specimens of Purple Copper Ore and Copper Pyrites. 283 



ner's supposition, and permit all these to be regarded as 

 distinct species, then, as their physical and crystallogra- 

 phic characters are apparently perfectly identical, we should 

 have a number of mineral species which it would be impos- 

 sible to distinguish from one another, otherwise than by a 

 complete chemical analysis, which the majority of mineralo- 

 gists are either incapable, or do not possess the opportunity 

 of doing. 



It has therefore been my opinion, on consideration of 

 these circumstances, that the mineral in reality cannot be 

 regarded as a combination of various sulphurets of copper 

 with sulphurets of iron, but that the iron replaces the copper ; 

 for although we have not as yet had these two metals proved 

 isomorphous when in combination with sulphur, still we have 

 many reasons for believing this to be the case ; and we know 

 that in their free state, as well as in combination with oxy- 

 gen, iron and copper are isomorphous,* and we therefore 

 can, without doubt, regard those minerals as combinations 

 of copper with sulphur, the iron playing a secondary and 

 non-essential part. 



As, however, the per-centage of sulphur is not constant, it 

 would appear that two sulphurets of copper had come into 

 play, and that the purple copper ore could be considered to 

 consist of a compound of the disulphuret of copper (in which 

 the iron replaces more or less of the copper) with protosul- 

 phuret of copper, — the proportion of the latter sulphuret 

 in the compound var^^ing without essentially altering the 

 physical properties of the mineral. 



On taking this view of its constitution, it will likewise 

 follow, that iron replaces the copper in the native disulphu- 

 ret of copper, and likewise in the mineral Digenite ; and as 

 these come in contact with the proposed classification, it 

 will be necessary to pay attention to them at the same time. 



* Frankenheim has likewise considered this to be the case in the following 

 combinations, as in the grey copper ore and in Tennantite, making its formula 

 (Cu, S, FeS,)'*A8a S3, instead of, as usually given (Fe S, Cu, S)* As, S,+ 

 2 Cuj S* As, S3 ; also in the case of silver Fahlerz making (Cu, S, Ag S, Fe S, 

 Zn S,)-* (Sb, S3, As, S3), instead of (Fe S, Zn S)* Sb, S3+2 (Cu, 8 Ag S)* Sb, S„ 

 assigned to it by Kof^e. 



