E. T. Allen — Composition of Natural Bornite. 113 



of a small quantity of chalcopyrite which in preparing the 

 sample for study has escaped detection." 



Finally, observations of two physical properties of bornite 

 confirm the chemical evidence. Murdoch* states that " the 

 mineralographic examination of polished surfaces of bornite 

 from at least 30 different localities has revealed only an exceed- 

 ingly slight variation in color and practically none in micro- 

 chemical behavior." While reliance on density alone as a 

 criterion of purity is unsafe, the determinations in the table 

 (p. 411) are confirmatory. All the determinations are in good 

 accord (5*061 to 5 - 079) except that on the Costa Rica speci- 

 men ; and while it may be possible that we have here another 

 crystalline form of the same composition, the low value for this 

 bornite is reasonably accounted for by Dr. Merwin's observa- 

 tion, viz. : that under the microscope it has a porous appearance. 



Aside, then, from those slight variations in composition 

 which are so common throughout the mineral kingdom, and 

 which are due to foreign admixtures or to solid solution, there 

 is, in my opinion, no satisfactory evidence that natural bornite 

 is variable in composition, or that it is ever of any other com- 

 position than that expressed by the formula Cu 5 FeS 4 . 



I wish to express my thanks to Dr. H. E. Merwin for metal- 

 lographic analyses ; to Drs. George P. Merrill and E. T. 

 "Wherry of the National Museum ; and to Professors Dana and 

 Ford of Yale University, for specimens of bornite ; and also to 

 Professor Graton of the Harvard Mining School for speci- 

 mens which had been carefully examined metallographically. 



*Loc. cit., p. 35. 



Geophysical Laboratory 



Carnegie Institution of Washington, 

 Washington, D. C, March 22, 1916. 



