74 spencer: chalcocite deposition 



by Stokes in deducing equation (12), and it is perhaps sufficiently 

 accurate to regard this chemist as their sponsor. As equation 

 (10) is to this extent plausible it may be suggested, not inappro- 

 priately, as offering a link between the two lines of investigation 

 due to Winchell and Tolman on the one hand and to Stokes on 

 the other. Equation (10) shows at least the possibility that SO2 

 may be a product of chemical reaction during the final stage of 

 oxidation as well as during earlier stages; or, in other words, it 

 indicates that SO2 may be evolved in the situs of chalcocite depo- 

 sition as well as in those situations where ferric sulfate undergoes 

 reduction, as suggested by equation (6), or where free oxygen is 

 being used as in equation (1). In this manner it may be possible 

 to avoid the objection raised by Lindgren. 



Although the principal object of the present communication 

 is attained in a tentative reconciliation of the suggestion due to 

 Winchell with the points urged against it by Lindgren, occasion 

 may be taken to present certain additional considerations per- 

 taining to the Stokes equation and analogous expressions. 



If the equations given are accepted as affording a working 

 hypothesis, it might be found that under experimental conditions 

 the attack of cupric sulfate on pyrite, or preferably on chalcopy- 

 rite, could be initiated in the presence of a moderate amount of 

 SO2 and that when thus started the conversion might continue 

 without further aid, proper environment being maintained by 

 the presence of SO2 currently evolved. There are good reasons 

 for the statement that chalcopyrite is less stable than pyrite in 

 solutions containing salts of copper. For the deposition of 

 chalcocite, the chemical mechanism involved must be essentially 

 identical in either case. 



When the pyrite and chalcopyrite equations are compared with 

 respect to volume relations, in the conversion to chalcocite by 

 molecular replacement the former is found to demand expansion 

 in the ratio 2:3, while the latter requires little if any volume in- 

 crease. Accepting both equations, we might anticipate that pyr- 

 ite would prove to be a less efficient agent than chalcopyrite in 

 chalcocite deposition. In the porphyry ore at Ely, Ne^^ada, 

 where grains of primary chalcopyrite and pyrite occur side by 

 side the former are always more deeply coated with chalcocite 



