268 ELEMENTARY CHEMICAL MICROSCOPY 



or shorter than the horizontal and is an axis of sixfold symmetry. Each hori- 

 zontal axis is one of twofold symmetry. 



Interaxes may serve as subordinate axes of symmetry. 



Cleavage lines usually intersect at angles of 60. 



Uniaxial. 



Optic axis coincident with c-axis. 



Crystals three-sided or six-sided, or long rectangles showing three faces. Three- 

 angled and six-angled forms usually isotropic (seen endwise). Long crystals lying 

 on their sides exhibit parallel extinction. 



Interference figure: symmetrical black cross with concentric spectrum colored 

 rings. Tetartohedral crystals are circular polarizing. 



Indices of refraction have same relations as in tetragonal system. 



ORTHORHOMBIC SYSTEM. 



Three axes at right angles to each other, of unequal length. Each axis is one 

 of twofold symmetry. Any axis may be made the vertical. 



Cleavage in direction of diametral planes. 



Biaxial. 



Optic axes: since any crystallographic axis according to convenience may be 

 made the c-axis no relationship may be formulated between the optic and crys- 

 tallographic axes. 



Extinction parallel J in all three positions of the crystals. 



Three indices of refraction, least index in direction of greatest elasticity, greatest 

 index in direction of least elasticity. 



MONOCLINIC SYSTEM. 



Three axes of unequal length. The a-axis and c-axis are oblique to each other. 

 The b-axis is perpendicular to the other two at their point of intersection. The 

 b-axis is an axis of twofold symmetry. 



Cleavage dependent upon crystal species. 



Biaxial. 



Extinction parallel in two positions, oblique in the third. (This does not apply 

 to sections through a crystal.) 



Three indices of refraction. 



TRICLINIC SYSTEM. 



Three crystallographic axes, all of unequal length and oblique to one another. 

 There are no axes of symmetry. 



Cleavage dependent upon crystal species. 



Biaxial. 



Extinction oblique in all three positions. 



Three indices of refraction. 



1 In biaxial crystals complete extinction is obtained only with monochromatic 

 light. 



