GUIDE TO THE MINERALOGIC COLLECTIONS 



25 



Hexagonal system 



There are in general many points of resemblance between 

 hexagonal crystals and those which are included in the tetra- 

 gonal system. This analogy is accentuated by the fact that the 

 molecular structure of the minerals in both systems as exhibited 

 in their optical properties show striking similarity. Hexagonal 

 as well as tetragonal crystals are said to be optically uniaxial; 

 that is, in every crystal of these two systems a section cut nor- 

 mal to the vertical axis will remain dark when viewed between 

 crossed nicols in the polarizing microscope; any other section 

 will show an interference color which changes to darkness or 

 " extinction " at regular intervals as the stage of the micro- 

 scope is rotated. 



Hexagonal crystals are referred to four crystallographic axes, 

 one of which is vertical and perpendicular to the plane of the 

 other three; this vertical axis, as in the tetragonal system, is 

 indicated by c. The three horizontal axes are interchangeable 

 and at 60° from each other; they are indicated by a. 



A Hexagonal division 

 Normal group 



The general symmetry of this group is shown in fig. 90 and 91. 

 The vertical axis is an axis of hexagonal symmetry and each 



Fig. 90 Fig. 91 



basal axis is an axis of binary symmetry; there are also three 

 axes of binary symmetry bisecting the angles between the cry- 

 stallographic axes. Crystals in this group are symmetric to a 

 plane of symmetry through the basal axes and to six planes of 

 symmetry passing through the vertical axis and each of the axes 

 of binary symmetry. The nomenclature of the forms is analo- 

 gous with that used in the normal group of the tetragonal 

 system, the forms being briefly stated as follows: 



