Silver and on the Crystallization of Polyhasite. 23 



is about 3. The specific gravity was taken with a chemical 

 balance on three different portions of carefully selected mate- 

 rial and gave 6'125, 6*160 and 6*166, the mean of these being 

 6*15. The luster is metallic and the color of the mineral and 

 the streak is black. The material, even in thin particles, is 

 opaque. In the ruby silvers the arsenical compound proustite 

 is more transparent than the antimony one pyrargyrite, and we 

 might, therefore, naturally expect pearceite to be more trans- 

 parent than polybasite, but that this is not the case may be due 

 to the fact that the variety of pearceite under examination con- 

 tains over 18 per cent of copper, while the published analyses 

 of polybasite indicate usually about 5 and never over 10 per 

 cent of this element. 



Pyrognostics cmd other tests. — Before the blowpipe, pearceite 

 decrepitates slightly and fuses at about one. Heated on char- 

 coal in the oxidizing flame, a slight coating of ASjOg is formed 

 and by addition of borax or sodium carbonate and continued 

 heating a globule of metallic silver is obtained. In the open 

 tube SOj is given off and a volatile sublimate of As^Og is 

 formed. In the closed tube the mineral fuses, yields a yellow 

 sublimate of sulphide of arsenic and above the latter a very 

 slight one of sulphur. The powder is readily oxidized and 

 dissolved by nitric acid, the solution yields with hydrochloric 

 acid an abundant precipitate of silver chloride and on addition 

 of ammonia in excess the blue color characteristic of copper is 

 obtained, while a slight precipitate of ferric hydroxide is 

 formed. 



Occurrence. — According to information received from Mr. 

 Bayliss, the pearceite crystals were found with quartz and cal- 

 cite lining a vug at only one place in the Drumlummon mine, 

 and although a diligent search has been made for similar crys- 

 tals in other parts of the mine none have been found. A few 

 chalcopyrite crystals were observed intimately associated with 

 the pearceite. High grade silver and gold ores are taken from 

 the Drumlummon mine, and on one of the specimens of the 

 ore argentiferous tetrahedrite, freibergite, was observed. 



2. The Crystallization of Polyhasite. 



Rose originally described polybasite as rhombohedral and it 

 was thus considered until 1867, when Des Cloizeaux^ observed 

 that the transparent plates showed in convergent polarized 

 light a biaxial and not a uniaxial interference figure and that 

 the mineral, therefore, could not be rhombohedral. The crys- 

 tals were then referred to the orthorhombic system, but the 

 close approximation to rhombohedral symmetry has always 



*Nouvelles Recherches, p. 85, 1867. 



