W. M. Thornton, Jr. — Enargite, Covellite, and Pyrite. 359 



(weight for weight) and potassium nitrate, and the melt 

 leached with water. An aliquot part (3/4) of the filtrate was 

 concentrated for the precipitation of arsenic as ammonium 

 magnesium arsenate. The precipitate was ignited to magnesium 

 pyroarsenate. A trace of arsenic was recovered from the fil- 

 trate by hydrogen sulphide and weighed as arsenic trisulphide. 

 In one -fourth the original solution the sulphur was determined as 

 barium sulphate in the usual manner. The residue, consisting 

 chiefly of copper oxide, was dissolved in sulphuric acid and a 

 little nitric acid ; and the copper precipitated with sodium 

 thiosulphate, ignited under hydrogen sulphide, and weighed as 

 cuprous sulphide. In the filtrate from the cuprous sulphide 

 the zinc was precipitated by ammonium sulphide and ignited 

 to the oxide. 



The covellite was dissolved in nitric acid ; and with the aid 

 of potassium chlorate the separated sulphur was converted into 

 one clean globule. The sulphur thus separated was weighed 

 as free sulphur. One-fifth of the filtrate was employed to 

 determine the remaining sulphur as barium sulphate. In two- 

 fifths of the original solution the copper was estimated as 

 above. 



This cuprous sulpharsenate is not among the commonly 

 occurring ores of copper. Cupric sulphide, as compared with 

 cuprous sulphide, or ordinary copper glance, is rare. This 

 association, therefore, is in the author's opinion a very interest- 

 ing one and deserving the brief notice here given. It may 

 also be mentioned that the minerals of this aggregate contain 

 no silver, which is quite the exception for ores of the Red 

 Mountain district. 



Virginia Geological Survey, 

 University of Virginia, 

 January 22, 1910. 



