GUIDE TO THE MINERALOGIC COLLECTIONS 



13 



Isometric system 



Crystals included in the isometric system can be referred to 

 three interchangeable axes at right angles to each other. 1 The 

 molecular structure of the mineral with respect to these axes 

 is revealed not only by the outward form of the crystal but by 

 the property, common to all isometric minerals, of transmitting 

 polarized light equally in all directions. By virtue of this prop- 

 erty a thin section of an isometric mineral cut in any direction 

 will remain dark when viewed in a polarizing microscope be- 

 tween crossed nicols or when observed in a similar way in the 

 tourmalin tongs. -There are five groups, differing slightly in 

 symmetry, included in the isometric system, three of which con- 

 tain nearly all the isometric minerals known. 



Any mineral of which definite crystals are found produces 

 forms which show the symmetry of a distinct group, and 

 it is impossible to find in nature a crystal whose symmetry would 

 place it in more than one group. 



Normal group 



Fig. 26 Fig. 27 Fig. 28 



The general symmetry of this group is shown in fig. 20 and 

 27. The crystallographic axes are axes of tetragonal symmetry 

 and any form belonging to the group, as for example the cube 

 shown in fig. 27. must be symmetric to the planes which inter- 



: In the ideal representation of an isometric crystal these axes are equal. 

 Such a condition, however, seldom occurs in nature, the crystal being dis- 

 torted in various directions. In the following brief outline, as well as in 

 the description of mineral species, the diagrams represent ideal crystals 

 and the reader's attention is directed to the symmetry and distribution of 

 the faces shown, which are invariable however much the actual crystal 

 may be distorted. 



