MINERALOGY 



V. Hexagonal prism of the first order ; a:ooa:a:ooc; (hohl). 

 Three possible positions of the poles now remain, the three 



angles of the triangle, each position yielding one of the three fixed 

 forms. The four forms already developed represent the variable 

 forms, there being a series of each. If the poles coincide with the 

 intermediate axes, Fig. 152, four faces of the most general form will 

 fall in one plane, producing an open form, the hexagonal prism of 

 the first order, bounded by 6 faces, all of which are parallel to the 

 c axis, Fig. 157. Each face cuts two lateral axes at the same 

 distance and is parallel to the third. 



VI. Hexagonal prism of the second order; 2a:2a:a:ooc; 

 (hh2ho). 



In this form the poles will coincide with the lateral crystallo- 

 graphical axes. It is bounded by 6 faces, each of which cuts two 

 lateral axes at the same distance, and the third at one half that 

 distance. The axes will therefore terminate in the center of the 

 faces, Fig. 158. 



VII. Basal pinacoid ; GO a : oo a : oo a : me, (oooi). 



If the poles are moved to the c axis the number of faces will be 

 reduced to a single pair of faces parallel to the equatorial plane. 

 They terminate the prisms as shown in Fig. 158. 



FIG. 158. Hexagonal Prism 

 of the Second Order (hhzho). 



FIG. 159. Beryl, a Combi- 

 nation of m (10TO), u (2021), 

 s (1121), p(10Tl),c (0001). 



The forms possible to combine on crystals of the dihexagonal 

 equatorial type are : 



