4 2 Van Hum — Discussion of t 'he Formulas of 



Sb, 120-20; Ag. L07-88; Cu, 63-57; Fe, 55-85; Zn, 65-37. The 

 following combining ratios were obtained: — 



Analysis. S (As,Sb)(Ag a Cu„Fe,Zn) 



1. Schemnitz, 1833 12-330 2 8-G90 



2. Arqueros, 1879 10-570 2 7*728 



3. Aspen, crystallized, 1892 11-786 2 8-830 



4. Aspen, massive, 1892 12-766 2 9-852 



5-6. Marysville, 1896 11-188 2 8*446 



7. Sierra Mo jada, 1911 . 10-800 2 7-886 



An inspection of these ratios shows that the proportions 

 obtained in 2, 6, and 7 conform more nearly to the formula 

 (Ag.,Cu.,),As„S n than to (Ag.,Cu.,) 9 As 2 S,., ; 4 does not conform 

 to either, which is probably due to the fact that 28-18 per cent 

 of impurities had to be deducted before the analysis was avail- 

 able for calculation at all. In addition to this, 2-57 per cent 

 of FeC0 3 was deducted, some of which probably belonged to 

 the pearceite. Analysis 3 lies between the two formulas, aud 

 slightly nearer the larger values, but since the same methods 

 employed in 4 were used, and 12-81 per cent of impurities had 

 to be deducted, the writer is inclined not to attach great impor- 

 tance to the ratios obtained in this analysis. It certainly could 

 not be used as convincing proof of the correctness of the 

 larger formula. The remaining analysis, 1, was made in 1833 

 and conforms more nearly to the larger formula, but the sul- 

 phur is considerably more than required, while the silver is 

 much less than is demanded by the larger formula, thus showing 

 a wide variation in results. Considering all facts in connection 

 with the analyses of pearceite. and especially the purity of 

 material used, the writer feels justified in suggesting the form- 

 ula 8(Ag,Cu),S.As„S, or (Ag,Cu) )C As.,S n instead of (Ag,Cu) 18 

 As 2 S 12 , which was first proposed. 



Polybasite. 



Since pearceite is regarded as being isomorphous with poly- 

 basite, and shows the same relation to it which is observed 

 between proustite and pyrargyrite, or between tennantite and 

 tetrahedrite, the writer became interested in polybasite analyses. 

 All authors with the exception of Rammelsberg (cited under 

 pearceite) accept the formula 9(Ag,Cu),S.Sb 2 S 3 or (Ag,Cu) 8 SbS 

 which was proposed by H. Rose in 1829. However Hintze* 

 in the table of polybasite analyses gives the theoretical compo- 

 sition of the formula proposed by Rammelsberg, Ag 16 Sb 2 S n , 

 in addition to that of Rose. The writer has been able to find 

 but ten analyses of this mineral which are arranged chronolog- 

 ically, beginning with the oldest, as follows : 



* Op. cit. page 1174. 



