W. E. Ford — Remarkable T%oins of Atacamite. 23 



tions, II, III, IV, and V. A crystal of tins type is ilhistrated 

 in figure 9, wliere the four twin individuals do not entirely 

 cover up the dome faces of the normal crystal. This figure 

 also shows the e face common to twins IV and V above the h 

 face of the normal crystal. At times the faces in twin position 

 completely envelope the ends of the normal crystal, allowing 

 only the faces of its prism zone to show. A doubly terminated 

 crystal of this type is shown in figure 10. Figure 11 is the same 

 as figure 10 with the lines of the back faces showing in order to 

 assist in forming a mental picture of these strange twin groups. 

 In addition to groups such as figured there are much more 

 complicated ones in which both types of twins are found with 



bewildering relations to each other. 



Fig. 9. 



Fig. 10. 



Fig. 11. 



ejsr.z 



An analysis was made of the atacamite crystals which agreed 

 closely with the theoretical composition of the mineral as 

 shown below. 



CI 16-55 



Cu U'82 



CuO 56-01 



H^O 12-69 



Total 100-07 



Theory 

 16-6 

 14-9 



55-8 

 12-'7 



1000 



