Van Horn and Cook — New Occurrence of Pearceite. 523 



ning of this article. At the three other localities cited a small 

 amount of antimony was determined. Although iron was pres- 

 ent in small amounts in three of the previous occurrences, and 

 zinc in one of them, neither of these elements was observed in 

 the Yeta Rica pearceite. A glance at the proportion of com- 

 bining weights found in this analysis shows an extremely close 

 approximation to the formula (Ag a Cu a ) 8 As a S„ rather than to 

 that of the accepted one, (Ag a Cu a ), As„S ia . The theoretical 

 composition of the Sierra Mojada pearceite was calculated for 

 the latter formula on the basis that the ratio of silver to copper 

 was 2744 : 1231, as follows : 



Atomic Molecular 

 weights weights 



Theo. 



Theo- Percent- percent, 

 retical age Differ- of 

 perc'tage found ence Ag 9 AsS« 



S = 32-07 S„ = 384-84 



17-27 17-46 + '19.. 15*54 



As = '74-96 As, = 149-92 



6-73 7-56 +'83.. 6'05 



9*74.4. 

 Ag— 107-88 Ag of 107-88X 18 = 1339-92 

 3975 



60-12 59-22 --90.. 78*41 



1 2^1 

 Cu— 63-57 Cu of 63-57x18= 353-88 

 3975 



15-88 15-65 --23.. 



2228-56 100-00 99"89 



100*00 



It is seen that the differences between the theoretical com- 

 position and that which was actually found are quite consid- 

 erable, so that the theoretical composition required by the 

 formula (Ag a Cu 2 ) 8 As 2 S n was likewise calculated, using the pro- 

 portion of silver to copper as above, with the following results : 















Theoretical 







Molecular 



Theoretical 



Percentage 



Differ- 



percentage of 







weights 



percentage 



found 



ence 



AgieASsSu 



s„ 



= 



352-77 



17-56 



17-46 



— •10 



15-84 



As, 



= 



149-92 



7-46 



7-56 



+ *10 



6-72 



A £,. 



= 



1191-92 



59-32 



59-22 



— •10 



77-44 



Ou 16 



= 



314-56 



15-66 



15-65 



— •01 





2009-17 



100-00 



99-89 



100-00 



A comparison of the results obtained above shows that the 

 analysis of Sierra Mojada pearceite conforms much more 

 closely in all respects to the formula (Ag 2 Cu a ) 9 As a S n or(AgCu),, 

 As a S u than to (Ag a Cu a ),As 2 S ia or ( AgCu) 18 As 2 S ia . A comparison 

 of other analyses of pearceite and polybasite will follow this 

 article, at a later date, in another paper by the senior author. 



