CHAPTER VI 



THE ORTHORHOMBIC, MONOCLINIC, AND TRICLINIC SYSTEMS 

 THE ORTHORHOMBIC SYSTEM 



CRYSTALS of this system possess three crystallographical axes; 

 all at right angles, none of which are interchangeable. The 

 vertical axis is represented by c. The longer lateral axis 

 or macro-axis is represented by b and is placed horizontally 

 from right to left, while the short or brachy-axis, fi, is at 

 right angles to b. Included in the system are three types all of 

 which have at least one axis of digonal symmetry. The forms 

 fall into three groups, according to the relation of their faces 

 to the axes. If the faces cut all three axes, it is a pyramid and 

 there will be no in its indices, as here there are no pyramids or 

 prisms of the second order ; if the face cuts two axes and is paral- 

 lel to the third, it is a prism and there will be one in its indices ; 

 when parallel to a lateral axis it is a dome and receives the name 

 of the lateral axis to which it is parallel, as macrodome. A dome is 

 a prism parallel to a lateral axis. When the face is parallel to two 

 axes, it is a pinacoid and there will be two zeros in its indices ; when 

 parallel to the lateral axes it is a basal pinacoid ; when parallel to c 

 and one of the lateral axes, it takes the name of the lateral axis to 

 which it is parallel, as brachypinacoid. 



CLASS, ORTHORHOMBIC, HOLOSYMMETRIC, OR HOLOHEDRAL 

 TYPE 8, DIDIGONAL EQUATORIAL 



Crystals of this type possess three didigonal axes, correspond- 

 ing to the crystallographical axes; three planes, the diametral 

 planes, and a center of symmetry, Fig. 217. The largest number of 

 faces possible on any crystal form of the type will be 8, one in each 

 octant into which the three planes of symmetry, Fig. 218, divide 

 space. 



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