GUIDE TO THE MINERALOGIC COLLECTIONS 







will occupy the same position in space. An axis of binary sym- 



metry is indicated by this sign 



120' 



Fig. 15 



Fig. 16 



Similarly a crystal is said to be symmetric to an axis of trig- 

 onal symmetry when it occupies the same position in space 



Fig. 17 



three times during one revolution about the axis, the coinciding 



being 



120° 



c 





Ufj- 



U H- 



1 a ; 



4 



\l 





c 



Fig. 19 



Fig 20 



calcite. c-c being an axis of trigonal symmetry, indicated by the 

 sign Compare model 7. 



