RELATION or INDIYIPI \L CRYSTALS 



143 



in the pla.iiiuclascs. Tin- latti-r are confined to the crystal face 

 ;uul are not caused l>\ ;i change in the molecular arrangement, and 

 will then-turf not appear on cleavage faces or ho indicated below 

 the surface. 



Twinning in the isometric system. In isometric minerals the 

 spinel law is the most common. In this method of twinning the 



1 i... L'87. Twins of 

 Albite. 



Fiu. 288. 



FIG. 289. Spinel Twins. 



trigonal axis, which is common to the five types of the system, is 

 the twinning axis, and the face of the octahedron 1 1 1 or the plane 

 parallel to it is the com- 

 position plane. For all 

 minerals crystallizing in 

 the isometric system this 

 is a possible form of 

 twinning. Fig. 289 is a 

 drawing in which mn is 

 the twinning axis, and Fig. 

 290 is a photograph of 

 spinel twins from Frank- 

 lin, New Jersey. 



Penetration twins with 

 the same axis as the twin- 

 ning axis occur in galena 

 and fluorite, Fig. 291, 

 twins of fluorite. 



The second law is where 

 the twinning axis is per- 

 pendicular to the face of the rhombic dodecahedron face, 110, and 

 is a possible twinning law in those types only where this axis is 



FIG. 290. Spinel Twins from Franklin, New 

 Jersey. 



