Van Horn — Silver, Copper, and Cobalt Minerals. 27 



Found Theoretical 



Ag 64-65 65*40 



Cu ... 0-70 



S 20-18 19-43 



As 15-25 15-17 



Sb Trace 



100-78 100-00 



Sp.gr 5-60 5-57-5-64 



The specific gravity was obtained by taking the average of 

 six determinations on three pieces, using both pyknometer and 

 Jolly balance. The analysis shows that the pure proustite 

 molecule is present, since but a trace of antimony was found. 

 A small amount of copper, which was also obtained in the 

 argentite, is present. This is quite unusual for proustite, but 

 when one considers that both argentite and proustite are more 

 or less associated in the same ore body with pearceite, 

 (Ag,Cu) 16 As 2 S n , one need not be surprised if they contain 

 copper, since copper-silver solutions were evidently prominent 

 in the deposition of ores at this particular place. 



Pearceite. — This very rare mineral was fully described* in 

 this Journal in 1911. It might, nevertheless, be mentioned that 

 the analysis of the mineral caused the conclusion that a more 

 logical formula for it might be (Ag,Cu) 16 As 2 S n , rather than 

 (Ag,Cu) 18 As 2 S 12 which was originally proposed. In accordance 

 with these facts, the formula of the isomorphous polybasite 

 should be (Ag,Cu) 16 Sb 2 S n , rather than (Ag,Cu) ]8 Sb 2 S 12 which 

 was proposed by Heinrich Hose in 1829. f 



Cerargyrite var. Embolite. — In recent investigations by 

 Prior and Spencer;): they suggest that cerargyrite be employed 

 as a group name, and that embolite should be regarded as a 

 sub-species of the group. The mineral occurs in well-defined 

 cubo-octahedral crystals on cracks and in pores of an iron- 

 stained limestone which is siliceous in places if judged by 

 analyses, although no quartz could be recognized even in thin 

 sections. This limestone is brecciated locally, and the embo- 

 lite is also found as irregular concretionary coatings on the 

 brecciated surfaces. Consequently the embolite is an impreg- 

 nation of the breccia, and not a replacement of the limestone. 

 Thin sections show the mineral to occur only in the cracks. 

 Both crystals and coatings have a bright sulphur-yellow color, 

 and possess a resinous to adamantine luster. It is associated 



* Op. cit. 



f A Discussion of the Formulas of Pearceite and Polybasite, by Frank E. 

 Van Horn, this Journal, xxxii, 40, 1911. 

 JMin. Mag., xiii, 174. 



