Silver and on the Crystallization of Polyhasite. _ 25 



normal biaxial one was seen. There is nothing in the external 

 aspect of the crystal to indicate twinning except perhaps the 

 uneven character of the base c, which may result from a twin- 

 ning about m, 110 and a slight deviation from 90° of CA^m. 

 The crystal did not give satisfactory reflections, but the angle 

 of c on the apparent rhombohedrons was approximately 61-|-° 

 and of the latter faces on to each other 80|-°. It is difficult to 

 explain this apparently rhombohedral development on the 

 assumption that the crystallization is orthorhombic, while it is 

 known that monoclinic minerals whose prismatic angles are 

 near 60°, the micas, chlorites and leadhillite for example, exhibit 

 in a marked degree a tendency to imitate rhombohedral sym- 

 metry, while orthorhombic compounds, chalcocite, aragonite 

 and witherite for example, do not show this tendency. It 

 seems reasonable, therefore, to assume that the crystallization 

 of polybasite, like that of pearceite, is monoclinic^ and the 

 apparently rhombohedral forms on fig. 3 have accordingly been 

 lettered j9 and f with the understanding that there is an uncer- 

 tainty regarding their identification. 



In the summer of 1891, while the author was engaged in 

 some mineralogical work in Colorado for the United States 

 Geological Survey, a collection of minerals w^as made at the 

 Yankee Boy mine, near Ouray, and thanks are due to Dr. 

 F. M. Endlich, manager of the mine, for his courtesy and the 

 interest which he took in making the collection as complete as 

 possible. On the specimens, associated wath pyrargyrite and 

 implanted upon quartz, were some small but remarkably per- 

 fect crystals of polybasite. The occurrence of the mineral 

 from this locality has been noted by Endlich,"^ the identifica- 

 tion being based in part upon measurements by the author of 

 the prismatic angle m^m = nearly 60°. 



The forms which were observed on a single crystal of poly- 

 basite, having a habit like fig. 4, are as follows, the orientation 

 being derived from the symmetrical arrangement of the faces 

 with reference to the prism ^, 310, which, however, was only 

 slightly developed and is not represented in the figure. 



c, 001 



Z/, 203 



0, 114 



w, 331 ? 



/, 310 



n°, lOl 



r, 112 



0°, 114 



m, 110 



n, 403 



p, 111 



t\ 112 



n, 101 



f, 201 



s, 221 



P\l\\ 



The crystal was too opaque to admit of optical tests being 

 made, but thinner crystals from the locality transmitted a deep 

 red light and showed indications of twinning. 



*This Journal, xl, p. 424, 1890. 



