60 



MINERALOGY 



hedron are selected, so that pairs taken intersecting in the diame- 

 tral planes (Fig. 37, the shaded faces) are then extended, they 

 will produce a new form, the diploid, Fig. 100, with 24 four-sided 

 faces, three of which are grouped around the trigonal and four 

 around the didigonal axes. Figure 100 a shows the symmetry and 

 poles of the form. 



na : a : oo a 

 II. Pyritohedron ; pentagonal dodecahedron ; 



ir(hlo) ir(hlo). 



When the pole is moved to the side of the triangle between the 

 didigonal axes, and in the plane of symmetry, a new form will be 

 produced, the pyritohedron, Fig. 101, bounded by 12 pentagonal 

 faces. 



Three faces are grouped around the trigonal axes, and the di- 

 digonal axes bisect the long edge between adjacent faces. The 



FIG. 101. The Pyritohedron, 

 ir(hlo). 



FIG. 102. Pyrite: Combi- 

 nation of (100) and IT (hlo). 



pyritohedron may be considered as derived from the tetrahexahe- 

 dron by extending alternate faces. 



III. Other forms. 



Other possible positions of the poles are identical in number 

 and positions with the forms of type 32 : Therefore the tetragonal 

 trisoctahedron, trigonal trisoctahedron, hexahedron, rhombic 

 dodecahedron, and octahedron may be found in combination with 

 the diploid and pyritohedron. They also reproduce the same 

 forms when the method of selection to form hemihedrons of this 

 class is applied to them. 



Combinations 



Geometrical holohedral forms of this type must possess the pyri- 

 tohedral symmetry. Pyrite, FeS2, crystallizes in all seven forms of 



