274 H. V. WINCHELL SYNTHESIS OF CHALCOCITE AXD ITS GENESIS 



SO2 8.30 



CaO Trace 



MnO , None 



Fe 41.20 



Cu 1.50 



Zn 0.20 



S 48.70 



99.90 



Dividing these results by the molecular weights, the molecular con- 

 stitution is represented as follows : 



Fe 0.736 



Cu. 0.024 



Zn 0.003 



S 1.522 



After standing for three months in an ordinarily well lighted room, 

 inclosed in a sealed jar to exclude the atmosphere, the formerly yellow 

 grains of pyrite were completely plated Avith a solid coating of a dark 

 blue-black mineral, and so closely resemble grains of solid chalcocite 

 that they can only be distinguished from the latter by breaking them 

 open, while in another jar which stood alongside, similarly sealed and 

 exposed to light and ordinary temperature, containing pyrite and copper 

 sulphate solution (but no SO2), the grains of pyrite were just as bright 

 and yellow as before they were immersed. Indeed, there has been no 

 visible alteration on the surface of grains which have now been thus im- 

 mersed in copper sulphate without SOg for two years, while in an adja- 

 cent jar containing SO2 there has been formed Avhat appears to be, and 

 undoubtedly is, a fine coating of chalcocite. 



From the first jar there were taken some of the larger grains for 

 analyses, with results as follows : 



SiOj 9.60 



CaO 



MnO 



Fe 40.10 



Cu 3.60 



Zn 



S 46.30 



99.60 

 CaO, MnO, and Zn not determined. 



The molecular constitution is now Fe, 0.716; Cu, 0.057, and S, 1.448. 

 There are .016 equivalents of sulphur left over (after calculating the iron 

 as FeS2) to unite with the copper. The exact theoretical amount to 

 form CU2S is .014, and the surplus sulphur may be combined with zinc, 



