410 E. T. Allen— Composition of Natural Bornite. 



the purest bornites they had met with in their extended inves- 

 tigations of mineral sulphides at the Harvard Mining School, 

 while several others obtained from different sources were care- 

 fully examined by Dr. H. E. Merwin of this laboratory. 



The following table includes analyses of all the bornites of 

 satisfactory purity which the author was able to get. 



It will be observed that the analyses agree closely with the 

 formula Cu 5 FeS 4 with the exception of that of the bornite 

 from North Carolina. The discrepancy in this case is satis- 

 factorily accounted for by the presence of a little chalcocite in 

 the mineral. Dr. H. E. Merwin in an independent micro- 

 scopic examination estimated the amount of the chalcocite as 

 2 to 3 weight per cent. Calculated on the basis of the copper 

 determination, which is very exact, and assuming the formula 

 of bornite to be Cu 5 FeS 4 , the quantity of the chalcocite is 

 found to be 3 - 45 weight per cent. If we take 5*076 as the 

 specific gravity of pure bornite and 5 - 8 as that of pure chalco- 

 cite,* the percentage of chalcocite in a mixture having a gravity 

 of 5'103, becomes 4 p 22 per cent. Finally, if we assume the 

 copper determination to be exact and accept the formula 

 Cu 5 FeS 4 , we calculate the gravity of the bornite present to be 

 5*074. There can, therefore, be no reasonable doubt that all 

 the specimens I have analyzed are essentially identical and are 

 well represented by the formula Cu 5 FeS 4 . 



My conclusion as to the composition of bornite consequently 

 coincided with Harrington's, and I had no intention of pub- 

 lishing these results separately until several other papers 

 appeared on the subject, in all of which the authors presented 

 different views. 



In the number of this Journal for June, 1914 (vol. xxxvii, p. 

 539), Kraus and Goldsberry, while admitting that some bornites 

 have the composition Cu 5 FeS 4 , maintain that other specimens 

 have a different composition, as did Cleve years ago. They 

 submit in evidence two analyses made by them on the bornite 

 crystals from Bristol, Connecticut. The analyses follow : 



I II Average 



Cu 65-42 65-91 65-665 



Fe 9-74 9-67 9-705 



S 24-79 24-51 24-656 



99-95 100-09 100-020 



Kraus and Goldsberry state, regarding the material for these 

 analyses, that they used portions of two different specimens, 

 viz : 4 and 1. No. 4 consisted of an aggregate of cubical crys- 



* Posnjak, Allen and Merwin, The Sulphides of Copper, Economic Geol- 

 ogy, x, 491, 1915. 



