition, pp. 139-140. 



