HKXACONAL SVSTKM 



lifinipyramid of the first order, (hohl), (ohhl), (hohlj, 

 (ohhl). 



l'l>per andjower hexagonal hemipyramid of the second order, 

 (hh2hl), (hh2hl). 



Plus and minus ditrigonal prism, (hkio), (khio). 



Plus and minus trigonal prism of the first order, (hoho), (ohho). 



Hexagonal prism of the second order, (hh2ho). 



I'pprr and lower base, (0001), (0001). 



Example. The common and important mineral tourmaline 

 belongs to this type ; Fig. 200 represents a combination of forms as 

 found on this mineral. 



CLASS, TRIGONAL TETARTOHEDRAL 



TYPE 11, TRIGONAL EQUATORIAL 



Symmetry. Crystals of this type 

 have an axis of trigonal symmetry, 

 the c axis, and one plane of symmetry 

 perpendicular to it, Fig. 201. The 

 class may also be considered as te- 

 tartohedral derived by superposing 

 type 19, scalenohedral hemihedral, 

 upon type 13, trigonal hemihedral. 



Forms 



I. Trigonal pyramids of the third FIG. 201. Type 11, Trigonal 

 order, Equatorial. 



r/l 



na: a:a:mc 



n i 



; (hkil), (khil), (ikhl), (ihkl). 



The faces represented by the poles in Fig. 201 bound the right 

 plus trigonal pyramid of the third order, a form having six isosceles 

 triangular faces. The a axes terminate asymmetrically in the equa- 

 torial edges, as represented in Fig. 202. There are four pyramids 

 of the third order: the plus and minus right, two congruent forms; 

 the plus and minus left, also two congruent forms. The rights and 

 lefts are enantiomorphic. Fig. 203 is a minus left form. 



