IMA \(io\.\l. SYSTI.M 







pyramid of the third order will fall in OIK- |>l:nic, producing ; , new 

 open form, the prism of the third order, \"\ K . 177, in whicli the a 



ax0B neither terminate in the edges or 



ill the center of the faee>, hut OR the 



line drawn between these two points. 



III. Other forms of this type are 

 like the hexagonal holohedral in shape. 

 The possible forms to be found in 

 combination will be : 



Plus and minus hexagonal pyramid 

 of the third order, ir(hkil), ir(khil). 



Hexagonal 

 pyramid of the 

 first order, 

 ir(hohl). 



Hex agonal 

 pyramid of the 

 second order, 

 ir(hh2hl). 



Plus and minus hexagonal prism of tho 

 * third order, Tr(hkio), ir(khio). 



Hexagonal prism of the first order, ir(hoho). 

 Hexagonal prism of the second order, 

 Tr(hh2ho). 



Hexagonal base, ir(OOOl). 

 Examples. Apatite, Ca 8 (FCl)(PO 4 )3, Fig. 178, shows a combina- 

 tion of the three pyramids, a prism, and the base. * 



Pyromorphiie, Pb 6 Cl(PO 4 ) 3 ; Mimetite, Pb B Cl(So 4 ) 3 , and Vana- 

 dinite, Pb B Cl(O 4 ) 3 , also crystallize in this group. 



1 I... 177. Hexagonal Prism of 

 the Third Order. 



FIG. 178. Apatite, 

 a Combination of 

 p (1011), u (1231), 

 s (1121), m(1010). 



CLASS, TRAPEZOIDAL (PLAGIOHEDRAL) HEMIHEDRONS 

 TYPE 16, HEXAGONAL HOLOAXIAL 



Symmetry. Crystals of this type possess all the axes of the di- 

 hexagonal equatorial type, but no planes, or center of symmetry. 

 They have therefore one axis of hexagonal symmetry, the c axis, 

 and six digonal axes corresponding to the lateral and intermediate 

 axes, lying in the position of the equatorial plane, Fig. 179. The 

 forms are plagiohedral and may be derived from the holohedral 

 forms by extending alternate faces around the poles, Fig. 180. 



