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NEW YORK STATE MUSEUM 



Trapezohedral group 



The forms in this group possess the lowest grade of symmetry 

 in the hexagonal system, having no plane of symmetry and no 

 center of symmetry; the vertical axis is, however, an axis of tri- 

 gonal symmetry and the three horizontal axes (a) are axes of 

 binary symmetry. 



Trapezohedron. The trigonal trapezohedron (fig. 116, 117) is a 

 characteristic form of this group and consists of six trapezohe- 



Fig. 116 Fig. 117 



dral faces each of which cuts all the axes. Four trapezohedrons 

 are possible; two plus, called respectively right-handed (fig. 116) 

 and left-handed (fig. 117), and two minus forms which are also 



right-handed and left-handed. The 21 faces of these four forms 

 constitute the planes of a dihexagonal pyramid. 



The two crystals of quartz shown in fig. 119, 120 show the 

 trigonal trapezohedron in combination with other forms of the 

 group; they are termed respectively right-handed and left- 

 handed forms. 



