96 



MINERALOGY 



Forms. Hexagonal trapezohedrons, 



a : a : me 



r/1. 



n i 



; T(hkil), r(khil). 



O 







The faces represented by the poles in Fig. 179 yield a form 

 bounded by 12 similar trapezoidal faces, Fig. 181, the right trape- 



zohedron. If the shaded faces are ex- 

 tended, the left trapezohedron, Fig 

 1-80, is formed. Six faces are grouped 

 around each extremity of the c axis, 

 making equal angles and equal polar 

 edges. The median edges are alter- 

 nately long and short; the crystallo- 

 graphical axes terminate in the 

 middle of the long edges. In look- 

 ing at a south polar edge, with the 

 crystal form vertical, if the long 

 median edge is to the right, it is a 

 right-handed form ; if to the left, it is a left-handed form. 



Other forms. All other possible positions of the poles, as the 

 sides and angles of the triangles in Fig. 179, will yield holohedral 



o 







FIG. 179. Type 16, Hexagonal 

 Holoaxial. 



FIG. 180. The Left Hexagonal 

 Trapezohedron. 



FIG. 181. The Right Hexago- 

 nal Trapezohedron, T (hkil). 



shapes. The possible forms therefore to be found in combination 

 on crystals of this type will be : 



Right and left hexagonal trapezohedrons, r(hkil), r(khil). 



Hexagonal pyramid first order, T(hohl). 



Hexagonal pyramid second order, T(hh2hl). 



