GUIDE TO THE MINERALOGIC COLLECTIONS 37 



all of which are inclined to one another. The axes are desig- 

 nated as in the orthorhombic system. The angle between 

 b and c is called a, that between '/ and c, ,' and that between 

 a and 6, y. These angles are distinct for every triclinic species 

 (fig. 149). 



The similarity in molecular structure between minerals of the 

 orthorhombic, monoclinic and triclinic systems indicated by 

 their crystallization is further accentu- 

 ated by their optical properties, errs- I 

 tals of all three systems being opti- ^W^"' 

 cally biaxial; that is, there are two v -— -"" ^""M 

 directions in which polarized light is i c 

 transmitted through them without double Fig - 149 

 refraction. Lines bisecting the angle between these optic axes 

 bear a close relation to the symmetry and outward form of the 

 crystal. 



Normal group 



Crystals occurring in this group are symmetric only to a 

 center, which is the point of intersection of the crystallographic 

 axes. This symmetry admits of forms occurring only in the 

 pairs of faces; 1 thus all prismatic and dome forms which in the 

 orthorhombic system are represented by four faces here occur 

 as hemiprisms and hemidomes. two faces alone being required 

 to satisfy the symmetry of the class. Similarly, pyramidal 



Fig. 160 Fig. 151 



forms which in the orthorhombic system consisted of eighl fares 

 are replaced by four complementary forms each consisl ing of two 

 parallel planes. Compare mode] 11, which shows a triclinic or 

 doubly inclined rhombic prism. With the above exceptions 



See p. 10, fig. 21. 



