COLLECTION.S IN ECONOMIC GEOLUiiY /VN'l) METALLURGY. 19 



(4) Crystalli/t'il and granular, in (luart/,, w itli otlior snlpliitics. (Joiiiargo Mine, 

 Reese River (listri(!t, Lander County, Nevada. (l.')1.5li. ) 



(.'>) CryHtaliized and j^iannlar, in cjnartz, with otht-r Hulpiiides. Allison Miiu', (Jar- 

 son district, OwylieaCounty, Idaho. (1480G.) 



(6) Intimately mixed with galena and blende, with a little (jnartz. Legal Tender 

 Mine, Prickly Pear district, Jett'erson County, Montana. (lll.'jG. ) 



PRUUSTITE. (RUBY SILVER, ARSENICAL SILVER ORE, LIGHTKED 



SILVER ORE.) 



Compositiou: Ag^AsS^t or SAg^S-fAsaSa. tSulph-arseuide of silver, 

 contaiiiiDg silver 65.5 per cent., arsenic 15.1 per ceut., aud sulphur 1{)A 

 per cent. 



It crystallizes in the rhombohedral system, aud also occurs massive. 

 It varies in hardness from 2 to 2.5 and in specific gravity from 5.42 to 

 5.56, is cochineal-red in color, transparent, and has an adamantine 

 luster. 



An interesting and im[)ortaut silver mineral, being found frequently 

 with stephauite and pyrargyrite, and like them being difficult to reduce. 



HBSSITE. (TELLURK! SILVER.) 



Composition: AgvTe. Telluride of silver, containing silver 62.8 per 

 cent, and tellurium 37.2 per cent. 



It crystallizes in the orthorhombic system aud occurs also massive. 

 It varies in hardness from 2 to 3.5 and in specific gravity from 8.3 to 

 8.6, has a lead-gray color and metallic luster, and can be readily cut 

 with a knife. 



An important silver mineral in a number of Colorado mines. 



TETRAHEDRITE, (FAIILERZ, GRAY COPPER OKE.) 



Composition: CuaSb^Sr or ICu^S-l-SboSa, with part of the copper re- 

 placed by iron, zinc, mercury, or silver and part of the antimony by 

 arsenic or bismuth, thus forming n, very complex composition. In the 

 varieties containing silver it sometimes amounts to 31 per cent., but is 

 more frequently less tliau 10 j)er ceut. 



It crystallizes in the isometric system, especially in tetrahedrons 

 (whence its name), and crystals are frequently very complex. It also 

 and very frequently occurs massive. It varies in hardness from 3 to 

 4.5 and in specific gravity from 4.5 to 5.11, ami in color from gray to 

 black, aud has a full metallic luster. 



Tetrahedrite, while essentially a copper mineral, is generally more 

 valuable for its silver contents, although very variable in richness, 

 and the extraction of the silver is intimately connected with the met- 

 allurgy of copper. 



To illustrate the occurrence of tetrahedrite four specimens are shown: 



(1) Massive, with a little quartz. Witteknit Mine, Invo Couniv, California. 

 (8-27-2.) 



(2) Massive, in (luartz, very much stained I>y carbonates of copper from the decom- 

 position of the tetrahedrite. Stewart's Wonder Mine, Panamint district, Inyo 

 County, California. (14dJl.) 



