E. T. Allen — Composition of Natural Bornite. 409 



Art. XXIX. — The Composition of Natural Bomite • by 



E. T. Allen. 



The analyses presented in this brief paper were originally 

 undertaken to determine the purity of material needed in an 

 investigation of the copper-iron sulphides. They were further 

 extended, however, when it was realized that the composition 

 of bornite was not fully settled. The formula Cu 3 FeS 3 which 

 is given in most of the text-books was deduced by Plattner 

 from an analysis of material from Cornwall, a locality where 

 bornite is intimately intergrown with chalcopyrite and notori- 

 ously impure.* 



As early as 1875 Clevef recognized that the above formula 

 agreed with but few analyses ; in fact it did not agree closely 

 witli any. Six analyses of bornite made under Cleve's direc- 

 tion pointed to the formula Cu 5 FeS 4 and in line with these he 

 found two others recorded in the literature. However, four 

 other analyses done in Cleve's laboratory led to more compli- 

 cated formulae and Cleve concluded that there must be several 

 similar minerals all of which had passed under the name of 

 bornite. In 1903 Harrington, £ in a critical consideration of 

 this question, made analyses of six different bornites from 

 widely separated American localities, and found that the results 

 all agreed with the formula Cu.FeS,. Harrington states that 

 his specimens were " carefully selected," and his statement is 

 corroborated by the uniformity of his results and his specific 

 mention of impurities in some of the specimens he examined, 

 especially specimens from Cornwall. But a lens, on which 

 Harrington was dependent at that time (1903), is not an 

 entirely safe instrument to decide the homogeneity of an 

 opaque mineral. Intimate intergrowths with other sulphides 

 are fonnd to be very common when bornite is examined by 

 the metallographic method. § 



My own analyses have been confined to specimens of bornite 

 which, after careful microscopical study on several polished 

 surfaces, have been shown to be practically free from other 

 sulphides. | Graton and Murdoch kindty furnished me with 



* Plattner, Pogg. Ann., xlvii, 351, 1839; Varrentrap, ibid., p. 372; B. J. 

 Harrington, this Journal (4), xvi, 151, 1903; Joseph Murdoch, Microscopical 

 Determination of Opaque Minerals, pp. 35 and 36. 



fGeol. For. Forh., ii, 526, 1875. {Loc. cit. 



§ Graton and Murdoch, Trans. Am. Inst. Min. Eng., xlv, p. 45, 1914. 

 Murdoch, loc. cit., pp. 35-6. 



|| This statement is not strictly true of the bornite from North Carolina, 

 but as will be seen later, the way in which the chemical and microscopical 

 analyses of this sample agree leaves no doubt that the material examined by 

 the two methods was practically identical. 



