GEOPHYSICAL LABORATORY. 139 



The change in the measured properties is thus shown to be a con- 

 tinuous one, and the sulphur in solution is found to lower the melting 

 temperature of chalcocite, as might be expected. 



The relation of these intermediate solutions to chalcocite and covel- 

 lite is in every way analogous to the relations of the various pyrrhotites, 

 FeS(S)x, to ferrous sulphide and pyrite, and since the pyrrhotites are 

 common in nature, chalcocite solutions might also be expected. Never- 

 theless, the field evidence shows that whereas the pyrrhotites are found 

 more commonly in nature than ferrous sulphide, CU2S is certainly more 

 common alone than with excess sulphur in solution. 



Thus far, chalcocite alone has not been obtained in the wet way from 

 cupric solutions, but mixtures of the covellite result. When covellite 

 is heated with an excess of alkali sulphide, a part of it is changed into 

 chalcocite with formation of polysulphide, while when polysulphide 

 is heated with chalcocite, part of the latter becomes covellite. The 

 reaction CU2S + Na2S2 = 2 CuS + Na2S is therefore reversible. 



At this point the investigation was interrupted until a method could 

 be worked out for the determination of covellite and chalcocite, each in 

 the presence of the other, and this has now been successfully accom- 

 plished. (Reviewed under (46) p. 156.) 



Simultaneously with the above investigation of the relations between 

 sulphur and copper, a similar study of the system copper, iron, and 

 sulphur was undertaken, and, although more complicated than the 

 first, is also well advanced. Chalcopyrite and bornite have both been 

 successfully prepared in the laboratory and a method for their deter- 

 mination is available. Also the composition of pure bornite has been 

 found not to be CusFeSs as commonly stated in the books, but Cu5FeS4. 



Conjointly with these experiments on the formation of the copper- 

 sulphide minerals and their relations, direct experiments have been in 

 progress for some time on the changes which actually take place during 

 the processes of secondary sulphide enrichment. 



In nature we find indications that the solutions from the oxidized 

 sulphide zone have produced a series of changes in the sulphides 

 beneath them, as follows: 



Pyrite changes to chalcopyrite FeS2 > CuFeS2 



Chalcopyrite changes to bornite CuFeS2 * Cu5FeS4 



Bornite changes to covellite Cu6FeS4 > CuS 



Covellite changes to chalcocite CuS > CU2S 



Chalcocite (rarely) changes to copper CU2S > Cu 



After more than a year of work, w^e now know that all these changes 

 can also be effected in the laboratory by the action of copper-sulphate 

 solutions on the sulphides, and will therefore be accessible to detailed 

 study. Most of these changes are very slow at low temperatures 

 and more rapid at higher ones. The laboratory work has therefore 

 been done for the most part at 200° C, but the same reactions have been 



