CRYSTAL SYSTEMS 267 



Never depend upon "observations made upon a single indi- 

 vidual. Check the readings by again making a crystal parallel 

 to the cross-hairs and turning the polarizer or analyzer until 

 the colors of field and crystal are identical; read the gradua- 

 tions on the nicol mounting; the angles observed should be 

 identical. 



CHARACTERISTICS OF THE SIX CRYSTAL SYSTEMS. SUMMARY. 



The chief characteristic features exhibited by individuals of 

 the six different crystal systems which will prove of assistance 

 in microchemical analysis may be summarized as follows: 



ISOMETRIC SYSTEM (Cubic System). 



The three crystallographic axes are all at right angles. Each axis is one of four- 

 fold symmetry. All axes are of like value, hence any axis may be made the c-axis. 



Cleavage usually parallel to the faces of the crystal and symmetrical with refer- 

 ence to the crystallographic axes. 



Optically isotropic, hence there is no change between crossed nicols. No inter- 

 ference figures. 



A single refractive index, independent of direction. 



TETRAGONAL SYSTEM. 



Two equal horizontal crystallographic axes at right angles to each other and to 

 the vertical. Vertical or c-axis either longer or shorter than the other two. c-axis 

 is one of fourfold symmetry. Each horizontal axis is one of twofold symmetry. 



Interaxes (lines) bisecting the interaxial angles between a- and b-axes may also 

 serve as subordinate axes of symmetry. 



Cleavage, rectangular. 



Uniaxial. 



Optic axis coincident with c-axis. Hence in one position isotropic; in other two, 

 parallel extinction. 



Crystals four-sided or eight-sided or lath-shaped or six-sided. Four- or eight- 

 sided crystals isotropic (seen on end). Crystals lying on their side give parallel 

 extinction. 



Interference figure: symmetrical cross with concentric rings. 



Index of refraction, e in direction parallel to optic axis; o> index in the plane 

 normal to the optic axis. 



HEXAGONAL SYSTEM. 



Vertical or c-axis is at right angles to the three horizontal axes at their point 

 of intersection. Horizontal axes intersect at angles of 60. c-axis may be longer 



