138 



CARNEGIE INSTITUTION OF WASHINGTON. 



Some details of the plan as at first proposed were contained in the 

 annual report of this laboratory for 1913 (Year Book No. 12, p. 129). 

 The present status of the investigation, or rather of the laboratory 

 portion of it, follows. 



What is secondary enrichment? It is now generally conceded that 

 when the sulphides of an ore body oxidize at the surface of the ground 

 and pass into the soluble form, they are carried down by percolating 

 waters and reprecipitated on the surface of the still unoxidized sulphides 

 below. The chemical reactions involved in this process and the phys- 

 ical conditions which favor them have been heretofore only matters of 

 conjecture. The laboratory problem involves the elucidation of these 

 processes, as well as the somewhat broader problem of the stability 

 relations between copper and the copper and iron sulphides : chalcocite 

 CU2S, covellite CuS, chalcopyrite CuFeS2, and bornite Cu5FeS4; it 

 involves the synthesis of these sulphides under known conditions, 

 including, of course, the conditions of natural formation where known 

 and the most important alterations which are found to occur. 



The most obvious manner of attack is to start from the pure chemical 

 elements copper and sulphur, and from these to endeavor to make, in 

 the laboratory and under measured conditions, the compounds of these, 

 or, in a word, to study the relations between pure copper and pure 

 sulphur over a sufficient temperature range to include all the natural 

 combinations of these two substances which are known. Heating 

 these two elements together results first of all in the formation of the 

 simple 1 : 1 compound CuS, corresponding to the mineral covellite, 

 which then appears stable up to about 360° C. Beyond this tempera- 

 ture (in H2S at atmospheric pressure) it begins to lose S, but micro- 

 scopic examination shows that the mass remains homogeneous, which 

 may be interpreted to mean that the covellite has lost its stability and 

 yielded place to a lower sulphide of copper carrying sulphur in solution. 

 Continued heating drives out this dissolved sulphur gradually, until in 

 vacuo at 1127° C. the charge melts as CU2S, corresponding to the nat- 

 ural mineral chalcocite. The progress of this reaction has been checked 

 by observation at several intermediate temperatures, at each of which 

 the charge is exposed until no further change occurs there, after which 

 the product is examined. The results are tabulated below : 



Temper- 

 ature. 



Dissolved Specific 

 sulphur, gravity (25°). 



Color. 



Product. 



°C. 



410 



485 



700 



1050 



lOGO 



1127 



p. ct. 

 2.89 

 2.32 

 1.75 

 1.66 

 1.66 

 



5.562 Light 



5.638 Darker 



5 . 666 Still darker 



5.675 Do 



Melting temperature in H^S (1 atm.) . 

 Melting temperature in vacuo 



Cu2S(S)x 

 Cu2S(S)x 

 Cu2S(S)x 

 Cu2S(S)x 

 Cu2S(S)x 



CU2S 



1 



