156 CARNEGIE INSTITUTION OF WASHINGTON- 



copper minerals may be formed? (2) what will happen to these minerals 

 when they are exposed in a given environment? The principal copper 

 minerals to which attention is being directed are chalcocite (CU2S), 

 covellite (CuS), chalcopyrite (CuFeS2), and bornite (Cu5FeS4). Of 

 these, chalcocite and covellite have been thoroughly studied during 

 the year just past, and an account of their formation under measured 

 conditions in the laboratory, together with their relation to each other, 

 their stability at various temperatures, their specific gravity, and other 

 properties, will be found reviewed on pages 165 and 166. 



One property of chalcocite is of considerable geological interest. It 

 shows a readily reversible change of state at about 91°, which has been 

 clearly brought out both in its thermal behavior and in the change in 

 its electrical resistance. All chalcocites, synthetic or natural, which 

 contain no excess of sulphur show this change. It has also been estab- 

 lished with reasonable certainty that this change accompanies a trans- 

 formation from the isometric to the orthorhombic crystal form. The 

 isometric form in measurable crystals is obtained by both wet and dry 

 methods at all temperatures from the melting-point down to 200°, 

 between which temperature and 91° no evidence of change can be 

 detected. 



All crystals of CU2S hitherto found in nature have been orthohrombic; 

 hence we infer that this chalcocite, whether primary or secondary, has 

 been formed below 91°, and this is the lowest temperature yet estab- 

 hshed on the ''geologic thermometer." It would seem probable, 

 nevertheless, that much massive chalcocite in nature must have been 

 formed above this temperature, and a thorough search for natural 

 isometric chalcocite ought to be made. 



Chalcopyrite and bornite are more complex substances and have 

 been much more difficult to deal with, but an apparatus has now 

 been developed for the systematic investigation of the whole system 

 Cu-Fe-S, with the aid of which all the known phases of the system, \'iz, 

 pyrite, pyrrhotite, chalcopyrite, bornite, chalcocite, and covellite, have 

 been prepared synthetically in such form that they can be handled 

 easily by the microscopist. The manner of studying these has been 

 through their dissociation-pressure curves. The method consists in 

 ascertaining for a given temperature two sulphur- vapor pressures near 

 together, at one of which a sulphide dissociates (loses sulphur), while 

 at the other it remains undissociated or its dissociation product absorbs 

 sulphur. Between these two pressures Ues the equilibrium pressure. 



In this way it is possible to discover the boundaries which limit the 

 domain of stable existence for each of these minerals. This information 

 is not alone of theoretical importance, but may find direct application 

 in copper-smelting practice. 



The sulphides which have been considered in connection with the 

 process of secondary enrichment are chalcocite, covellite, chalcopyrite. 



