CHAPTER IV 



TETRAGONAL SYSTEM 



I'm; tetragonal system embraces all those crystals referable to 

 tin re axes at right angles, two of which, the lateral axes, are equal 

 and interchangeable, designated by the letter a. The third or 

 vertical axis, designated by c, is not interchangeable with the lateral 

 axes. The most general parameter of the system is na : a : me, 

 in \\hich the value of n may vary from unity to infinity; m may 

 vary from zero to infinity. With the c axis held vertical and one 

 of the lateral axes in the plane of the paper, the other at 90 to it, 

 the extremities are designated + or as in the isometric sys- 

 tem, the upper right octant being positive. Seven of the 32 

 types are included in the tetragonal system. 



CLASS, TETRAGONAL HOLOHEDRAL (HOLOSYMMETRIC) 

 TYPE 27, DITETRAGONAL EQUATORIAL 



Symmetry. Crystals of this type possess one axis of ditetrag- 

 onal symmetry, the c axis ; four didigonal axes, two of which are 



Fro. 111. The Planes of 

 Symmetry in Type 27. 



FIG. 112. The Ditetragonal 

 Pyramid. 



the lateral axes; the other two, the intermediate axes, bisect the 

 angle between the a axes. All four didigonal axes lie in one plane, 

 the equatorial plane, at 90 to the vertical axis. There are five 

 F 65 



