CRYSTALLOGRAPHY 



27 



In all cases there arc two forms of hemihedrons possible to l>e 

 drri\ ! from each holohrdron, for the white faces in each case could 

 le cxtciitlcd to obliterate the shaded faces; one of these is the +, 

 the other the hemihedron, in classes I and II. In class III they 

 are ri^ht and left forms. 



Tetartohedral forms. In some types of crystals with still 

 lower symmetry, only one quarter of the face of the general form 

 may be required by the symmetry to complete the form; such 

 fon us are termed tetartohedrons. The faces extended to form te- 

 tartohedrons must in each case modify the extremities of inter- 

 changeable axes in the same manner. If in Fig. 41 the shaded faces 

 are extended, the tetartohedral form of Fig. 42 will be produced, 

 having 12 faces. This is the right positive form, designated 



R 



o 



or irk (hkl). If the three unshaded faces in the 



upper right octant and the corresponding faces in other octants are 

 extended, the + left form will be produced. These two forms are 



Fio. 41. (a:2a:3a.) 



mirror images of each other ; there is no way in which they can be 

 revolved into congruent positions. It is like a left glove on a right 

 hand ; such forms are enantiomorphic. Two other forms are 

 possible: the minus right produced by extending the faces R, 

 Fig. 41, and the minus left produced by extending L. There 

 are always four tetartohedral forms possible, the rights and the 

 lefts ; the rights are congruent with each other and the lefts are 

 also congruent, but the rights are enantiomorphic with the lefts. 



