352 palmer: bornite as silver precipitant 



not been definitely settled. Analyses of bornite from different 

 sources have led to several empirical formulas for it, such as, 



CugFeSa, Cu5FeS4, and CuioFeoSg. 



The constitution of bornite has been a favorite subject of 

 speculation. The views of Rammelsberg and Groth illustrate 

 the vacillations and uncertainties likely to follow sole reliance 

 on compovsition as indicative of constitution. In 1841 Rammels- 

 berg assigned to bornite the constitutional formula, 3Cu2S.Fe2S3, 

 by which all the copper is represented as present in the cuprous 

 state. Later he considered all the iron of bornite to be ferrous 

 iron and the copper to be partly cuprous and partly cupric, 

 and to express these supposed conditions he ascribed to bornite 

 the constitutional formula, Cu2S.CuS.FeS. Groth has con- 

 sidered bornite to be strictly a cuprous substance and has adopted 

 the formula, 3Cu2S.Fe2S3 which Rammelsberg had abandoned. 



It has been known for a long time that bornite immersed in 

 a silver nitrate solution quickly develops on its surface a crop 

 of silver crystals, but beyond the mere appearance of silver on 

 bornite this striking phenomenon seems to have received no 

 attention from mineralogists. Believing that a study of the 

 capacity of bornite as a silver precipitant might throw light 

 not only on the chemical composition of the mineral, but also 

 on the intramolecular relations of its constituents, the reaction 

 of a specimen of bornite from Virgilina, Virginia, and dilute 

 silver sulphate solution has been studied quantitatively. The 

 specimen was massive, its freshly broken surfaces having a 

 uniform color. Metallographic examination of polished sur- 

 faces showed the presence of a few small grains of chalcopyrite 

 and a few gashes of a dark grey mineral suggestive of chalcocite. 

 The mineral was broken into small fragments, and from these 

 were chosen only those pieces in which, under the magnifier, 

 no chalcopyrite could be detected. The granular pieces were 

 washed with alcohol, and from these a second selection of bornite 

 was made. In this way, it was believed, material representative 

 of the true bornite in the specimen was obtained. An analysis 

 of the material selected for study gave the content 



