m.l.ATloN or l\DI\ IDl'Al. CRYSTALS 



1 I.') 



a\i- i> perpendicular to :i pyramid face and the pyramid fun- 

 is the composition plane. This law is a very common one in 

 the trigonal types, and is the same as where the twinning axis 

 i- perpendicular to a scalenohedral or rhombohedral face. In 

 calcite the common form of twinning is where the twinning axis 

 is normal to the rhomhohedron e (0112), Fig. 294. 



FIG. 294. Calcite Twins in which e (0112) is the Composition Face. 

 Guanajuato, Mexico. 



In types of alternating symmetry and trigonal types the vertical 

 axis is a trigonal axis, and is therefore a possible twinning axis with 

 the base as the composition plane; Fig. 295 is such a twin of cal- 

 cite. In the polar types supplementary twinning, as in the tetrag- 

 onal system, is a possible law. There are very few minerals of 

 these types ; the most common is tourmaline, in which there are no 

 twins; and in nephelite the polar symmetry is shown by the etch 

 figures only, and all crystals must be considered as examples of 

 supplementary twinning. 



In the holoaxial types, of which quartz is an example, twinning 

 by reflection is the rule, as in the Brazilian twins, where the plane 

 of reflection is parallel to the prism face (1120), Fig. 296, and x 

 is a reflection of x'. 



