Silver and on the Crystallization of Polyhasite. 19 



I. H. Rose, Ag^ : Cu, : Zn : Fe = 335 : 24 : 9 : 6. 



II. Penfield, alter deducting 12-81 per cent of impurities, 

 mostly PbS, Ag, : Cu, : Zn = 263 : 1 17 : 43. 



III. S. H. Pearce, after deducting 28*18 per cent of impurities, 

 mostly PbS, Ag^ : Cu : Zn = 276 : 102 : 49. 



I. II. III. Theory for 



Schemnitz. Theory. Aspen, Colo. Theory. Aspen. Theory. AggAsSe- 



8. 16-83 16-19 18-13 18-13 17*73 18*02 15-50 



As 6-23 6-32 7-01 7*08 6-29 7*03 6*05 



Sb 0-25 .... -30 0-18 



Ag 72*43 73-47 56*90 57*07 59*73 59*06 78-45 



Cu 3*04 3*08 14-85 14-91 12-91 12-77 



Zn 0-59 0-60 •2-81 2-81 3-16 3*12 



Fe 0-33 0-34 



99*70 100-00 100-00 100-00 100-00 lOO'OO 100-00 



Crystallization. 



The crystallization of pearceite is tnonoclinie but with a 

 close approximation to rhombohedral symmetry. The habit is 

 commonly hexagonal with the basal planes prominent and the 

 zones of bevelling forms between them often highly modi- 

 fied. The material from which crystallographic data could be 

 obtained came wholly from a single specimen where the crys- 

 tals were implanted upon a gangue of quartz and imbedded in 

 calcite, and were obtained by dissolving the latter in dilute 

 acid. Unfortunately the crystals had grown close together, 

 thus interfering more or less with one another, and they also 

 were cracked, probably owing to the severe shocks received in 

 the processes of blasting and mining ; consequently when lib- 

 erated by dissolving the calcite they fell to pieces, so that usu- 

 ally only parts of crystals were available for measurement. 

 The faces had a beautiful metallic luster, and when free from 

 striations and vicinal planes gave excellent reflections on the 

 goniometer. The determination of the crystalline form and 

 the axial ratio proved to be a difficult matter owing to the 

 fragmentary character of the crystals, their grouping, often in 

 nearly parallel position, a probable twinning and their close 

 approximation to rhombohedral symmetry, and it was not 

 until many measurements had been made upon a series of crys- 

 tals that a satisfactory solution of the problem was obtained. 



As fundamental measurements, the following were selected : 



m/sw, 110/^110 = 60° 2' 

 c /v, d, 001/^102 = 25 3 

 - y\ a, 001/^100 = 89 51 



c 



