TI:TIU<;ONAL SVSTKM 



75 



tuples. In combinations the two new forms, the 3rd order 

 pyramid and prism will give the crystals an asymmetric appearance 

 a- in l-'ig. 135a. 



. Scheelite; CaW0 4 : (101) (111) (313). Fig. 136. 



CLASS, TETRAGONAL TRAPEZOHEDRAL (PLAGIOHEDRAL) HEMI- 



HEDRONS 



TYPE 23, TETRAGONAL HOLOAXIAL 



Symmetry. As the name implies, this type has all the axes of 



symmetry of the tetra- 

 gonal system, but no 

 planes or center. The 

 c axis is a tetragonal 

 axis and the lateral 

 and intermediate axes 

 are digonal axes of 

 symmetry. As in all 

 holoaxial types, there 

 are right and left 

 enantiomorphic forms. 



\ 



10. 137. Type 23 : Tetragonal 

 Holoaxial. 



Forms 



r/1 



I. Tetragonal trapezohedron; 

 T(hkl) r(khl). 

 If alternate faces 



na: a: me 



Trapezohedron. 



of the di- 

 tetragonal pyramid around the 

 north pole, represented by circles 

 in Fig. 137 and shaded in Fig. 

 138, and faces alternating with 

 these around the south pole, rep- 

 resented by + in Fig. 137, are 

 extended, the right trapezohedron, 

 Fig. 139, will result; if the un- 

 shaded faces of Fig. 138 are ex- 

 tended, the left trapezohedron 

 will result, Fig. 139 a. 



The form is bounded by eight 

 trapezoidal faces, four of which 

 are grouped around each extrem- 



Fio. 138. 



Fia. 139 a. The 

 Left Tetragonal 

 Trapezohedron, 

 T (212). 



