HEX K5ONAL SYS 



101 



litliiuni, KLiSO 4 ; lithium, arnmon um, LiNHjSOij and lithium 

 and rubidium, LiKbSO 4 . Fig. 189 shows the appearance of these 

 combinations. 



('I.ASS, TRIGONAL HKMIIIKDIIONS 

 TYPE 13, DITRIGONAL EQUATORIAL 



Symmetry. Crystals of this type possess one ditrigonal axis, 

 the c axis, three didigonal axes, the intermediate lateral axes, and 

 four planes, three of which intersect in the c axis and each con- 

 tains one of the didigonal axes. The fourth is at right angles 

 to these and contains the a axes. Fig. 190 illustrates this sym- 



Fiu. 190. Type 13, Ditrigonal 

 Equatorial. 



IMC. 101. The Plus Di- 

 trigonal Pyramid (hkll). 



metry. The forms may be considered as hemihedrons, derived 

 from type 20 by extending all the faces in alternate dodecants 

 around the north pole and dodecants below, which intersect with 

 these in the equator, as the shaded faces in Fig. 191. 



I. Ditrigonal pyramid, 



Forms 

 n 



na: 



a : a : me 



n i 



(hkil), (ihkl). 



The faces represented by the poles in Fig. 190 or shown in Fig. 

 191, in their relation to the hexagonal pyramid, when extended, 

 form the plus ditrigonal pyramid. A form bounded by 12 scalene 

 triangles, meeting in six equal equatorial edges. There are 12 

 polar edges, six around each extremity of the c axis, alternate 

 edges are similar both as to length and angle. Fig. 192 represents 

 the ditrigonal pyramid. 



