76 MINERALOGY 



ity of the c axis. The equator is represented by eight zigzag edges. 

 The lateral axes bisect opposite edges. 



II. Other forms. All other positions of the poles will yield 

 holohedral shapes. The trapezohedron is the only new form of the 

 type, and it has never been found on a crystal. All substances 

 crystallizing in this type have been placed here as a result of a study 

 of their etch figures. 



Forms possible to combine in the type are : 



The right and left tetragonal trapezohedron, r(hkl) r(khl). 



The tetragonal pyramid of the first order, (hhl). 



The tetragonal pyramid of the second order, (ohl). 



The ditetragonal prism, (hko). 



The tetragonal prism of the first order, (hho). 



The tetragonal prism of the second order, (oho). 



The tetragonal base, (001). 



Examples. There are no minerals of this type. The artificial 

 nickel sulphate, NiS0 4 , 6 H 2 0, is placed here, also the sulphate of 

 strychnine. 



CLASS, TETRAGONAL SPHENOIDAL (TETARTOHEDRAL) 

 TYPE 22, TETRAGONAL ALTERNATING 



The c axis in this type is a tetragonal alternating axis; there is 

 no plane or center of symmetry. The forms of the type may be 



considered as derived from the holohe- 

 dral forms by an extension of one quarter 

 of the faces selectecT as illustrated in Fig. 

 140, which also shows the alternating 

 character of the c axis. 



I. Sphenoid of the third order ; r/1 



na : a : mC ; TTK(hkl), TTK(khl), TTK(hkl), 

 4_ 



TTK(khl). 



The faces represented by the poles of 

 Fig. 141 produce the + R sphenoid of 

 FIG. 140. The Plus Left Te- the third order, Fig. 142. The + L form 

 tragonai Sphenoid of the is shown in Fig. 140, in its relation to the 

 ditetragonal pyramid, where the four 



faces selected include the left-hand face of the + octant. There 

 are four sphenoids of the third order possible, bearing the same 

 relation to each other as in the tessera! polar type. The lateral 



