GUIDE TO THE MINERALOGIC COLLECTIONS 



27 



Pyramidal group 



The symmetry of this group resembles that of tin- pyramidal 

 group of the tetragonal system in that crystals of this type are 

 symmetric to the horizontal plane of symmetry shown in fig. 

 90. The vertical axis (c) is an axis of hexagonal symmetry. 

 Fis:. 101 gives an idea of the general arrangement of faces about 



LAJ 



Fig. 101 Fig. 102 Fig. 103 



this axis. The third order pyramid and prism, indicated in plan 

 in fig. 102, are of frequent occurrence in this group, as well as 

 the pyramids and prisms of the first and second order described 

 above. 



The crystal of vanadinite shown in fig. 103 illustrates a combi 

 nation of pyramidal forms. Apatite and pyromorphite are com- 

 mon minerals in this group. 



B Rhombohedral division 

 The groups which come under this division differ from the 

 hexagonal forms hitherto discussed in the essential feature of a 

 vertical axis of trigonal symmetry which gives to the Termina- 

 tion of rhombohedral crystals a trigonal as dis- 

 tinct from a hexagonal aspect. Compare fig. 

 104, which shows a termination of a rhombo- 

 hedral crystal, with the hexagonal terminations 

 shown in fig. 91 and 101. Compare also model 



8 with model 7. 



Rhombohedral group 



Forms in this group are characterized by a vertical axis of 

 trigonal symmetry and three horizontal axes <»f binary sym- 

 metry, these ;ix«-s being identical with the crystallographic axes. 

 They are also symmetric to ihr<-<- planes which interseel in the 



vertical axis ;is shown in ti^. L05. 



Pig. 1'4 



