Silver and on the Crystallization of Polyhasite. 27 



expected, a very close relation to one another in everything 

 which concerns their crystallization. The tendency of arsenic 

 is to make the vertical axis of its compound a little longer 

 than that of the antimony one, as shown by the following 

 ratios and by the angles G/^p or p° : 



a : b : c /? c ^p andjp°. 



Pearceite, l-'7309 : 1 : 1-6199 89° 51' 61° 49' 61° 56' 



Polyhasite, 1-7309:1:1-5796 90° O'" 61 16 



Perhaps the next most interesting relation of these minerals, 

 which imitate rhombohedral symmetry, is their very close 

 similarity to the rhombohedral ruby silvers, proustite AggAgSj 

 and pyrargyrite AggSbSg, as shown by the relations of the 

 vertical axis and the angles. 



c y^p or p° c ^s 



Pearceite, ic = 0*8099 61° 49'-61° 56' Proustite, c = 0-8039 61° 41^' 



Polybasite, ic = 0-7898 6! 16 - Pyrargyrite, c = 0-7891 61 15 



Difference = -0201 Difference = -0148 



The vertical axes are longer in the arsenic than in the anti- 

 mony compounds and to about the same extent in the two 

 classes. _In proustite and pyrargyrite s is the —2 rhombohe- 

 dron, 0221. 



The orthorhombic sulphides, chalcocite Cu^S and stromeyer- 

 ite CuAgS, also exhibit a close and striking similarity to the 

 minerals in question, best seen when the positions of the for- 

 mer are changed by a simple interchange of the a and h axes, 

 making thus the obtuse angles of the prisms at the extremities 

 of the h instead of the a axis, as follows : 



Chalcocite, 1-7176 



Stromeyerite, 1-7176 



Pearceite, 1-7309 



Polybasite, 1-7309 



Although widely separated in our chemical classification, the 

 differences in chemical composition of the two former from the 

 two latter is not so very great, for Cu and Ag being isomorph- 

 ous we have as the general composition of the former K'2S 

 and of the latter R'^S-f-l-As^Sg respectively SbjSg. 



On examining the whole group of sulpharsenites and sul- 

 phantimonites it is quite interesting to note that the majority 

 of them which are well crystallized exhibit prismatic angles 

 of nearly 60° and show forms in other zones which can be 

 referred to vertical axes nearly like those of the chalcocite and 

 polybasite groups. These relations are shown in the following 

 table, where the compounds are arranged according to varia- 



b : 



1 



1 



c 

 1-6663 

 1-6603 



90° 0' 

 90 



llO/sllO 

 60° 25' 

 60 25 



C ^p OT p° 



62° 35^' 

 62 30 



1 

 1 



1-6199 

 1-5796 



89 51 



90 



60 2 

 60 2 



61 49-61° 56' 

 61 16 



