64 MINERALOGY 



90 it is superimposed on R, likewise lefts are congruent, but 

 the rights are enantiomorphic with the lefts, for in no way can the 

 face R be revolved around the digonal axis to bring it into a 

 congruent position with the face + L or L. There are always 

 four tetartohedral forms. They may be derived by superimposing 

 one hemihedral type of selection on another and extending the faces 

 remaining, Fig. 41, page 27. 



Other Forms 



The six other possible forms of the type are derived by a consid- 

 eration of the position of the poles in the triangle, Fig. 107 : 



Pole on the side between the digonal axes = + pyritohedron. 



Pole on the side between the digonal and trigonal axes = 

 tetragonal tristetrahedron. 



Pole on the side between trigonal axes = trigonal tristetrahe- 

 dron. 



Pole on the digonal axes = hexahedron. 



Pole on the trigonal axes = tetrahedron. 



Pole on the angle between the digonal axes = rhombic dodeca- 

 hedron. 



Combinations 



Apparent holohedral and hemihedral forms of more than one 

 type may be found combined on the same crystal ; when this is 

 observed, all forms must be considered as being of tetartohedral 

 symmetry. 



Examples : Minerals crystallizing in the type are rare. 



Ullmannite from one locality crystallizes in pyritohedra and in 

 tetrahedra from another, which would indicate that it is tetarto- 

 hedral. There are a number of artificial salts, as barium nitrate, 

 sodium chlorate, strontium nitrate, and sodium bromate, which 

 crystallize in this type. ' 



