18 Penfield and Pearce — Polyhasite and Tennantite, etc. 



Tennantite or arsenical tetraheclrite. <; Gray copper'''' in part of 



the miners. 



Among the specimens of massive polybasite, just described, 

 there were a few which contained patches of a steel gray min- 

 eral, differing only slightly in color from the blacker polybasite. 

 ISTo crystals were observed and it was necessary to pick very 

 carefully, in order to secure sufficient pure material for the 

 following analysis. The mineral gave a reddish streak, only a 

 little darker in color than that of hematite. The analysis by 

 Penfield is as follows : 



Theoretical composition 

 Specific gravity, 4-56 where Cu 2 : Ag 2 : Zn = 2S2 : «3 : 106 



S 25-04 25-66 



As 17-18^-150 =-1145 ) 17*18 



Sb 0-13-^-244 ='0005 \ 



Cu 35-72-^-126-8= -282 "| 36"29 



Ag 13-65^-216 = -063 j 13-86 



Zn 6-90-i- 65 = -106 }■ -463 7'01 



Fc 0-42-=- 56 = -008 I 



Pb 0-86^-207 = -004 J 



99-90 100-00 



The ratio of the metals to (As 9 + Sb s )=-463: -115 or 4*00: 

 0*99, almost exactly that required by the formula 4Cu,S, 

 As 2 S 3 . A part x>f the Cu 2 is replaced by Ag 2 and Zn. It is 

 not certain whether the small quantities of Fe and Pb are 

 impurities or whether they belong to the mineral. The 

 analysis is interesting as it shows an unusually high percentage 

 of silver. • 



Although tennantite has been known to occur in the United 

 States, we can find no analyses, and very little mention of it 

 in scientific literature. 



According to information received from Dr. Pearce, and 

 from our own experience, polybasite and tennantite are not 

 rare silver ores in Colorado. Polybasite occurs well crystal- 

 lized in the mines about Georgetown, at the Yankee Boy mine 

 near Ouray, in the Marshall Basin near Telluride and probably 

 at a number of mines in the Red Mountain District. Tennan- 

 tite has been observed well crystallized at the mines about 

 Central City and at the Freeland Lode and Crocett Mine near 

 Idaho Springs. 



Mineral ogical Laboratory of the Sheffield Scientific School. 

 New Haven, Dec, 1891. 



