Ki:i,ATI<>\ OF I \ 'I >IYI 1 M'AL CRYSTALS 



137 



pn-m ami tin- rhombohedroiL Striations are very characteristic 



of .( -rtaiu crystal faces in various mineral species. The cube face 



IK;. '2~'.i. Crystals of Pyrito showing Striations on the Cube and 

 Pyritohedron. 



in pyrite is striated at right angles to a pair of edges, Fig. 273, 

 representing an oscillatory growth between the cube and the pen- 

 tagonal dodecahedron or py- 

 ritohedron. In tourmaline, 

 Fig. 274, the prism faces are 

 striated lengthwise the crys- 

 tal, which represents an os- 

 cillation between the trigonal 

 and hexagonal prisms. 



Twins. In a large num- 

 ber of unions of crystals, all 

 crystallographic equivalent 

 directions are not parallel, as 

 in parallel growths; some 

 may be parallel and others 

 at an angle, as if rotated 

 around an axis 180, or as 

 if reflected across a plane. 

 Fig. 275 is a diagrammatical representation of 6 molecules. In 

 a, b, and c the equivalent directions are all parallel, as in a simple 

 crystal, but x, y, and z are reversed, as if reflected over the plane 



Fio. 274. Tourmaline from Pala, Cali- 

 fornia, showing Longitudinal Striations on 

 the Prism. 



