96 F. R. VAN HORN PROUSTITE AND ARGENTITE FROM COLORADO 



presence of native silver, although none was perceptible. Silver, however, 

 was observed on the argentite described later. Professor A. W. Smith 

 very kindly recalculated the analysis on the assumption that the antimony 

 replaces arsenic. It was found that .93 per cent of antimony is equiva- 

 lent to .58 per cent of arsenic, which makes the total of the analysis come 

 still lower than the original, as follows : 



Ag 67.60 



As 14.43 



S 17.40 



99.43 



This calculation shows the arsenic to be .67 per cent low when com- 

 pared with the theoretical amount, which is 15.1 per cent. 



Using the arsenic of the above as a basis of calculation of proportions 

 in the theoretical formula of proustite, AggAsSg, we find the following 

 requirements : 



Required Found 



Ag 62.62 67.60 4.98 excess. 



As 14.43 14.43 



S 18.54 17.40 1.14 deficit. 



95.59 99.43 



The above calculation proves at least an error in the determination of 

 sulphur, but also indicates that native silver may probably be present. 

 According to the last results, the original substance analyzed may have 

 had the following composition : 



Proustite 95.59 



Silver 4.98 



100.57 



Occurrence of the Argentite 



The argentite specimens, which vary from 2 to 3 inches in width, are 

 generally massive and finely granular, Ijut in some cases are quite coarsely 

 granular. As was indicated in the description of the proustite occur- 

 rence, the argentite seems to replace the proustite in the central portion of 

 the vein for a short distance. In two instances a reduction of the argen- 

 tite to native silver in wire-like forms was observed. The coarsely gran- 

 ular material is evidently purer, but all specimens are completely sectile 

 and malleable. That the mineral, however, is not pure is shown by the 



