HEXAGONAL SYSTKM 



will remain. The>e are the rhoml>ohedrons of the second order, 

 Fig. 1S.">, derived from the hexagonal pyramid of the second order. 



III. Rhombohedrons of the first order. 



If the pole> :u-e moved into the plane containing the intermediate 

 azee, the resulting form is the rhombohedron of the first order. 



The three rhombohedrons differ only in the position of the 

 Literal axes: in the first order they end in the central point of the 

 zigzag edges; in the second order they end in the median point 

 of the line connecting the central points of the zigzag edges; in 

 the third order they end asymmetrically on this same line between 

 the above two points. 



Other forms. When the poles are moved 

 to the equatorial plane the first, second, and 

 third order prisms are formed. 



The possible forms to combine on crystals 

 of this type will be : 



Plus and minus right and plus and minus 

 left rhombohedrons of the third order, (khil), 

 (hkil), (ihkl), (ikM). 



Plus and minus rhombohedrons of the sec- 

 ond order, (hh2hl), (2hhhl). 



Plus and minus rhombohedrons of the first 

 order, (hkil), (khil). 



Plus and minus prisms of the third order, 

 (hoho), (ohho). 



Hexagonal prism of the second order, 

 (hh2ho). 



Hexagonal prism of the first order, (hoho). 



Hexagonal base, (0001). 



Examples. Dolomite, MgCa(CO 3 ) 2 ; 

 Willemite, Z 

 type, Fig. 186. 



FIG. 186. Phenacitc. a 

 Combination of the 

 Negative Right Rhom- 

 bohedron of the Third 

 Order and the Prism of 

 the First Order. 



4 



Phenacite, Be 2 Si0 

 and Dioptase, H 2 CuSi0 4 , crystallize in this 



CLASS, PYRAMIDAL HEMIMORPHIC 

 TYPE 14, HEXAGONAL POLAR 



Symmetry. Crystals of this type possess an axis of hexagonal 

 symmetry, the c axis, and no plane or center of symmetry. It 

 is a polar development of the hexagonal equatorial type. Fig. 187 

 shows the symmetVy. 



