GUIDE TO THE MIXERALOGIC COLLECTIONS 9 



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

 metry is indicated by this sign ^m . 



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 Fig. 18 



three times during one revolution about the axis, the coinciding 

 positions being 120° apart. Fig. 15 and 16 show a crystal of 



Fig 20 



calcite, <■-<■ being an axis of trigonal symmetry, indicated by the 

 sign j^. ( Jompare model 7. 



