14 



MINERALOGY 



360 by a revolution of the solid around an axis, then it is said to 

 be symmetrical in respect to an axis, or to possess an axis of sym- 

 metry. In Fig. 14, if from any point a, in 

 the solid, a line be drawn perpendicular to 

 the axis at o, when revolved about the 

 axis o as a center the point a will describe 

 a circle ; if on this circle it meet a point 

 a', the conditions surrounding which are 

 exactly the same as those surrounding the 

 point a, then o is an axis of symmetry. 

 The crystal will, in this instance, after a 

 rotation of 180 become congruent, or 

 will appear as if it had not been revolved. 

 Symmetry in respect to an axis is the 

 symmetry of revolution. 



An axis of symmetry is a digonal axis, 

 if the crystal becomes congruent after a 

 Such an axis is represented by the conven- 



FIG. 14. A Digonal Axis 

 of Symmetry : Gypsum. 



revolution of 180 C 



tional sign as at o, Fig. 14. A didigonal 



axis is a digonal axis at the intersection of 



two planes of symmetry. 



If the crystal becomes Congruent after 



a revolution of 120, the axis is a trigonal 

 axis, represented 

 as at o, Fig. 15. 

 A ditrigonal axis 

 is a trigonal axis 

 at the intersec- 



FIG. 15. Trigonal Axis of 

 Symmetry : Tourmaline. 



tion of three planes of symmetry. 



If it becomes congruent every 90 C 

 axis is a tetragonal 

 axis and is repre- 

 sented as at o, 

 Fig. 



the 



FIG. 16. Tetragonal Axis 

 of Symmetry : Scheelite. 



"' 16. A dite- 



tragonal axis is a tetragonal axis at the in- 

 tersection of four planes of symmetry. 



If it becomes congruent every 60 it is a 

 hexagonal axis and is represented as at o, 

 Fig. 17. A dihexagonal axis is a hexagonal 



, , , . , , . FIG. 17. Hexagonal 



axis at the intersection of six planes of sym- Axis of Symmetry : Apa- 

 metry. The above four axes are all the . tite. 

 possible axes of direct rotation to occur in crystals. 



