HKXACONAL SVSTK.M 



X!) 



pyramid, (hkil). 

 pyramid of the first order, (hohl). 



llrxaii'Dnal pyramid of the second order, (hh2hl). 



Dihexa^ttnal prism, (hkio). 



Hexagonal prism of the first order, (hoho). 



Hexagonal prism of the second order, (hhaho). 



Hexagonal base, (0001). 



Examples. Few minerals crystallize in this type; as a rule the 

 holosymmetric class in other systems is 

 the most important class in the system. 



Beryl, Be&A^SiOsV Fig. 159, repre- 

 sents a combination of five forms on a 

 crystal of beryl. 



Hanksite, 9 NasSO 4 . 2 NaaCOs . KC1, 



FIG. 160. Hanksite, c (0001), 



Fig. 160, represents a combination of three forms on a crystal of 

 hanksite. 



CLASS, RHOMBOHEDRAL HEMIHEDRONS 

 TYPE 19, DIHEXAGONAL ALTERNATING 



Symmetry. Crystals of this type possess one dihexagonal al- 

 ternating axis, the c axis; three ^fcligonal axes, the lateral crys- 

 tallographical axes, three planes of symmetry, intersecting in the 

 c axis and each containing one of the intermediate lateral axes, and 



Fia. 161. Type 19, Dihex- 

 agonal Alternating. 



Fio. 162. The Positive Scale- 

 nohedron, k hhil 



also a center of symmetry, Fig. 161. It is to be noted here that the 

 equatorial plane of type 20 is lost and the equatorial edge in the 

 two new forms of the type is represented by a zigzag edge. Forms 

 of this type may be derived from the holohedral forms of type 20 



