18 S. L. Penfield — PearceiU^ a Sulpharsenite of 



the following analysis was made by Mr. F. C. Knight under 

 Dr. Pearce's immediate supervision. 



Found. Ratio. Theoretical composition where 



143: ]9. 







, 



-^- . Ag2 



: Cu2 



: Fe = 255 



s 



17-71^ 32 = 



•553 



11^95 





17-96 



As,._- 



7-39-j- 75 = 



•098 



2-11 





7-02 



Ag ... 



55-17-^216 = 



•255 ] 







55-61 



Cu.__- 



18-11-M27 =3 



•143 



-•417 9-00 





18-34 



Fe._.. 



r05^ 56 = 



•019 ) 







1^07 



Insol. . 



•42 











99-85 100-00 



Dr. Pearce recognized that the mineral belonged to the 

 polybasite class, where arsenic played the role usually taken by 

 antimony, and forwarded the specimens, together with the 

 analysis, to the author for an expression of opinion. As may 

 be seen from the ratio, the proportion of S : As : (Ag^ + Cu^H- 

 Fe) is very nearly 12 : 2 : 9, which is that demanded by the 

 polybasite formula, and taking the metals in the same propor- 

 tion as they are found in the analysis, Ag^ : Cu^ : Fe = 255 : 

 143 : 19,^ and calculating the theoretical composition, results 

 agreeing very satisfactorily with the analysis are obtained. 



Although recognizing that antimony and arsenic are isomor- 

 phous and may mutually replace one another, it is custom- 

 ary and has been found convenient in mineralogy to consider 

 the sulphantimonites and sulpharsenites as distinct species, and 

 to designate them by different names, and the author proposes 

 that hereafter the name polybasite shall be restricted to the 

 antimony compound AgySbSg, and to make of the correspond- 

 ing arsenic compound AggAsSg a distinct species. For the 

 arsenical mineral he takes pleasure in proposing the name 

 jpearceite 2i^ a compliment to his friend. Dr. Richard Pearce, of 

 Denver, whose keen interest in mineralogy and connection 

 with one of the large smelting and refining works of Colorado 

 have made him known both to scientific men and to those 

 interested in the development of the mining industries of the 

 Pocky Mountain region. The author furthermore takes 

 pleasure in expressing his thanks to Mr. Bayliss, who has taken 

 a great interest in the investigation and naming of the mineral, 

 and has most generously placed at his disposal all of the avail- 

 able material. 



It seems best to give at this point the analyses of pearceite, 

 already referred to, which have previously been published as 

 arsenical varieties of polybasite. In the theoretical composi- 

 ition given with each the ratio of the metals is the same as in 

 the accompanying analysis. 



