HEXAGONAL SYSTEM 



MI.; 



l.y >i\ faces. Alternate >. did angles are equal, three 

 less than 120 and tliree greater. The crystallographical axes 



l>i>rct the edges. 



/a : oo a : a : oo c\ 

 IV. Trigonal prism of the first order, ( - 



(hoho), (ohho). 



When the poles are moved on the primitive circle to coincide with 

 the didigonal axes, the resulting form is the trigonal prism of the 

 first order, Fig. 195, bounded by three equal faces, the lateral 

 axes terminating, two in each face, as indicated in Fig. 195. 



Other forms. Other possible positions of the poles will produce 

 apparent holohedral forms, i.e. the hexagonal pyramid and prism 

 of the second order and the base. 



Forms possible to combine on 

 crystals of this type will be 



Plus and minus ditrigonal pyra- 

 mids, (hkil), (ihkl). 



Plus and minus trigonal pyramids 

 of the first order, (hohl), (ohhl). 



Hexagonal pyramid of the sec- 

 ond order, (hh2ho). 



Plus and minus ditrigonal prisms, (hklo), (ihko). 



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



Hexagonal prism of the second order, (hh2ho). 



Hexagonal base, (0001). 



Example. There is only one example of a substance crystalliz- 

 ing in this type, the mineral benitoite, BaTiSi 3 9 , Fig. 196. 



CLASS, DITRIGONAL HEMIMORPHIC 

 TYPE 12, DITRIGONAL POLAR 



Symmetry. The c axis is an axis of ditrigonal symmetry, which 

 is also polar, with three planes of symmetry intersecting in it. The 

 forms are derived from the ditrigonal equational type by a polar 

 development of the c axis, Fig. 197. 



Forms 

 I. Ditrigonal hemipyramids, 



FIG. 196. Benitoite, Combina- 

 tionof p(10ll), p'(Oril), m(ioro), 

 r(10T2), c(0001). 



U/l 



na: 



a : a : me 



n i 



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



